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Hot Water Demand Calculator (GPM)

Design simultaneous hot water flow from editable fixture flows and a documented diversity factor, with the heat input it takes to hold it.

Inputs

Fixtures that could run together

The worst simultaneous case you are designing for.

Showerheads made after 1 Jan 1994 are capped at 2.5 gpm at 80 psi (10 CFR 430.32(p)). Water-saving heads run 1.5-2.0 gpm.

Bathroom sinks at the same time.

Lavatory faucets made after 1 Jan 1994 are capped at 2.2 gpm at 60 psi (10 CFR 430.32(o)); aerators are often swapped down to 1.0-1.5 gpm.

Usually one.

Also capped at 2.2 gpm at 60 psi by 10 CFR 430.32(o).

A filling tub takes a large share of the flow.

Tub fillers are not covered by the EPAct faucet limit, so read the actual rate; filler spouts commonly deliver 4-8 gpm.

Washer fill, dishwasher fill, a hose bib or anything else running at the same moment. Leave 0 if nothing else runs.

Design basis

100 percent is the conservative one- and two-family default: everything you listed really can run at once. Lower it only with a documented judgement for a larger system, and keep the number with the design.

Groundwater temperature, not air temperature. Cold-season water runs 37-45 F in the northern states, near 50 F in the mid-Atlantic and 65-75 F in the Deep South. The 58 F used by the DOE test procedure is a rating condition, not a design value.

The temperature the design flow has to arrive at.

Result

Design simultaneous flow
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Heat input to hold that flow
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Connected flow if everything runs at once
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Electric input to hold that flow
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Gas input to hold that flow
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Where this stops applying
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Calculation details
Design flow in metric
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Kitchen faucets
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Lavatory faucets
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Cold-season temperature rise
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Equivalent full-rate showers
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Showers
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Tub fillers
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What to check

  • Fixture counts and the actual flow rate of each fixture
  • The diversity factor you design to, and the reason for it
  • Cold-season groundwater temperature and the delivery temperature

Standards referenced

Related calculators

Screening estimate only. The result panel lists the limits that apply to this calculation. Verify nameplate data, rates, groundwater temperatures and installer quotes before deciding, and have gas, venting and electrical installations checked against the local code by a licensed contractor.

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Keep the assumptions with the result.

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