20100708-PSD CEU 169-Hot Water Re Circulation
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Recircling Domestic Hot 9 Water Systems
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Continuing Education rom Plumbing Systems & Design
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CONTINUINg eDUCa eDUCaTION TION
Recirculating Domestic Hot Water Systems INTRODUCTION
obtaining hot water at xtures has become as critical an issue as the It has been determined through eld studies that the correct sizing energy losses caused by hot water temperature mai ntenance systems. and operation o water heaters depend on the appropriateness o the o reduce the wasting o cooled hot water signicantly, the engineerhot water maintenance system. I the hot water maintenance system ing community has reevaluated the permissible distances or unciris inadequate, the water heater sizing criteria are wrong and the culated, dead-end branches to periodically used plumbing xtures. temperature o the hot water distributed to the users o the plumb- Te new allowable distances or uncirculated, dead-end branches ing xtures is below acceptable standards. Additionally, a poorly represent a trade-o between the energy utilized by the hot water designed hot water maintenance system wastes large amounts o maintenance system and the cost o the insulation, on the one hand, energy and potable water and creates time delays or those using the and the cost o energ y to heat the excess cold water makeup, the cost o plumbing xtures. Tis chapter addresses addresses the c riteria or establish- wasted potable water, and extra sewer surcharges, on the other hand. ing an acceptable time delay in delivering hot water to xtures and Furthermore, engineers should be aware that various codes now limit the limitations o t he length between a hot water recirculation system the length between the hot water maintenance system and plumbing and plumbing xtures. It also d iscusses the temperature drop across xtures. Tey also should be aware o the potential or liability i an a hot water supply system, types o hot water recirculation system, owner questions the adequacy o their hot water system design. and pump selection criteria, and gives extensive inormation on the What are reasonable delays in obtaining hot water at a xture? insulation o hot water supply and return piping. For anything beside very inrequently used xtures (such as those in industrial acilities or certain xtures in oce buildings), a delay o 0 to 10 sec is normally considered acceptable or most residential occupancies and public xtures in oce buildings. A delay o 11 to 30 sec BaCkgROUND In the past, the plumbing engineering community considered the is marginal but possibly acceptable, and a time delay longer than 31 sec is normally considered unacceptable and a signicant waste o prompt delivery o hot water to xtures either a requirement or a project or a matter o no concern. Te plumbing engineer’s decision water and energy. Tereore, when designing hot water systems, it is was based primarily on the type o acility under consideration and prudent or the designer to provide some means o getting hot water to the xtures within these acceptable time limits. Normally this means the developed length rom the water heater to the arthest xture. Previous reerence material and proessional common practices that there should be a ma ximum distance d istance o approxi mately 25 t (7.6 (7.6 m) have indicated that, when the distance rom the water heater to the between the hot water maintenance system and each o the plumbing arthest xture exceeds 100 t (30.48 m) water should be circulated. xtures xtu res requiring hot water, the distance depending on the water fow However, this recommendation is subjective, and, unortunately, rate o the plumbing xture at the end o the line and the size o the some engineers and contractors use the 100-t (30.48-m) criterion line. (See ables 1, 2, and 3.) Te plumbing designer may want to stay as the maximum length or all uncirculated, uninsulated, dead-end under this length limitation because the actual installation in the eld hot water branches to xtures in order to cut the cost o hot water may dier slightly rom the engineer’s design, and additional delays distribution piping. Tese long, uninsulated, dead-end branches to may be caused by either the routing o the pipe or other problems. xtures create considerable problems, such as a lack o hot water at Furthermore, with the low xture discharge rates now mandated by xtures, inadequately sized water heater assemblies, and thermal national and loca l laws, it takes considerably longer to obtain hot water temperature temperature esca lation in showers. showers. rom non-temperature maintained hot water lines than it did in the past, when xtures had greater fow rates. For example, a public lavaTe 100-t (30.48-m) length criterion was developed in 1973 ater the Middle East oil embargo, when energy costs were the paramount tory with a 0.50 or 0.25 gpm (0.03 or 0.02 L/sec) maximum discharge concern and water conservation was given litt le consideration. consideration. Since rate would take an excessive amount o time to obtain hot water rom the circulation o hot water causes a loss o energy due to radiation 100 t (30.48 m) o uncirculated, uninsulated hot water piping. (See able 3.) Tis table gives c onservative approximat ions o the amount o and convection in the ci rculated system and such energy losses have have able 3.) to be continually replaced by water heaters, the engineering commu- time it takes to obtain hot water at a xtu re. Te times are based on the nity compromised between energy loss and construction costs and size o the line, the xture fow rate, and the times required to replace the cooled o hot water, to heat the pipe, and to oset the convection developed the 100-t (30.48-m) maximum length criterion. energy lost by the insulated hot water line.
LeNgTh aND TIme CRITeRIa Recently, due to concern about not only energy conservation but also the extreme water shortages i n parts o the countr y, the 100-t 100-t (30.48-m) (30.48-m) length criteria ha s changed. Water wastage caused by the long delay in
Note: All decimal equivalencies in the metric calculations are rounded. Tereore, the metric conversions shown in the text may vary slightly rom the answers shown in the metric equations.
Domestic Water Heating Design Manual II, Chapter 14: “Recirculating D omestic Hot Water Systems,” © American Society of Plumbing Engineers, 2006
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Table 1 Water Contents and Weight of Tube or Piping per Linear Foot Nominal Diameter (in.)a ½ ¾ 1 1¼ 1½ a
Copper Pipe Type L Water Wgt. (gal/t) (lb/t) 0.012 0.285 0.025 0.445 0.043 0.655 0.065 0.884 0.093 1.14
Copper Pipe Type M Water Wgt. (gal/t) (lb/t) 0.013 0.204 0.027 0.328 0.045 0.465 0.068 0.682 0.100 0.94 0
Steel Pipe Sc hedule 40 Water Wgt. (gal/t) (lb/t) 0.016 0.860 0.028 1.140 1.140 0.045 1.680 0.077 2.280 0.106 2.720
CPVC Pipe Schedule 40 Water Wgt. (gal/t) (lb/t) 0.016 0.210 0.028 0.290 0.045 0.420 0.078 0.078 0.590 0.106 0.710
Pipe sizes are indicated or mild steel pipe sizing.
