Tankless Water Heater Size Calculator

Showerheads made after 1 Jan 1994 are capped at 2.5 gpm at 80 psi (10 CFR 430.32(p)).

Faucets made after 1 Jan 1994 are capped at 2.2 gpm at 60 psi (10 CFR 430.32(o)).

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

Dishwasher fill, clothes washer fill, a hose bib or anything else that runs at the same moment. Leave 0 if nothing else runs.

Enter 0 to use the fixture total above. Use this when you already have a flow figure from a fixture-unit takeoff or from the manufacturer's worksheet.

This is the design case for tankless. Groundwater runs about 37-45 F in the northern states in winter, near 50 F in the mid-Atlantic and 65-75 F in the Deep South. Note that the 58 F used by the DOE UEF test procedure (10 CFR 430 Subpart B Appendix E 2.3) is a rating condition, not a cold-season design value.

Warm-season groundwater runs roughly 15-20 F warmer than winter, which is why a unit that keeps up in July can fail in January.

The fixture temperature you are designing to.

From the spec sheet: the gallons per minute the unit is published to deliver at its rated temperature rise. Leave 0 to skip the screen.

The rise the rated flow is quoted at - usually printed next to the GPM, for example '6.6 GPM at 35 F rise'.

The flow below which the burner does not fire. Typically 0.5-0.75 gpm; small draws such as a single lavatory can fall under it and get no hot water at all.