Hot Water Recirculation Pump Sizing Calculator
Enter the controlling loop length, copper tube size, insulation, temperatures, and allowance for fittings. The calculator estimates heat loss, required GPM, friction head, total head, and the pump duty point to compare with circulator curves.
Required recirculation flow (GPM)0.24
- Estimated loop heat loss (Btu/h)
- 1,205.97
- Pipe friction head (ft)
- 0.02
- Total required head (ft)
- 1.02
- Pump duty point
- 0.24 GPM at 1.02 ft head
- Water velocity (ft/s)
- 0.16
- Maximum flow at velocity limit (GPM)
- 3.02
- Velocity check
- Within selected limit
Thermal and hydraulic details
| Quantity | Value |
|---|---|
| Heat loss per foot (Btu/h ft) | 8.04 |
| Mean loop water temperature (F) | 125 |
| Effective hydraulic length (ft) | 195 |
| Pipe resistance (h ft F/Btu) | 0 |
| Insulation resistance (h ft F/Btu) | 5.82 |
| Surface resistance (h ft F/Btu) | 1.02 |
Static building elevation is excluded because it cancels in a filled closed recirculation loop.
For branched systems, size the hydraulically controlling circuit and balance the other branches.
Entered component head dominates pipe friction; verify valve and equipment loss data.
How to use this calculator
- Choose the Type L copper size used by the controlling recirculation loop.
- Enter the complete developed supply-and-return loop length before fitting allowance.
- Enter maintained water temperature, ambient temperature, allowed temperature drop, and insulation data.
- Add fitting allowance, Hazen-Williams C-factor, known component head, and velocity limit.
- Compare the calculated GPM at head with potable-water circulator curves.
How the recirculation pump size is calculated
A domestic hot water return pump is usually sized from the heat lost by the maintained loop. The pump flow has to replace that heat without allowing the return temperature to fall more than the selected design drop. This calculator follows that approach, then adds pipe friction, entered component losses, and a velocity check.
The selected Type L copper size supplies the inside diameter d and outside diameter D. The radii in feet are ri = d / 24 and ro = D / 24. If insulation thickness is t inches, the outside insulation radius is rs = ro + t / 12. For bare pipe, rs = ro and insulation resistance is zero.
The calculator uses cylindrical thermal resistance per foot. Copper resistance is Rp = ln(ro / ri) / (2 x pi x kc), with kc = 223 Btu/(h ft F). Insulation conductivity is converted with ki = K / 12, so Ri = ln(rs / ro) / (2 x pi x ki). The outside surface resistance is Ro = 1 / (ho x 2 x pi x rs). The mean loop temperature is Tm = Ts - deltaT / 2, and heat loss per foot is q = max(0, Tm - Ta) / (Rp + Ri + Ro).
Total loop heat loss is Q = q x L. Required flow is G = Q / (500 x deltaT), where 500 represents water density, minutes per hour, and specific heat in common plumbing units. Effective hydraulic length is Le = L x (1 + E / 100). Hazen-Williams head is Hf = 4.52 x Le x G^1.85 / (C^1.85 x d^4.87). Total required head is H = Hf + Hc, where Hc is known component loss.
What moves the result most
Insulation and allowed temperature drop usually control the flow. Better insulation reduces heat loss directly. A smaller allowed temperature drop requires more flow because the same heat loss must be carried by fewer degrees of water-temperature change.
Head often comes from valves and other components rather than straight copper friction, especially at the low flows common in small recirculation loops. The velocity check is separate from head: it reports whether the required flow stays under the selected continuous-circulation limit for the chosen tube size.
What this calculator leaves out
Static building height is not added because static head cancels in a filled closed loop. The model also does not balance branched systems. For branches, calculate the hydraulically controlling circuit, then balance the other returns. The heat-loss estimate is steady state and does not include uninsulated fittings, pipe supports, wind, intermittent controls, mixing devices, thermal disinfection, scald protection, or code requirements.
Worked example
Suppose the controlling loop uses 3/4 in Type L copper, has 150 ft of developed supply and return length, maintains 130 F supply water in 70 F surroundings, and allows a 10 F loop drop. With 1/2 in insulation at k = 0.25, outside surface coefficient 2, 30% fitting allowance, C = 140, 1 ft of component head, and a 2 ft/s velocity limit, the mean loop temperature is 125 F.
The calculated heat loss is 8.04 Btu/h per ft, or about 1,206 Btu/h for the loop. Required flow is 0.24 GPM. The effective hydraulic length is 195 ft, pipe friction is 0.02 ft, total required head is 1.02 ft, and the pump duty point is 0.24 GPM at 1.02 ft head. Velocity is 0.16 ft/s, which is within the 2 ft/s limit.
Common questions
How is a domestic hot water recirculation pump sized?
A common design method estimates heat loss from the maintained loop, then divides that loss by 500 times the allowed temperature drop to get GPM. The pump is then selected from a curve that can deliver that flow at the calculated total head.
Why is building height excluded from recirculation pump head?
In a filled closed recirculation loop, water going up is balanced by water coming down. The circulator has to overcome friction and component pressure losses, not the static height of the building.
What temperature drop should a hot water return loop use?
Many preliminary DHW recirculation estimates use a 5 F to 10 F loop drop, but the right value depends on the required return temperature and the system design. A smaller drop increases required flow, while a larger drop lowers flow and may reduce delivered return temperature.
What velocity is safe for continuously circulating hot water in copper pipe?
Continuous hot-water circulation is commonly kept at low velocity to reduce erosion and noise risk. This calculator defaults to 2 ft/s and lets you test another project limit if the designer or manufacturer specifies one.
Should supply and return pipe lengths both be included?
Yes. Enter the developed length around the complete controlling loop, including both the maintained supply path and the return path. Leave elbows, tees, valves, and similar fittings for the equivalent-length allowance unless you already included their equivalent lengths.
How do I choose a pump from the calculated GPM and head?
Use the pump duty point, stated as GPM at feet of head, on manufacturer curves for potable-water-rated circulators. Select a bronze or stainless model whose curve passes through or above the duty point and also satisfies minimum-flow and control requirements.