Table 1(M) Water Contents and Weight of Tube or Piping per Meter Nominal Diameter (mm)a DN15 DN20 DN25 DN32 DN40 a
Copper Pipe Type L Water Wgt. (L) (kg) 0.045 0.129 0.129 0.095 0.09 5 0.202 0.163 0.297 0.246 0.401 0.352 0.517
Copper Pipe Type M Water Wgt. (L) (kg) 0.049 0.204 0.102 0.328 0.170 0.465 0.4 65 0.257 0.682 0.379 0.940
Steel Pipe Schedule 40 Water Wgt. (L) (kg) 0.061 0.390 0.106 0.517 0.170 0.170 0.762 0.291 1.034 0.401 1.233
CPVC Pipe Schedule 40 Water Wgt. (L) (kg) 0.061 0.099 0.106 0.132 0.170 0.191 0.295 0.268 0.401 0.322
Pipe sizes are indicated or mild steel pipe sizing.
Table 2 Approximate Fixture and Appliance Water Flow Rates Maximum Flow Ratesa GPM L/Sec 2.0 1.3 0.5 0.03 0.25 gal/cycle 0.946 L/cyc le 2.5 0.16 2.5 0.16
Fittings Lavatory aucet Public non-metering Public metering Sink aucet Shower head Bathtub aucets Single-handle Two-handle Service sink aucet Laundry tray aucet Residential dishwasher Residential washing machine a
2.4 minimum 4.0 minimum 4.0 minimum 4.0 minimum 1.87 aver 7.5 aver
0.15 minimum 0.25 minimum 0.25 minimum 0.25 minimum 0.12 0.12 aver 0.47 aver
Unless otherwise noted.
Table 3 Approximate Time Required to Get Hot Water to a Fixture Fixture Flow Rate (gpm) Piping Pipin g Length (t)
Delivery Time (sec) 1.5 2.5
0.5
4.0
10
25
10
25
10
25
10
25
Table 3(M) Approximate Time Required to Get Hot Water to a Fixture Delivery Time (sec) Fixture Flow 0.03 0.10 0.16 Rate (L/sec) Piping Pipin g Length (m) Copper Coppe r Pipe
3.1
7.6
3.1
7.6
3.1
7.6
0.25 3.1
7.6
DN15 DN15 DN22
25 63a 48a 119a
8 16
21 40a
5 10
13 24
3 6
8 15
Steel Pipe DN15 Sched. 40 DN20
63a 157a 91a 228a
21 30
52a 76a
13 18
31a 46a
8 11
20 28
CPVC Pipe DN15 Sched. 40 DN20
64a 159a 95a 238a
21 32
53a 79a
13 19
32a 48a
8 12
20 30
Note: Table based on various xture fow rates, piping materials, and dead-end branch lengths. Calculations are based on the amount o heat required to heat the piping, the water in the piping, and the heat loss rom the piping. Based on water temperature o 60°C and an air temperture o 21.1°C. a
Delays longer than 30 sec are not acceptable.
ResULTs Of DeLays IN DeLIveRINg ResULTs hOT WaTeR TO fIxTURes As mentioned previously, when there is a long delay in obtaining hot water at the xture, there is signicant wastage o potable water as the cooled hot water supply is simply discharged down the drain unused. Furthermore, plumbing engineers concerned about total system costs should realize that the cost o this wasted, previously heated water must include: the original cost or obtaining potable water, the cost o previously heating the water, the nal cost o the waste treatment o this excess potable water, which results in larger sewer surcharges (source o supply to end disposal point), and the cost o heating the new cold water to bring it up to the required temperature. Furthermore, i there is a long delay in obtaining hot water at the xtures, the aucets are turned on or long periods o time to bring the hot water supply at the xture up to the desired temperature. Tis can cause the water heating system to run out o hot water and make the heater sizing inadequate, because the heater is unable to heat all the extra cold water brought into the system through the wastage o the water discharged down the drain. In addition, this extra cold water entering the hot water system reduces the hot water supply temperature. Tis exacerbates the problem o insucient hot water because to get a proper blended temperature more lower temperature hot water water will be used to achieve the nal mi xed water temperature. (See Chapter 1, able 1.1.) Additionally, this accelerates the downward spiral o the temperature o the hot water system. Another problem resulting rom long delays in getting hot water to the xtures is t hat the xtures operate or longer longer than expected periods o time. Tereore, the actual hot water demand is greater than the demand normally designed or. Tereore, when sizing the water heater and the hot water piping distribution system, the designer should be aware that the lack o a proper hot water maintenance system can seriously impact the required heater size.
Copper Coppe r Pipe
½ in. ¾ in.
25 48a
63a 119a
8 16
21 40a
5 10
13 24
3 6
8 15
Steel Pipe Sched. 40
½ in. ¾ in.
63a 91a
157a 228a
21 30
52a 76a
13 18
31a 46a
8 11
20 28
meThODs Of DeLIveRINg ReasONaBL ReasONaBLy y PROmPT hOT WaTeR sUPPLy
CPVC Pipe Sched. 40
½ in. ¾ in.
64a 95a
159a 238a
21 32
53a 79a
13 19
32a 48a
8 12
20 30
Hot water water maintenance systems are as var ied as the imagi nations o the plumbing engineers who create them. Tey can be grouped into three basic categories, though though any actual i nstallation may be a combination o more than one o these types o system. Te three basic categories are 1. Circulation systems. 2. Sel-regulating heat trace systems. 3. Point-o-use water heaters (include booster water heaters).
Note: Table based on various xture fow rates, piping materials, and dead-end branch lengths. Calculations are based on the amount o heat required to heat the piping, the water in the piping, and the heat loss rom the piping. Based on water temperature o 140°F and an air temperture o 70°F. a
Delays longer than 30 sec are not acceptable.
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CONTINUING EDUCATION: Recirculating Recirculating Domestic Hot Water Systems Circulation Systems or Commercial, Industrial, and Large Residential Projects A circulation system is a system o hot water supply pipes and hot water return pipes with appropriate appropriate shuto valves, balancing valves, circulating pumps, and a method o controlling the circulating pump. Te diagrams or six basic circulating systems are shown in Figures 1 through 6.
Fixture 1 Upeed Hot Water Water System with Heater at Bottom o System. * See text or requirements or strainers.
Figure 2 Downeed Hot Water System with Heater at Top o System. * See text or requirements or strainers.
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Figure 3 Upeed Hot Water Water System with Heater at Bottom o System. * See text or requirements or strainers.
Figure 4 Downeed Hot Water System with Heater at Top o System. * See text or requirements or strainers.
