Tankless Water Heater GPM: How Many Fixtures at Once?

Where the fixture flows come from

Start with the federal maximums for new fixtures. Rules in 10 CFR 430.32 set these maximums for products manufactured after January 1, 1994:

• Showerheads: 2.5 gallons per minute, measured at a flowing pressure of 80 psi.
• Lavatory and kitchen faucets: 2.2 gallons per minute, measured at 60 psi.

EPA's WaterSense program labels showerheads that use no more than 2.0 gpm, against 2.5 gpm for a standard showerhead.

Read those as ceilings under test conditions, not as the flow at your customer's tap. Actual flow depends on the pressure in the house, the fixture and any aftermarket or older parts. Where you can, as DOE suggests (see the sizing guide), time a bucket for one minute at each fixture, the same way you would for any flow check, and use the measured number. Body sprays, rain heads and multi-head valves add up, so list every head that opens when the valve does.

Add up what runs together

Pick the realistic worst moment, then add the flows. A tankless unit has no stored reserve: whatever runs at that moment has to be heated on the way through.

Now turn each total into heat. With a 50 °F incoming temperature and 120 °F at the tap, the rise is 70 °F. Output in Btu/h is 500 × gpm × rise (a rounded constant, explained in the sizing guide), kW is Btu/h ÷ 3,412, and gas input is output ÷ the efficiency on the spec sheet. The gas figures below assume 95%, an example; use the figure for the unit you are quoting. Electric resistance is taken as 100%, also an assumption.

The last row is a lot of heat. 252,000 Btu/h is 500 × 7.2 × 70. As electricity, 73.86 kW is 307.7 amps at 240 V by P = V × I. That is arithmetic only; circuits, conductors and breakers come from the electrical code and the manufacturer's instructions, not from this page.

What a unit can deliver

Turn the question around. A unit of a given input has a fixed heat output, so the flow it can deliver falls as the rise goes up: max gpm = input × efficiency ÷ (500 × rise). The table uses five example sizes, not any product line, at 120 °F out.

An 80 °F rise is a 40 °F supply, which is colder than the 50 °F default and the kind of winter water the sizing guide shows for northern cities. Warmer supply helps: at a 50 °F rise (70 °F supply) the 36 kW unit delivers 4.91 gpm.

The worked comparison: two showers at 2.5 gpm need 5.0 gpm. At a 70 °F rise the 199,000 Btu/h gas example delivers 5.40 gpm, so it covers them with 0.40 gpm to spare. The 36 kW electric example delivers 3.51 gpm and cannot. Put 5.0 gpm through it and the water leaves 49.1 °F warmer than it came in, not 70 °F, or 99 °F instead of 120 °F with 50 °F supply.

Add the kitchen faucet and the 7.2 gpm load beats every unit in the table at a 70 °F rise. The 199,000 Btu/h gas example would raise it only 52.5 °F, the 36 kW electric example 34.1 °F.

Gas vs. electric, and what to do about a shortfall

In these examples the difference is input. The 5.0 gpm load is 51.29 kW of heat into the water, more than one 36 kW unit, while the 199,000 Btu/h gas example covers it.

When the numbers fall short, the choices are about the load, not the arithmetic:

• Reduce the design load. A WaterSense showerhead cuts two showers from 5.0 gpm to 4.0 gpm, which is 140,000 Btu/h at a 70 °F rise.
• Ask the manufacturer whether more than one unit can be installed together, and what its installation instructions require.
• Tell the customer plainly which fixtures cannot run together at the winter temperature.
• Compare with a tank. A tank covers a short peak from stored water, which a tankless unit cannot do; see the first-hour rating method in the sizing guide.

Some of this is not sizing at all. Venting, gas supply, combustion air, relief valves and electrical service are governed by the adopted codes and the manufacturer's instructions, and the local building department is the authority having jurisdiction. This is general information, not code or engineering advice.

A shower is not all hot water

Everything above uses the fixture's full flow as hot flow. That is the safe side: a shower mixes hot with cold at the valve, so the heater supplies only part of its flow. With a 50 °F supply, a 120 °F heater and a 105 °F shower, the hot share is (105 − 50) ÷ (120 − 50) = 0.786. A 2.5 gpm shower then draws 1.96 gpm through the heater.

That helps, but there is no standard shower temperature to assume, so we do not build it into the defaults. The water heater size calculator does not include a mixing option, so apply this fraction yourself. If you do, take the temperature the customer actually runs and say so on the quote.

Bottom line

List the fixtures that can run together, add their measured flows, multiply by 500 and by the rise at the coldest water you expect, and compare the Btu/h with what the unit can deliver at that rise. Do the check with the unit's own published flow-and-rise data before you quote it, because the sizes above are examples. A spec sheet's rated maximum flow is measured at a 67 °F rise, so read the unit's flow-and-rise chart at your rise.

Run your own fixtures in the water heater size calculator. To turn the unit you pick into a price, use how to price a water heater replacement. To check whether the pipe feeding the unit can carry the flow, see water velocity and pipe size.