1. Domestic Hot Water Recirculation Design
ASPE Washington, DC Chapter
November 15, 2016
Robert Evans, CPD, GPD, LEED AP BD&C
ASPE Washington, DC Chapter

2. Domestic Hot Water Recirculation
2012 International Plumbing Code
IPC Hot or tempered water supply to fixtures. The developed length of hot or tempered water piping, from the source of hot water to the fixtures that require hot or tempered water, shall not exceed 50 feet ( mm). Recirculating system piping and heat-traced piping shall be considered to be the sources of hot or tempered water.
2012 International Green Construction Code
Maximum allowable pipe length method. The maximum allowable pipe length from the source of hot or tempered water to the termination of the fixture supply pipe shall be in accordance with the maximum pipe length columns in Table Where the length contains more than one size pipe, the largest size shall be used for determining the maximum allowable length of the pipe in Table
ASPE Washington, DC Chapter

3. Domestic Hot Water Recirculation
Maximum allowable pipe volume method. The water volume in the piping shall be calculated in accordance with Section The maximum volume of hot or tempered water in the piping to public lavatory faucets, metering or nonmetering, shall be 2 ounces (0.06 L). For fixtures other than public lavatory faucets, the maximum volume shall be 64 ounces (1.89 L) for hot or tempered water from a water heater or boiler; and 24 ounces (0.7 L) for hot and tempered water from a circulation loop pipe or an electrically heat-traced pipe.
Water volume determination. The volume shall be the sum of the internal volumes of pipe, fittings, valves, meters and manifolds between the source of hot water and the termination of the fixture supply pipe. The volume shall be determined from the liquid ounces per foot column of Table The volume contained within fixture shutoff valves, flexible water supply connectors to a fixture fitting, or within a fixture fitting shall not be included in the water volume determination. Where hot or tempered water is supplied by a circulation loop pipe or an electrically heat-traced pipe, the volume shall include the portion of the fitting on the source pipe that supplies water to the fixture.
ASPE Washington, DC Chapter

4. Domestic Hot Water Recirculation
ASPE Washington, DC Chapter

5. Domestic Hot Water Recirculation
ASHRAE Standard
Temperature Maintenance Controls. Systems designed to maintain usage temperatures in hot-water pipes, such as recirculating hot-water systems or heat trace, shall be equipped with automatic time switches or other controls that can be set to switch off the usage temperature maintenance system during extended periods when hot water is not required.
Circulating Pump Controls. When used to maintain storage tank water temperature, recirculating pumps shall be equipped with controls limiting operation to a period from the start of the heating cycle to a maximum of five minutes after the end of the heating cycle.
ASPE Washington, DC Chapter

6. Domestic Hot Water Recirculation
2015 INTERNATIONAL ENERGY CONSERVATION CODE
C Circulation systems. Heated-water circulation systems shall be provided with a circulation pump. The system return pipe shall be a dedicated return pipe or a cold water supply pipe. Gravity and thermo-syphon circulation systems shall be prohibited. Controls for circulating hot water system pumps shall start the pump based on the identification of a demand for hot water within the occupancy. The controls shall automatically turn off the pump when the water in the circulation loop is at the desired temperature and when there is no demand for hot water.
ASPE Washington, DC Chapter

7. Domestic Hot Water Recirculation
Credit - Jason Hatcher, City of Oakland M&P Inspector
ASPE Washington, DC Chapter

8. Domestic Hot Water Recirculation
HEAT LOSS TABLE FOR ½" FIBERGLASS PIPE INSULATION
1/2" COPPER PIPE = 15 BTU/HR
1" COPPER PIPE = 19 BTU/HR
1-1/2" COPPER PIPE = 25 BTU/HR
2" COPPER PIPE = 28 BTU/HR
3" COPPER PIPE = 38 BTU/HR
HEAT LOSS TABLE FOR 1" FIBERGLASS PIPE INSULATION
1" COPPER PIPE = 10 BTU/HR
1-1/2" COPPER PIPE = 12 BTU/HR
2" COPPER PIPE = 15 BTU/HR
3" COPPER PIPE = 20 BTU/HR
ASPE Washington, DC Chapter