Figure 5 Combination Upeed and Downeed Downeed Hot Water System System with Heater at Bottom o System. Note: This piping system increases the developed length of the HW system over the upfeed systems shown in Figures 14.1 and 14.3. * See text for requirements for strainers. JULY/AUGUST 2010
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CONTINUING EDUCATION: Recirculating Recirculating Domestic Hot Water Systems
Figure 6 Combination Combinatio n Downeed and Upeed Hot Water System with Heater at Top o System.
Note: This piping system increases the developed length o the HW system over the downeed systems shown in Figures 14.2 and 14.4. * See text or requirements or strainers.
Sel-Regulating Heat Trace Over approximately the last 20 years, sel-regulating heat trace has come into its own because o the problems o balancing circulated hot water systems and energy loss in the return piping. For urther discussion o this topic, see Chapter 15.
Point-o-Use Point -o-Use Heaters Tis concept is applicable when there is a single xture or group o xtures that is located ar rom the temperature maintenance system. In such a situation, a small, instantaneous, point-o-use water heater—an electric water heater, a gas water heater, or a small underxture storage type water heater o the magnitude o 6 gal (22.71 L)—can be provided. (See Figure 7.) Te point-o-use heater will be very cost-eective cost-eective because it wil l save the cost o r unning hot water piping to a xt ure that is a long dista nce away rom the temperature maintenance system. Te plumbing engineer must remember, however, that when a water heater is installed there are various code and installation requirements that must be complied with, such as those pertaining to & P relie valve discharge. Instantaneous electric heaters used in point-o-use applications can require a considerable amount o power, and may require 240 or 480 V service.
POTeNTIaL PROBLems IN CIRCULaTeD hOT hOT WaTeR maINTeNaNCe sysTems
Figure 7 Instantaneous Point-o-Use Point-o-Use Water Water Heater Piping Diagram. Source: Courtesy o Chronomite Laboratories, Inc.
Te ollowing are some o the potential problems with circulated hot water maintenance systems that must be addressed by the plumbing designer.
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Water Velocities in Hot Water Piping Systems
a DeLay IN OBTaININg hOT WaTeR aT DeaD-eND LINes
For copper piping systems, it is very important that the circulated hot water supply piping and especially the hot water return piping be sized so that the water is moving at a controlled velocity. High velocities in these systems can cause pinhole leaks in t he copper piping piping in as short a period as six months or less.
Keep the delay in obtaining hot water at xtures to within the time (and branch length) parameters given previously to avoid unhappy users o the hot water system and to prevent lawsuits.
fLOW BaLaNCINg DevICes Te ollowing are the more common types o bala ncing device.
Balancing Systems Fixed Orifces and Venturis It is extremely important that a circulated hot water system be balanced or its specied fows, including all the various individual loops Tese can be obtained or specic fow rates and simply inserted into within the circulated system. Balancing is required even though an the hot water return piping system. (See Figure 8.) However, extreme insulated circulated line usually requires very little fow to main- care should be taken to locate these devices so they can be removed tain satisactory system temperatures. I the individual hot water and cleaned out, as they may become clogged with the debris in the circulated loops are not properly balanced, the circulated water will water. It is recommended, thereore, that a strainer with a blowtend to short-circuit through the closest loops, creating high veloci- down valve be placed ahead o each o these devices. Additionally, a ties in that piping system. Furthermore, the short-circuiting o the strainer with a ne mesh screen can be installed on the main water circulated hot water will result in complaints about the long delays line coming into the building to help prevent debris buildup in the in getting hot water at the remotest loops. I the hot water piping is individual strainers. Also, a shuto valve should be installed beore copper, copper, high velocities can create velocity erosion which wi ll destroy and ater these devices so that an entire loop does not have to be the piping system. drained in order to service a strainer or balancing device. Because o the problems inherent in manually balancing hot water circulation systems, systems, many proessionals incorporate actory preset fow control devices in their hot water systems. systems. While the i nitial cost o such a device is higher than the cost o a manual balancing valve, a preset device may be less expensive when the eld labor cost or balancing the entire hot water system is included. When using a preset fow control device, however, the plumbing designer has to be ar more accurate in selecting the control device’s capacity as there is no possibility o eld adjustment. Tereore, i more or less hot water return fow is needed during the eld installation, a new fow control device must be installed and the old one must be removed and discarded. Isolating Portions o Hot Water Systems It is extremely important in c irculated systems that shuto valves be provided to isolate an entire circulated loop. Tis is done so that i individual xtures need modication, their piping loop can be isolated rom the system so the entire hot water system does not have to be shut o and drained. Te location o these shuto valves should be given considerable thought. Te shuto valves should be accessible at all times, so they should not be located in such places as the ceilings o locked oces or condominiums.
Maintaining the Balance o Hot Water Systems o ensure that a balanced hot water system remains balanced ater the shuto valves have been utilized, the hot water return system must be provided with a separate balancing valve in addition to the shuto valve or, i the balancing valve is also used as the shuto valve, the balancing valve must have a memory stop. (See the discussion o “balancing valves with memory stops” below.) With a memory stop on the valve, plumbers can return a system to its bala nced position ater working on it rather than have t he whole piping piping system remain unbalanced, which would result in serious problems.
Providing Check Valves at the Ends o Hot Water Water Loops Te designer should provide a check valve on each hot water return line where it joins other hot water return lines. Tis is done to ensure that a plumbing xture does not draw hot return water instead o hot supply water, which could unbalance the hot water system and cause delays in obtaining hot water at some xtures.
Figure 8 Fixed Orifces and Venturi Venturi Flow Meters. Meters. Source: Courtesy o Gerand Engineering Co.
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CONTINUING EDUCATION: Recirculating Recirculating Domestic Hot Water Systems Factory Preset Automatic Flow Control Valves Te same admonition about about strainers and valves g iven or “xed orices and venturis” above applies to the installation and location o these devices. (See Figure 9.)
Flow Regulating Valves Tese valves can be used to determine the fow rate by reading the pressure drop across the valve. Tey are avai lable rom various manuacturers. (See Figure 10.)
