Hot Water Return Sizing Chart
Sizing the return line for a hot water recirculation system — the loop that keeps hot water at the fixtures so no one runs the tap waiting for it. The return only carries the small flow that replaces the heat lost through the supply piping, so it is sized from that heat loss and then checked against the copper velocity limit that keeps it from eroding.
The recirculation flow formula
Recirc GPM = heat loss (BTU/hr) ÷ (500 × ΔT °F)
The 500 is 60 minutes per hour times 8.33 pounds per gallon — the standard water-side heat conversion. ΔT is the temperature you let the water lose going around the loop; 10°F is the common default, and a wider ΔT means a smaller flow but a cooler return (kept above the Legionella floor). Because the flow only offsets pipe heat loss, the numbers are small — which is why return lines and recirc pumps are modest.
Copper velocity limits
| Copper tube service | Max velocity |
|---|---|
| Cold water | 5–8 ft/s |
| Hot water below 140°F | 4–5 ft/s |
| Hot water above 140°F | 2–3 ft/s |
| Recirculation loops | 2–3 ft/s |
Size to the flow, then check velocity
The return line is not sized by a fixed rule but by the two constraints together: it has to carry the computed recirculation GPM, and it has to do so without exceeding the velocity limit. In practice that lands most return lines at 1/2" to 3/4" copper — often one or two sizes below the supply they serve — but that is a consequence of the math, not a rule to apply blindly.
- Recirculation flows are small — typically low single-digit GPM even for a whole building.
- Residential and small-commercial return lines are commonly 1/2" to 3/4" copper.
- The return is generally one or two sizes smaller than the hot-water supply it serves — a heuristic; the real constraint is the computed GPM and the velocity cap.
- Keep recirculation velocity at or below about 3 ft/s in copper to avoid erosion-corrosion and pinhole failures at fittings.
Convert the loop's flow and head with the flow rate calculator.
Common questions
How do you size a hot water recirculation return line?
Size it to carry the recirculation flow that offsets the heat lost through the hot water supply piping. The flow is heat loss (BTU/hr) ÷ (500 × ΔT), where 500 is 60 minutes × 8.33 lb/gal and ΔT is the allowable temperature drop around the loop (10°F is the common default). The flows are small — a 10,000 BTU/hr loss at ΔT 10°F is only 2 GPM — so return lines are typically 1/2" to 3/4".
What is the velocity limit for hot water recirculation?
Keep it at or below about 2–3 ft/s in copper. Hot, fast water attacks copper through erosion-corrosion, especially at fittings, and recirculation loops run hot water continuously — so an oversized pump or an undersized return is the classic cause of pinhole leaks. Design the loop to stay under 140°F and under 3 ft/s.
How big is a recirculation pump?
Small. Because the recirc flow only replaces pipe heat loss, whole-building systems run in the low single-digit GPM, and residential recirc pumps are tiny. Size the pump to the calculated GPM and the loop’s head, not by rule of thumb — an oversized pump pushes the velocity up and starts eroding the copper.
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