9. Domestic Hot Water Recirculation
ASPE Washington, DC Chapter

10. Domestic Hot Water Recirculation
1. Calculate piping heat loss using ½" pipe insulation:
3" pipe (77 feet) = 38 X 77 = 2926 BTU/HR
2" pipe (40 feet) = 28 X 40 = 1120 BTU/HR
1-1/2" pipe (140 feet) = 25 X 140 = 3500 BTU/HR
1" pipe (120 feet) = 19 X 120 = 2800 BTU/HR
Total heat loss = 9826 BTU/HR
ASPE Washington, DC Chapter

11. Domestic Hot Water Recirculation
Calculate piping heat loss using 1" pipe insulation:
3" pipe (77 feet) = 20 X 77 = 1540 BTU/HR
2" pipe (40 feet) = 15 X 40 = 600 BTU/HR
1-1/2" pipe (140 feet) = 12 X 140 = 1680 BTU/HR
1" pipe (120 feet) = 19 X 120 = 1200 BTU/HR
Total heat loss = 5020 BTU/HR
ASPE Washington, DC Chapter

12. Domestic Hot Water Recirculation
Assume heat loss of return piping is 2/3 heat loss of supply piping. 2/3(9826) = 6544 BTU/HR 2/3(5020) = 3343 BTU/HR
Total heat loss is BTU/HR with ½" insulation, or 8363 BTU/HR with 1" insulation.
2. Allow for temperature drop during circulation (▵T)
20 degree drop - conversion factor is GPM
10 degree drop - conversion factor is 5000 GPM
5 degree drop - conversion factor is 2500 GPM
ASPE Washington, DC Chapter

13. Domestic Hot Water Recirculation
Divide heat loss by conversion factor:
16370/10000 = 1.64 GPM - minimum required circulation rate
3. Determine pump head requirements:
A. Select a pump which will operate efficiently at the flow rate calculated - Bell & Gossett model LR-20BF circulator at GPM has a discharge head of 12.5 feet.
B. Determine the longest run of return piping (approximately feet). This number needs to be converted to equivalent length to account for fittings if they are not added separately. Using 30% for fittings we have a total equivalent length of 299 feet.
ASPE Washington, DC Chapter

14. Domestic Hot Water Recirculation
C. Divide pump head by the longest return pipe equivalent length and you will have the uniform friction head.
12.5 feet / 299 = 0.042
D. Multiply by 100 to convert the friction loss to feet per feet X 100 = 4.2 ft/100 ft
E. Multiply ft/100 ft by .433 to obtain friction loss in psi/100 ft ft/100 ft X = PSI/100 ft uniform friction loss.
F. Size return piping using PSI/100 ft friction loss.
ASPE Washington, DC Chapter

15. Domestic Hot Water Recirculation
ASPE Washington, DC Chapter

16. Domestic Hot Water Recirculation
Return to step 1 and recalculate loss for return pipe.
Heat loss in the return piping was assumed to be 6544 BTU/HR. Actual heat loss, based on 230 feet of ½" piping is Adding to the heat loss of the supply pipe ( = BTU/HR).
By step /10000= GPM. The corrected minimum circulation rate is 1.33 GPM (down from 1.64). The head at 1.33 GPM is still approximately 12.5 feet.
5. Completing the same calculation for a 10 degree ▵T shows the uniform friction loss to be 1.69'/100ft. For a 5 degree ▵T, the friction loss is 1.47'/100ft.
ASPE Washington, DC Chapter

17. Domestic Hot Water Recirculation
ASPE Washington, DC Chapter

18. Domestic Hot Water Recirculation
Energy conservation for pumps in dollar value
What if you can reduce the required horsepower of a pump from 2HP to 1HP, from 1HP to 1/2HP, or from 5HP to 2HP?
If you assume an energy cost of 13 cents/kWhr and the pump running 24/7:
1HP = 0.746kW
0.746kW x 24 hr/day x 365 days/year x $0.13/kWhr = $850 per year savings
For a reduction of 1HP to 1/2HP –
0.373kW x 24 hr/day x 365 days/year x $0.13/kWhr = $425 per year savings
For a reduction of 5HP to 2HP –
2.238kW x 24 hr/day x 365 days/year x $0.13/kWhr = $2,549 per year savings
ASPE Washington, DC Chapter