Balancing Valves Valves with Memory Stops Tese valves can be adjusted to the proper setting by installi ng insertable pressure measuring devices (Pete’s Plugs, etc.) in the piping system, which indicate the fow rate in the pipe line. (See Figure 11.)
sIzINg hOT WaTeR WaTeR ReTURN PIPINg s ysTems aND ReCIRCULaTINg PUmPs Te method or selecting the proper size o the hot water return piping system and the recirculating pump is airly easy, but it does require engineering judgment. First, the plumbing engineer has to design the hot water supply and hot water return piping systems, keeping in mind the para meters or total developed developed length1, prompt delivery o hot water to xtures, and velocities in pipe lines. Te plumbing engineer has to make assumptions about the sizes o the hot water return piping.
Figure 10 Adjustable Adjustab le Orifce Flow Control Valve. Source: ITT Industries. Used with permission.
Figure 11 Adjustable Balancing Valve with Memory Stop. Source: Courtesy o Milwaukee Valve Co.
1
Figure 9 Preset Sel-Limiting Sel-Limiting Flow Control Control Cartridge. Source: Courtesy o Griswold Controls.
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See American Society o Plumbing Engineers, 2000, Cold-water systems, Chapter 5 in ASPE Data Book , Volume 2, or piping sizing methods. WWW.PSDMAGAZINE.ORG
Ater the hot water supply and hot water return systems are designed, the designer should make a piping diagram o the hot water supply system and the assumed return system showing piping sizing and approximate lengths. From this piping diagram the hourly heat loss occurring in the circulated portion o the hot water supply and return systems can be determined. (See able 4 or minimum required insulation thickness and able 5 or approximate piping heat loss.) Next determine the heat loss in the hot water storage tank i one is provided. (See able 6 or approximate tank heat loss.) Calculate the total hot water system energy loss (tank heat loss plus piping heat loss) in British thermal units per hour (watts). Tis total hot water system energy loss is represented by q in Equation 1 below. Note: Heat losses rom storage type water heater tanks are not normally included in the hot water piping system heat loss because the water heater capacity takes care o this loss, whereas pumped hot water has to replace the piping convection losses in the piping system.
Table 4 Minimum Pipe Insulation Thickness Required Insulation Thickness Thickness for Piping (in.) Runouts 2 in. or Lessa
1 in. or Less
1¼–2 in.
2½–4 in.
5 & 6 in.
8 in. or Larger
½
1
1
1½
1½
1½
Table 5(M) Approximate Insulat ed Piping Heat Loss and Surface Temperature Temperature Nominal Insulation Heat Loss Surace Pipe Size Thickness (W/m) Temperature (mm) (mm) (°C) DN15 25 7.7 20 DN20 25 9.6 21 DN25 25 9.6 21 DN32 25 12.5 21 DN40 25 12.5 21 DN50 or less 13a 23.1 or less 23 DN50 25 25 15.4 21 DN65 38 38 11.5 19 DN80 38 38 15.4 20 DN100 38 18.3 21 DN150 38 26.0 21 DN200 38 30.8 21 DN250 38 36.5 21 Note: Figures based on average ambient temperature o 18°C and annual average wind speed o 12 km/h. a
Uncirculating hot water runout branches only.
Note: Data based on berglass insulation with al l-service jacket. Data will change depending on actual type o insulation used. Data apply to recirculating sections o hot water systems and the rst 3 t rom the storage tank o uncirculated systems.
Table 6 Heat Loss from Various Size Tanks Tanks with Various Insulation Thicknesses
a
Uncirculated pipe branches branches to individual xtures (not exceeding exceeding 12 t in length). For lengths longer than 12 t, use required insulation thickness shown in table.
Insulation Thickness (in.)
Table 4(M) Minimum Pipe Insulation Thickness Required Insulation Thickness for Piping (mm) Runouts DN32 or Lessa
DN25 or Less
DN32–DN50
DN65–DN100
DN125 & DN150
DN200 or Larger
13
25
25
40
40
40
Note: Data based on berglass insulation with al l-service jacket. Data will change depending on actual type o insulation used. Data apply to recirculating sections o hot water systems and the rst 0.9 m rom the storage tank o uncirculated systems.
Tank Size (gal)
Approx. Energy Loss rom Tank at Hot Water Temperature 140°F (Btu/h)a
1 50 468 1 100 736 2 250 759 3 500 759 3 1000 1273 Source: From Sheet Metal and Air Conditioning Conditionin g Contractors National Association (SMACNA) Table 2 data. a For unred tanks, ederal standards limit the loss to no more than 6.5 Btu/h/t 2 o tank surace.
a
Uncirculated pipe branches to individual xtures (not exceeding 3.7 m in l ength). For lengths longer than 305 mm, use required insulation thickness shown in table. Table 6(M) Heat Loss from Various Size Tanks with Various Insulation Thicknesses
Table 5 Approximate Insulate d Piping Heat Loss and Surface Temperature Nominal Insulation Heat Loss Surace Pipe Size Thickness (Btu/h/ Temperature (in.) (in.) linear t) (°F) ½ 1 8 68 ¾ 1 10 69 1 1 10 69 1¼ 1 13 70 1½ 1 13 69 2 or less ½a 24 or less 74 2 1 16 70 2½ 1½ 12 67 3 1½ 16 68 4 1½ 19 69 6 1½ 27 69 8 1½ 32 69 10 1½ 38 69 Note: Figures based on average ambient temperature o 65°F and annual average wind speed o 7.5 mph. a
Insulation Thickness (mm)
Tank Size (L)
Approx. Energy Loss rom Tank at Hot Water Temperature 60°C (W)a
25.4 200 137 25.4 400 216 50.8 1000 222 76.2 2000 222 76.2 4000 373 Source: From Sheet Metal and Air Conditioning Conditionin g Contractors National Association (SMACNA) Table 2 data. a For unred tanks, ederal standards limit the loss to no more than 1.9 W/m 2 o tank surace.
Uncirculating hot water runout branches only. JULY/AUGUST 2010
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CONTINUING EDUCATION: Recirculating Recirculating Domestic Hot Water Systems (Equation 1)
q = 60rwc∆ [q = 3600rwc∆]
where 60 = min/h 3600 = sec/h q = piping heat loss, Btu/h (kJ/h) r = fow f ow rate, gpm (L/sec) w = weight o heated water, lb/gal (kg/L) c = specic heat o water, water, Btu/lb/°F (kJ/kg/K) ∆ = change in heated water temperature (temperature o leaving water minus temperature o incoming water, represented in this manual as T – c, °F [K]) Terefore q
[q
= c (gpm × 8.33 lb/gal)(60 min/h)(°F temperature temperature drop) = 1(gpm) × 500 × °F temperature drop = c(L c(L/s /sec ec• •1 1kg kg/L /L)( )(36 3600 00s sec ec/h /h)( )(K Kte temp mper erat atur ure edr drop op)) = 1(L/ 1(L/se secc)• )•15 15 077 077kJ kJ/L /L/s /sec ec/K /K • K temp tempe eratu raturredr edrop op]]
able pump heads. It is quite common that a plumbing designer will make wrong initial assumptions about the sizes o the hot water return lines to establish the initial heat loss gure (q). I that is the case, the plumbing engineer will have to correct the hot water return pipe sizes, redo the calculations using the new data based on the correct pipe sizing, and veriy that all the rest o the calculations are now correct.
