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The total standby heat loss from the entire hot water recirculation loop.
This is the sum of the heat loss along the whole piping run, accounting for insulation level, pipe length, and the temperature difference to the surrounding air. Use a pipe heat loss calculation or manufacturer's insulation heat loss table for your specific piping layout.
The maximum temperature drop allowed between the water heater and the farthest fixture on the loop.
A smaller allowable drop requires a higher recirculation flow rate to keep the water hotter along the full length of the loop; a larger allowable drop reduces the required flow but delivers cooler water at the far end.
Required recirculation flow
0.379 gal/min
Heat loss depends on your pipe insulation level, length, and ambient temperature — calculate it from your specific piping layout (or use a manufacturer's heat loss table) rather than assuming a generic value.
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Domestic Hot Water Recirculation Flow Calculator: 0.3787 gal/min — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Required recirculation flow = pipe heat loss ÷ (water density × specific heat × allowable temperature drop), the standard method for sizing a domestic hot water recirculation system to compensate for standby heat loss along the loop
Inputs used
- Total Loop Heat Loss (W)
- 500
- Allowable Temperature Drop
- 9 °F
Confidence note: Heat loss depends on your pipe insulation level, length, and ambient temperature — calculate it from your specific piping layout (or use a manufacturer's heat loss table) rather than assuming a generic value.
What this calculation does not cover
- Treats the allowable temperature drop as a free choice, and it is a hygiene limit before it is a comfort one. The temperature that suppresses Legionella growth has to hold at the coldest point on the loop, which is the return — so the drop, which this page will accept as far as 15 K (27 °F), is fixed by that floor and by the flow temperature above it, not by how lukewarm the far tap is allowed to feel.
- One total flow says nothing about where it goes. A loop with several risers or branch returns sends nearly all of it up whichever path offers least resistance, and the others go cold while the pump delivers exactly the total calculated here. Balancing valves — thermostatic, or manually set and commissioned — are what split it, and the branch flows they are set to are a separate calculation from this one.
- The flow still has to fit the pipe. A recirculation return is limited by water velocity rather than by whether the flow will physically pass: hot water moving too fast erodes a copper pipe wall from the inside, and returns show it first because they run continuously for the life of the building. Sizing the return off this flow means checking it against the velocity limit for the pipe material, which can force a larger return than the flow alone suggests.
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This result is a specification — 0.379 gal/min — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.1.1
Regulatory standards & verification citations1
- Required recirculation flow = pipe heat loss ÷ (water density × specific heat × allowable temperature drop), the standard method for sizing a domestic hot water recirculation system to compensate for standby heat loss along the loop
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