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Water Heater Size Calculator (First-Hour Rating)

Size a tank from the hot water the fixtures draw in the busiest hour, then screen it against the nameplate first-hour rating.

Inputs

Hot water used in the busiest hour

Count showers that overlap in the busiest hour, not showers per day. A two-bathroom household commonly starts the day with two.

Minutes the shower actually runs. A low-flow showerhead at 2.5 gpm for 8 minutes draws 20 gal.

Showerheads made after 1 Jan 1994 are capped at 2.5 gpm at 80 psi (10 CFR 430.32(p)). Read the rate off the showerhead or its packaging. Water-saving heads run 1.5-2.0 gpm.

Set 0 if nobody fills a tub in the busiest hour.

Only the hot portion counts. A standard tub holds 30-50 gal of water but usually only half is drawn hot; a large soaking tub draws more.

Set 0 unless laundry runs during the same hour as the showers.

Hot-wash gallons from the washer's spec sheet. Top-load standard washers use more than front-loaders.

Hand washing, cleaning, dishwasher and anything else that draws hot water in the same hour. Leave 0 if the fixtures above cover it.

Temperatures and mixing

Groundwater temperature, not air temperature. The DOE test procedure uses 58 F (10 CFR 430 Subpart B Appendix E 2.3). Winter groundwater runs 40-45 F in the northern states and 70-75 F in the Deep South.

The dial setting on the tank. The DOE test procedure solves at 125 F (Appendix E 2.4). Setting the tank to 140 F is the usual Legionella-risk approach, but it needs a mixing valve so the fixtures still get safe water.

Turn on if the tank is stored hotter than the fixtures need and a thermostatic mixing valve tempers the water down. This is what makes extra storage temperature turn into extra usable gallons.

What the mixing valve delivers. Use the same value as the tank setting when there is no mixing valve.

The unit you are considering

Printed on the rating plate and on the yellow EnergyGuide label. Leave 0 to skip the comparison. The first-hour rating is defined in 10 CFR 430 Subpart B Appendix E 1.5 as the gallons a non-flow-activated heater can deliver in the hour after a full heat-up.

Result

Peak-hour hot water demand
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Storage needed if no recovery credit
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Nominal tank size to price out
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Mixing valve effect
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Where this stops applying
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Calculation details
Reference daily hot water use
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Delivery temperature rise
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Headroom against nameplate FHR
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Laundry in the busiest hour
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Showers in the busiest hour
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Storage temperature rise
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Tub fills in the busiest hour
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Usable gallons per tank gallon
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What to check

  • How many showers, tub fills and laundry loads share the busiest hour
  • Groundwater temperature, from your utility or a measurement
  • The nameplate or EnergyGuide first-hour rating of the unit you are considering

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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