EXAMPLE 1—CALCULATION TO DETERMINE REQUIRED CIRCULATION RATE 1. Assume that the hot water supply piping system has 800 t (244 m) o average size 1 ¼ in. (DN32) pipe. From able 5, determine the heat loss per linear oot (meter). o nd the total heat loss, multiply length times heat loss per oot (meter): 800 t × 13 Btu/h/t = 10,400 Btu/h supply supply piping piping losses (244 (244m m• •12 12.5 .5W W= = 3050 3050W Wsu supp pply lyp pip ipin ing glo loss sses es))
2.
Assume that the hot water return piping system or the system in no. 1 above has 100 t (30.5 m) o average ½ in. (DN15) piping and 100 t (30.5 m) o average ¾ in. (DN20) pipe. From able 5 determine the heat loss per linear oot (meter):
(Equation 2) gpm ≈ system heat loss (Btu/h) 500 × °F temperature drop
100 t × 8 Btu/h/t = 800 Btu/h Btu/h piping loss
(30.5 m • 7.7 W/m = 23 2 35 W piping loss)
[L/sec ≈ system heat loss (kJ/h) ] 15 077 • K temperature drop In sizing hot water circulating systems, the designer should note that the greater the temperature drop across the system, the less water is required to be pumped through the system and, thereore, the greater the savings on pumping costs. However, i the domestic hot water supply starts out at 140°F (60°C) with, say, a 20°F (6.7°C) temperature drop across the supply system, the xtures near the end o the circulating hot water supply loop could be provided with a hot water supply o only 120°F (49°C). In addition, i the hot water supply delivery temperature is 120°F (49°C) instead o 140°F (60°C), the plumbing xtures will use greater volumes o hot water to get the desired blended water temperature (see Chapter 1, able 1.1). Tereore, the recommended hot water system temperature drop should be o the magnitude o 5°F (3°C). Tis means that i the hot water supply starts out rom the water heater at a temperature between 135 and 140°F (58 and 60°C), the lowest hot water supply temperature provided by the hot water supply system could be between 130 and 135°F (54 and 58°C). With multiple temperature distribution systems, it is recommended that the recirculation system or each temperature distribution system be extended back to the water heating system separately and have its own pump. Using Equation 2, we determine that, i there is a 5°F (3°C) temperature drop across the hot water system, the number to divide into the hot water circulating system heat loss (q) to obtain the minimum required hot water return circu lation rate in gpm (L/sec) (L/sec) is 2500 (500 (500 ×5°F),(45213[15071•3°C]). For a 10°F (6°C) temperature drop that number is 5000 (rom Equation2,500×10°F=5000)(90426[fromEquation2,15071•6°C=90 426]). However, this 10°F (6°C) temperature drop may produce hot water supply temperatures that are lower than desired. Ater Equation 2 is used to establish the required hot water return fow rate, in gpm (L/sec), the plumbing designer can size the hot water return piping system based on piping fow rate velocities and the avail-
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100 t × 10 Btu/h/t = 1000 Btu/h piping piping loss 1800 Btu/h piping loss (30.5 m • 9.6 W/m = 293 W piping loss) 528 W piping loss
3.
Determine Determin e the hot water storage tank heat loss. Assume the system in no. 1 above has a 200-gal (757-L) hot water storage tank. From able 6 determine the heat loss o the storage tank @ 759 Btu/h (222 W).
4.
Determine Determin e the hot water system’s system’s total heat losses by totaling totalin g the various losses: A. Hot water supply piping losses 10,400 Btu/h B.
Hot water return piping losses
1,800 Btu/h
C.
Hot water storage tank losses
759 Btu/h
otal system heat losses
12,959 Btu/h
otal system piping heat losses (A + B) = 12,200 Btu/h [A. Hot water supply piping losses
3050 W
B.
Hot water return piping losses
527 W
C.
Hot water storage tank losses
222 W
otal system heat losses 3799 W otal system piping heat losses (A + B) = 3577 W] From Equation 2, using a system piping loss o 12,200 Btu/h (3577 W) and a 5°F (3°C) temperature drop, 12,200 Btu/h = 4.88 gpm (say 5 gpm) 5°F temperature dierence × 500 required hot water return retur n circulation rate
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3577 W = 0.29 (say 0.3) L/sec 3°Ctemp.dierence 3°Ctemp.dierence•41 •4188.32kJ/ 88.32kJ/m3 m3 requiredhotwater requiredhotwater return circulation rate
Recalculation o Hot Water System Losses 1.
Assume that the hot water water supply piping system has 800 t (244 m) o average size 1¼ in. (DN32) pipe. From able 5 determine the heat loss per linear oot (meter): 800 t × 13 Btu/h/t = 10,400 Btu/h Btu/h piping loss (244 (244m m• •1 12. 2.5 5W/ W/m m = 3050 3050W Wp pip ipin ing glo loss ss))
2.
Assume that the hot water return piping system or the system in no. 1 above has 100 t (30.5 m) o average ½ in. (DN15) pipe, 25 t (7.6 m) o average ¾ in. (DN22) pipe, and 75 t (22.9 m) o average 1 in. (DN28) pipe. From able 5, determine the heat loss per linear oot (meter): 100 t × 8 Btu/h/t Btu/h/t = 800 Btu/h piping loss 25 t × 10 Btu/h/t = 250 Btu/h piping loss 75 t × 10 Btu/h/t = 750 Btu/h piping loss 1800 Btu/h piping loss [30. [30.5 5m m• •7 7.7 .7W W/m /m =23 =235 5W Wpi pipi ping ngl los osss 7.6 7.6m m• •9. 9.6 6W/ W/m m = 73W 73Wp pip ipin ing glo loss ss 22.9 m • 9.6 W/m = 220 W piping loss
528 W piping loss] 3.
Determine the hot water storage tank heat loss. Assume the system in no. 1 above has a 200-gal (757-L) hot water storage tank. From able 6 determine the heat loss o the storage tank @ 759 Btu/h (222 W).
