How to size a tankless water heater

A tankless heater is sized by two numbers you have to measure, not by a bedroom count. The first is the gallons per minute you will actually run at the same time. The second is the temperature rise, which is the set point minus the incoming water temperature.

The Department of Energy’s sizing guide uses a clear example: a hot faucet at 0.75 gallons per minute plus a shower at 2.5 gallons per minute is 3.25 gallons per minute. If you do not know the incoming temperature, assume 50°F.

If you want 120°F at the outlet, the rise is 70°F. You then need a heater that can deliver at least 3.25 gallons per minute at a 70°F rise, not a heater whose brochure leads with a flow rate measured at a much smaller rise.

That last distinction is where shopping goes wrong. A unit advertised at 9 gallons per minute may be telling the truth at a 35°F rise and unable to hold 120°F at 4 gallons per minute when the well is 40°F.

Read the chart that matches your rise. The how-it-works page shows the formula and a worked 199,000 Btu/h example in how a tankless heater works. This page is the household side of the same arithmetic.

Add the fixtures you refuse to stagger

List the hot-water uses that will overlap in real life, not the uses that could overlap if everyone conspired. Two showers and a dishwasher at 7 a.m. count if that is the house you have. Two showers that never happen together do not.

Use the fixture’s actual flow, not a default from 1998, if you can. A showerhead marked 2.0 gallons per minute is not a 2.5. A tub filler is a large flow and will dominate the sum if someone fills a tub while a shower runs.

The Department of Energy points out that low-flow fixtures are how you reduce the number the heater must meet. Reducing demand is legitimate sizing. Pretending the shower is 1.5 gallons per minute when you measured 2.4 with a bucket and a phone timer is not.

The bucket test is simple. One gallon bucket or a known pitcher, hot tap fully open at the fixture you care about, seconds to fill, converted to gallons per minute.

Do it on the shower and on the kitchen faucet if those are the overlap. Aerators clog and the test changes. Retest when you clean them.

Minimum activation flow is a separate limit, often near half a gallon per minute. A fixture below that flow will not wake the heater at all. The symptom page covers that pattern in tankless symptoms.

Rise is a winter number

The 50°F assumption is a national placeholder. Groundwater in a warm coastal city can be far warmer. Groundwater in a northern January can be near 40°F or lower. The rise at a 120°F set point is then 80°F, not 70.

Using the formula from the how-it-works page, flow capacity falls as rise increases, because the same Btu are spread across more degrees. A unit that delivers about 4.8 gallons per minute at a 70°F rise in the worked example falls to about 4.2 at an 80°F rise.

If your simultaneous demand is 4.5, the unit is fine in the placeholder climate and short in the real winter. Size for the coldest month you care about, not for September when the salesperson visited.

The Energy Saver guide also notes that, historically, a 70°F rise was associated with about 5 gallons per minute through gas demand heaters and about 2 gallons per minute through electric ones. Current gas units with higher input beat the old gas figure.

Electric whole-house units that beat the old electric figure do it with very large amperage.

Gas or electric is the constraint. Do not select an electric tankless flow rate the panel cannot feed. The nameplate current and a load calculation are an electrician’s work, described as a boundary in electrical safety.

Dishwashers that heat their own water do not need a 140°F rise from the tankless unit. The Energy Saver guide mentions 140°F for dishwashers without internal heaters. Most current dishwashers have their own heater.

Check the appliance manual before you oversize the water heater for a dish cycle. A mixing valve and a higher set point are a scald conversation, not a default. See temperature and scalding.

Gas supply and vent, or the chart is fiction

A 199,000 Btu/h heater needs a gas line and a regulator that can pass that input while the pressure at the unit stays inside the data-plate window. A line that was sized for a 40,000 Btu/h tank will not do it.

The symptom of an undersized line is a heater that lights and then goes lukewarm or locks out when it tries to modulate up. Measuring inlet pressure under full fire is a manometer test.

It is not a guess from pipe diameter remembered across the basement. Budget the new line in the installed price. The cost guide is where tankless jobs separate from tank jobs, and the gas line is a common reason.

The vent is the other reason. Non-condensing and condensing units use different materials.

The old atmospheric vent is usually ineligible. Equivalent length of elbows counts against the maximum. Condensing gas heaters explains the plastic vent and the condensate drain.

A sizing exercise that ignores a vent the wall cannot accept is not finished. Two smaller units in cascade are sometimes the right answer when one unit cannot cover the rise and the vent route, and they cost like two installs.

Get that price before you accept a single unit that only works if nobody showers during the laundry.

A worked household

Assume a 2.0 gallon-per-minute shower and a 1.0 gallon-per-minute kitchen faucet that genuinely overlap, and a winter inlet of 42°F with a 120°F set point. Demand is 3.0 gallons per minute.

Rise is 78°F. You need the manufacturer’s chart cell at 75°F or 80°F rise, not the 35°F cell, showing at least 3.0 gallons per minute. If the chart shows 3.5 at that rise, you have a margin for a slightly longer shower and you do not have a margin for a third fixture.

Say so in the quote. If a teenager will open a second shower, add that flow and accept the larger unit or the cascade. The Department of Energy’s instruction is to list the devices you expect to use at one time and add them. The discipline is in the phrase “at one time.”

Checklist: tankless sizing

• Overlapping fixtures listed. Flows measured or read from the fixture, not remembered.
• Winter inlet temperature known or conservatively assumed. Rise written down.
• Manufacturer chart read at that rise, not at the flattering column.
• Result at or above the overlapping flow, with a small margin.
• Gas input checked against the pipe and the regulator. Vent material and length checked.
• Electric models checked against the panel. No whole-house unit on a tank-element circuit.
• Service valves included so descaling is possible. Hardness considered.
• Installed price compared with a tank that meets the first-hour worksheet, if a tank would serve.

Recirculation does not increase the gallons per minute the exchanger can heat during an overlap. It shortens the wait. Do not pay for a larger heater to solve a wait that a small pump would solve, and do not buy a pump to solve an overlap the heater cannot heat. The two problems feel similar in a morning rush and they are not.

Sources

U.S. Department of Energy, Energy Saver, “Sizing a New Water Heater,” for the 0.75 plus 2.5 gallon-per-minute example, the 50°F incoming assumption, the 120°F and 140°F set-point notes, and the historical 5 gpm gas and 2 gpm electric figures at a 70°F rise.

U.S. Department of Energy, Energy Saver, “Which Water Heater Is Right for You?,” for the 2-to-5 gallon-per-minute whole-house context and the note that more than one heater may be required. Manufacturer flow charts and gas-pressure requirements in the installation manual (Rinnai, Navien, Rheem). The Btu-to-gpm formula used in the companion article is the standard heat-capacity relation, 8.33 pounds per gallon times 60 minutes.