4.
Determine the system’s total heat losses:
A.
Hot water supply losses
B.
Hot water return losses
C.
Hot water storage tank losses otal system heat losses
10,400 Btu/h 1,800 Btu/h 759 Btu/h 12,959 Btu/h
otal system piping heat losses (A + B) = 12,200 Btu/h [A. Hot water supply losses
3050 W
B.
Hot water return losses
528 W
C.
Hot water storage tank losses
222 W
otal system heat losses
3800 W
otal system piping heat losses (A + B) = 3578 W]
Note: Te recalculation determined that the hot water system heat losses remai ned unchan ged and that 4.8 8 (say 5) 5) gpm (0.29 [say 0.3] L/ sec) is the fow rate that is required to maintain the 5°F (3°C) temperature drop across the hot water supply system. It should be stated that engineers use numerous rules o thumb to size hot water return systems. Tese rules o thumb are all based on assumptions, however, and are not recommended. It is recommended that the engineer perorm the calculations or each project to establish the required fow rates because, with all the various capacities o the pumps available today, exact sizing is possible, and any extra circulated fow caused by the plumbing engineer using a rule o thumb equates to higher energy costs, to the detriment o the client.
esTaBLIshINg The heaD CaPaC esTaBLIshINg CaPaCITy ITy Of The hOT WaTeR CIRCULaTINg PUmP Te hot water return circulating pump is selected based on the required hot water return fow rate (in gpm [L/sec]), calculated using Equation 2, and the system’s pump head. Te pump head is normally determined by the riction losses through only the hot water return piping loops and any losses through balancing valves. Te hot water return piping riction losses usually do not include the riction losses that occur in the hot water supply piping. Te reason or this is that the hot water return circulation circulation fow is needed only to keep the hot water supply system up to the desired temperature when there is no fow in the hot water supply piping. When people use the hot water at the xtures, there is usually sucient fow in the hot water supply piping to keep the system hot water supply piping up to the desired temperature without help rom the fow in the hot water return piping. Te only exception to the rule o ignoring the riction losses in the hot water supply piping is a situation where a hot water return pipe is connected to a relatively small hot water supply line. “Relatively small” here means any hot water supply line that is less than one pipe size larger than the hot water return line. Te problems created by this condition are that the hot water supply line will add additional riction to the head o the hot water circulating pump, and the hot water circulating pump fow rate can deprive the last plumbing xture on this hot water supply line rom obtaining its required required fow. It is recommended, thereore, thereore, that in such a situation the hot water supply line supplying each hot water return piping connection point be increased to pre vent these potential problems, i.e., use ¾ in. (DN22) hot water supply piping and ½ in. (DN15) hot water return piping, or 1 in. (DN28) hot water supply piping and ¾ in. (DN22) hot water return piping, etc. When selecting the hot water circulating pump’s head, the designer should be sure to calculate only the restrictions encountered by the circulating pump. A domestic hot water system is normally considered an open system (i.e., open to the atmosphere). When the hot water circulating pump is operating, however, it is assumed that the piping is a closed system. Tereore, Tereore, the designer should not include static heads where none exists. For example, in Figure 1, the hot water circulating pump has to overcome only the riction in the hot water return piping not the loss o the static head pumping the water up to the xtures because because in a closed system the static head loss is oset by the static head gain in the hot water return piping.
hOT WaTeR CIRCULaTINg PUmPs Most hot water water circulating pumps are o the centri ugal ty pe and are available as either in-line un its or small systems or base-mounted base-mounted units or large systems. Because o the corrosiveness o hot water systems, the pumps should be bronze, bronze tted, or stainless steel. Conventional, iron bodied pumps, which are not bronze tted, are not recommended.
CONTROL fOR hOT WaTeR CIRCULaTINg PUmPs Tere are three major methods commonly used or controlling hot water circulating pumps: manual, thermostatic (aquastat), and time clock control. Sometimes more than one o these methods are used on a system. 1.
A manual manual control runs the hot water water circulating pump continuously when the power is turned on. A manual control JULY/AUGUST 2010
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CONTINUING EDUCATION: Recirculating Recirculating Domestic Hot Water Systems should be used only when hot water is needed all the time, 24 h a day, or during all the periods o a building’s building’s operation. Otherwise, it is not a cost-eective means o controlling the circulating pump because it will waste energy. Note: Te method or applying the “on demand” concept or controlling the hot water circulating pump is a manual control. It can be used very successully or residential and commercial applications. 2.
A thermostatic thermostatic aquastat is a device that is inserted inserted into into the the hot water return line. When the water in the hot water return system reaches the distribution temperature, it shuts o the circulating pump until the hot water return system temperature drops by approximately 10°F [5.5°C]. With this method, when there is a large consumption o hot water by the plumbing xtures, the circulating pump does not operate.
3.
A time time clock is used used to turn the pump on during during specic hours o operation when people are using the xtures. Te pump would not operate, or example, at night in an oce building when nobody is using the xtures.
4.
Oten an aquastat aquastat and a time clock clock are are used used in conjunction so that during the hours a building is not operating the time clock shuts o the circulating pump, and during the hours the building is in use the aquastat shuts o the pump when the system is up to the desired temperature.
See able 4 or the minimum required insulation thicknesses or all systems. I the insulated piping is installed in a location where it is subjected to rain or other water, the insulation must be sealed with a watertight covering that will maintain its tightness over time. Wet insulation not only does not insulate, it also releases considerable heat energy rom the hot water piping, thus wasting energy. Furthermore, the insulation on any outdoor lines that is not sealed watertight can be plagued by birds or rodents, etc., pecking at the insulation to use i t or their nests. In time, the entire hot water supply and/or return piping will have no insulation. Such bare hot water supply and/or return piping will waste considerable energy and can seriously aect the operation o the hot water system and water heaters. Te minimum required insulation thicknesses given in able 4 are based on insulation having thermal resistivity (R) in the range o 4.0 to4.6ft2×h×(°F/Btu)×in.(0.028to0.032m2•[°C/W]•mm)ona fat surace at a mean temperature temperature o 75°F (24°C). Minimum insulation thickness shall be increased or materials having R values less than 4.0 ft2×h×(°F/Btu)×in.(0.028m2•[°C/W]•mm)ormaybereducedfor materials having R values greater than 4.6 t2 × h × (°F/Btu) × in. (0.032 m2•[°C/W]•mm). 1. For materials material s with thermal resistivity greater than 4.6 t2 × h×(°F/Btu)×in.(0.032m2•[°C/W]•mm),theminimum insulation thickness may be reduced as ollows: 4.6 × able 4 thickness = New minimum thickness Actual R (0.032•Table4thickness = New minimum thickness) Actual R
aIR eLImINa eLImI NaTION TION In any hot water return circulation system it is very important that there be a means o elimi nating any entrapped air rom the hot water return piping. Air eli mination is not required in the hot water supply piping because the discharge o water water rom the xtures wi ll elimi nate any entrapped air. I air is not eliminated rom the hot water return lines, however, it can prevent the proper circulation o the hot water system. It is imperative that a means o air elimi nation be provided at all high points o a hot water return system. Te plumbing engineer must always give consideration to precisely where the air elim ination devices are to be located and drained. For example, they should not be located in the unheated attics o buildings in cold climates. I the plumbing engineer does not consider the location o these devices and where they will drain, the result may be unsightly piping in a building or extra construction costs.
INsULaTION Te use o insulation is ver y cost-eective. It means paying one time to save the later cost o signicant energy lost by the hot water supply and return piping system. Also, insulation decreases the stresses on the piping due to thermal expansion and contraction caused by changes in water temperature. Furthermore, the proper use o insulation eliminates the possibility o someone getting burned by a hot, uninsulated water line. See able 5 or the surace temperatures o insulated lines (versus 140°F [60°C] or bare piping). It is recommended that all hot water supply and return piping be insulated. Tis recommendation exceeds some code requirements. 12 Plumbing Systems & Design
JULY/AUGUST 2010
2.
For materials with thermal thermal resistivity resistivity less than 4.0 4.0 t2 × h × (°F/Btu)×in.(0.028m2•[°C/W]•mm),theminimuminsu lation thickness shall be increased as ollows: 4.0 × able 4 thickness = New minimum thickness Actual R (0.028 •Table4thickness = New minimum thickness) Actual R
CONCLUsION In conclusion, an inappropriate hot water recirculation system can have serious repercussions or the operation o the water heater and the sizing o the water heating system. In addition, it can cause the wastage o vast amounts o energy, water, and time. Tereore, it is incumbent upon the plumbing designer to design a hot water recirculation system so that it conserves natural resources and is in accordance with t he recommendations recommendations given in thi s chapter. chapter.
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BIBLIOgRaPhy 1.
American Society o Heating, Rerigerating, and Air Conditioning Engineers. 1993. Pipe sizing. Chapter 33 in Fundamentals Handbook.
2.
American Society o Heating, Rerigerating, and Air Conditioning Engineers. 1993. Termal and water vapor transmission data. Chapter 22 in Fundamentals Fundamentals Handbook.
3.
4.
5.
6.
American Society o Heating, Rerigerating, and Air Conditioning Engineers. 1995. Service water heating. Chapter 45 i n Applications Handbook. American Society o Heating, Rerigerating, and Air Conditioning Engineers. Energy conservation in new building design. ASHRAE Standards, 90A–1980, 90B–1975, and 90C–1977. American Society o Heating, Rerigerating, and Air Conditioning Engineers. Energy ecient design o new low rise residential buildings. ASHRAE Standards, 90.2–1993. American Society o Heating, Rerigerating, and Air Conditioning Engineers. New inormation on service water heating. echnical Data Bulletin. Vol. 10, No. 2.
7.
American Society o Mechanical Engineers. Engineers. Plumbing xture ttings. ASME A112.18.1M–1989.
8.
American Society o Plumbing Plumbing Engineers. Engineers. 2000. 2000. Cold water systems. Chapter 5 in ASPE Data Book, Volume 2.
9.
American Society o Plumbing Plumbing Engineers. Engineers. 1989. 1989. Piping systems. Chapter 10 in ASPE Data Book.
17. Cohen, Arthur. Arthur. Copper Development Association. 1993. Historical perspective o corrosion by potable waters in building systems. Paper no. 509 presented at the National Association o Corrosion Engineers Annual Conerence. 18. Copper Development Associati on. 1993. Copper ube ube Handbook. 19. International Association o Plumbing and Mechanical Mechanical Ocials. 1985. Uniorm Plumbing Code Illustrated raining Manual. 20. Konen, Tomas P. 1984. An experimental study o competing systems or maintaining service water temperature temperature in residential buildings. In ASPE 1984 Convention Proceedings. 21. Konen, Tomas P. 1994. Impact o water conservation conser vation on interior plumbing. In echnical Proceedings o the 1994 ASPE Convention. 22. Saltzberg, Edward. Edward. 1988. 1988. Te plumbing plumbing engineer as a orensic engineer. In echnical Proceedings o the 1988 ASPE Convention. 23. Saltzberg, Edward. 1993. o combine or not to combine: An in–depth review o standard and combined hydronic heating systems and their various pitalls. Paper presented at the American Society o Plumbing Engineers Symposium, Symposium, October 22–23. 24. Saltzberg, Edward. Edward. 1996. 1996. Te eects o hot water circulation systems on hot water heater sizing and piping systems. echnical presentation given at the American Society o Plumbing Engineers convention, November 3–6.
10. American Society o Plumbing Engineers. Engineers. 1989. Position paper on hot water temperature limitations.
25. Saltzberg, Edward. 1997. In press. New methods or analyzing hot water systems. Plumbing Engineer Magazine.
11. American Society o Plumbing Engineers. Engineers. 1989. Service hot water systems. systems. Chapter 4 in ASPE Data Book.
26. Saltzberg, Edward. 1997. In press. Prompt delivery o hot water at xtures. Plumbing Engineer Magazine.
12. American Society o Plumbing Engineers. Engineers. 1990. Insulation. Chapter 12 in ASPE Data Book.
27. Sealine, Sealin e, David A., od od Windsor, Al Fehrm, and Greg Wilcox. 1988. Mixing valves and hot water temperature. In echnical Proceedings o the 1988 ASPE Convention.
13. American Society o Plumbing Engineers. 1990. Pumps. Chapter 11 in ASPE Data Book.
28. Sheet Metal Metal and Air Conditioning Contractors National Association. 1982. Retrot o Building Energy Systems and Processes.
14. American Society o Plumbing Engineers. Engineers. 2000. Energy conservation in plumbing systems. Chapter 7 in ASPE Data Book, Volume 1.
29. Steele, Alred. Engineered Plumbing Design. 2d ed.
15. American Water Works Association. Associati on. 1985. Internal corrosion o water distribution systems. Research Foundation Foundation cooperative research report.
30. Steele, Alred. 1988. emperature emperature limits in service servic e hot water systems. In echnical Proceedings o the 1988 ASPE Convention.
16. Cohen, Arthur. Copper Development Association. Associati on. 1978. Copper or hot and cold potable water systems. Heating/ Heating/ Piping/Air Conditioning Magazine. May.
31. Wen-Yung, Wen-Yung, W. Chan, and Milton Meckler. 1983. Pumps and pump systems. In American Society o Plumbing Engineers Handbook.
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CONTINUING EDUCATION: Recirculating Recirculating Domestic Hot Water Systems
Continuing Education rom Plumbing Systems & Design Do you nd it dicult to obtain continuing education units (CEUs)? Trough Trough this special section in every issue o PS&D, ASPE can help you accumulate the CEUs required or maintaining your Certied in Plumbing Design (CPD) status.
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Te technical article you must read to complete the exam is located at www.psdmagazine.org. Just click on “ Plumbing Systems & Design Continuing Education Article and Exam” at the top o the page. Te ollowing exam and application orm also may be downloaded rom the website. Reading the article and completing the orm will allow you to apply to ASPE or CEU credit. I you earn a grade o 90 percent or higher on the test, you will be notiied that you have logged 0.1 CEU, which can be applied toward CPD renewal or numerous regulatory-agency CE programs. (Please note that it is your responsibility to determine the acceptance policy o a particular agency.) CEU inormation will be kept on le at the ASPE oce or three years. Note: In determining your answers to the CE questions, use only the material presented in the corresponding continuing education article. Using inormation rom other materials may result in a wrong answer.
About This Issue’s Article The July/August 2010 continuing education article is “Recirculating Domestic Hot Water Systems,” Chapter 14 rom Domestic Water Heating Design Manual II . This chapter addresses the criteria or establishing an acceptable time delay in delivering hot water to xtures and the limitations o the length between a hot water recirculation system and plumbing xtures. It also discusses the temperature drop across a hot water supply system, types o hot water recirculation systems, and pump selection criteria and provides extensive inormation on the insulation o hot water supply and return piping. You may locate this article at www.psdmagazine.org. Read the article, complete the ollowing exam, and submit your answer sheet to the ASPE oce to potentially receive 0.1 CEU.
9 6 1
CE Questions — “Recirculating Domestic Hot Water Systems” (PSD 169) 1. What aspect o a circulated system causes energy loss in the circulation o hot water? a. convection b. pressure c. radiation d. both a and c
7. Which o the ollowing is a common type o balancing device? a. automatic fow control valve b. fow-regulating fow-regulati ng valve c. pressure-regulating pressure-regulating valve d. both a and b
2. What delay period in obtaining hot water at a fxture is considered most acceptable? a. zero to 10 seconds b. 11 to 30 seconds c. more than 30 seconds d. no delay is acceptable
8. What is the required insulation thickness or a 3-inch runout? a. 0.5 inch b. 1 inch c. 1.5 inches d. 2 inches
3. What is the approximate time required to deliver hot water to a 1.5-gpm fxture 10 eet rom the hot water maintenance system using ½-inch Schedule 40 steel pipe? a. 8 seconds b. 16 seconds c. 21 seconds d. 30 seconds
9. What is the approximate heat loss or a 1½-inch pipe with 1 inch o insulation? a. 8 Btuh/linear oot b. 10 Btuh/linear oot c. 13 Btuh/linear oot d. 16 Btuh/linear oot
4. Which o the ollowing is a type o hot water maintenance system? a. sel-regulating sel-regulating heat trace system b. circulation circulation system c. point-o-use point-o-use water heater d. all o the above 5. High velocities in copper piping systems can cause ________ in less than six months. a. corrosion b. pinhole leaks c. water hammer d. decreased fow 6. A ________ should be provided in circulated systems to isolate an entire loop. a. balancing valve b. control valve c. shuto valve d. check valve
14 Plumbing Systems & Design
JULY/AUGUST 2010
10. In the total hot water system energy loss calculation, what does r stand or? a. piping heat loss b. fow rate c. weight o water d. specic heat o water
D S P
11. What is the maximum recommended hot water system temperature drop? a. 1°F b. 5°F c. 10°F d. 15°F 12. What is the recommended recommended material or a hot water water circulating pump? a. bronze b. iron c. stainless steel d. both a and c
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Plumbing Systems & Design Continuing Education Application Form This orm is valid up to one year rom date o publication. The PS&D Continuing Education program is approved by ASPE or up to one contact hour (0.1 CEU) o credit per article. Participants who earn a passing score (90 percent) on the CE questions will receive a letter or certifcation within 30 days o ASPE’s receipt o the application orm. (No special certifcates will be issued.) Participants who ail and wish to retake the test should resubmit the orm along with an additional ee (i required). 1. Photocopy this orm or download it rom www.psdmagazine.org. www.psdmagazine.org. 2. Print or type your name and address. Be sure to place your ASPE membership number in the appropriate space. 3. Answer the multiple-choice continuing education (CE) questions based on the corresponding article ound on www.psdmagazine.org and the appraisal questions on this orm. 4. Submit this orm with payment ($35 or nonmembers o ASPE) i required by check or money order made payable to ASPE or credit card via mail (ASPE Education Credit, 2980 S. River Road, Des Plaines, IL 60018) or ax (847-296-2963). Please print or type; this inormation will be used to process your credits. Name________________________________________________________________________________________________________ Title _________________________________________________ ASPE Membership No. ____________________________________ Organization __________________________________________________________________________________________________ Billing Address ________________________________________________________________________________________________ City _________________________________________ State/Province ________________________ Zip ______________________ Country ______________________________________________ E-mail _________________________________________________ Daytime telephone ____________________________________ Fax ____________________________________________________ PE State _____________________________________________ PE No. _________________________________________________
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Recirculating Domestic Hot Water Systems (PSD 169) Questions appear on page 14. Circle the answer to each question.
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appril Qution Recirculating Domestic Hot Water Systems (PSD 169)
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