Heater technologies

Whole house tankless water heater

Size a whole-house heater from the fixtures that run together and the winter inlet temperature. Check low-flow use and the home supply too.

A residential bathroom with a shower and basin that can create overlapping water demand
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Choose a whole house tankless water heater for the taps that run together. Floor area does not show that demand. Record the peak routine and measure flow. Use a winter inlet range backed by local evidence. Then check the exact model and home supply. A whole-house label does not mean every tap can run at once.

Size whole-house tankless for the hot-side flow that runs at once. Use the required rise. Mixed flow includes water that does not pass through the heater. Check the low flow that starts heating as well as the peak curve. Then check supply and location rules. If the home cannot support that load, compare storage or another setup before changing the routine.

Whole house tankless water heater sizing starts with a routine

Make a fixture list for the busiest real overlap. A shower while another person fills a basin is one case. Two showers while a kitchen tap runs is another. Write the times and intended use next to the list. Do not count every fixture as active by default or omit a regular draw because a smaller total looks convenient.

Use measured or specified flow bounds. The home calculator begins with no active fixtures so that every row is a deliberate input. It can accept a low and high flow when uncertainty is real. If you do not know a fixture's flow, mark it unknown until measured. Do not substitute zero.

Count only the water supplied by the heater. A mixed shower flow includes cold water. If you measure the total stream, enter it as mixed with the desired temperature. If you have a genuine hot-side measurement, use that instead. This distinction can change the required heater GPM without changing how much water leaves the showerhead.

Record a second routine for occasional peak use. Guests may add a draw. A planned bath or changed morning schedule can create another task too.

Label it as a separate case. An average routine may hide the peak that led you to replace the heater. Keep that peak visible while comparing models.

Measure flow without turning labels into household facts

A safe container-and-time check estimates fixture flow. Divide collected US gallons by minutes. A gallon in half a minute is 2 GPM.

Record the conditions with the reading. Tap position and other open taps can change flow. The supply conditions matter too, so one reading should not stand in for every household routine.

Avoid burns and spills. Do not lift heavy containers to get a number. You need no exposure to hot water for this worksheet.

Use a safe flow check or a fixture specification. State any limits of that value. Do not disconnect pipes or alter a valve just to measure flow for the page.

A fixture's stated maximum is a rating, not a reading from your home. Label it as an assumption if you use it. A range can keep that uncertainty visible. Gather a better reading when needed. The tool should preserve what you know, rather than present a product label as a household measurement.

If measurements differ, investigate the conditions before averaging them into one confident value. Two readings taken with different simultaneous uses may describe two cases. Keep those conditions together. A sizing record is more useful when someone can see what actually ran during the measurement.

Winter rise sets the capacity check in °F

Subtract the cold inlet from the outlet setpoint. With an assumed 120°F outlet and 50°F inlet, the rise is 70°F. A 60°F inlet reduces that to 60°F. For the same hot-side flow, the colder condition requires more heating duty.

Use a measured or stated inlet range for the real supply. A room-temperature reading from water that stood in indoor pipes may not represent the winter source. Record where and when the reading was taken. The calculator does not infer inlet temperature from a location label or require a ZIP code.

Compare the model's flow at that rise. Do not use its largest advertised GPM without the associated condition. Stiebel Eltron's flow table illustrates the relation between rated output and rise. The table applies to its listed versions; it does not show another maker's performance.

Also review how the model handles demand beyond its limit. Some equipment may reduce flow to hold temperature. A result that supplies less water is different from meeting every requested draw. Keep that trade-off visible if it forms part of a supported equipment choice.

Worked overlap: mixed shower and hot-side tap

Assume a shower uses 2 GPM mixed to 105°F. A tap draws 1 GPM entirely from the heater. Set the heater to 120°F and use a 50°F cold inlet. The shower's hot fraction is 55 divided by 70, or about 1.57 GPM from the heater. Add the tap to get about 2.57 GPM of hot-side flow.

At those conditions, the shower needs 55044 Btu/h and the tap needs 35028 Btu/h. Combined thermal duty is 90072 Btu/h, or about 26.4 kW. These are derived values for stated inputs. They are not typical fixture claims or a nameplate rating.

With a warmer inlet, recompute the shower's hot fraction. Do not carry the colder hot-side flow into the warmer case without adjusting it. The worksheet computes paired inlet and flow cases before taking result ranges. That prevents unrelated endpoints from becoming a fabricated worst case.

This math includes no pipe losses or generic safety margin. The exact curve and installation evidence still govern the choice. A system may need a specific adjustment.

If so, state its reason and source. Do not add a percentage just to make a result look cautious. Keep that extra calculation distinct from the basic water heat balance.

Gas and electrical supply can change the choice

For electric tankless, use the electric calculator to state a power case. Total current needs a stated voltage and heating efficiency. Unknown input values stay unknown.

That current does not choose branch breakers. The result also cannot tell you the spare service capacity. Keep the real circuit checks separate.

For gas tankless, use the correct fuel version and supported input rating when planning supply. A thermal-output number is not burner input. The calculator leaves input unknown unless a suitable steady-state efficiency is supplied. Do not substitute UEF just because it appears on a product label.

The supply check needs exact model instructions. Check circuits and gas delivery where relevant. Include exhaust and water connections, then any drains.

A nearby gas fitting is only one visible part of that work. Spare breaker space is also limited evidence. Licensed trades handle the real supply and installation under those instructions.

If one supply arrangement is impractical, compare alternatives. The electric-versus-gas page separates those constraints. A storage arrangement or a targeted local unit may change the task. Do not hide a supply shortfall by silently deleting a regularly used fixture from the calculator.

Low-flow uses and distribution still matter

A whole-house system serves small draws as well as large ones. Check activation and supported operating flow for the exact model. A high peak-flow rating does not show that a brief warm-water handwash works at the intended fixture setting.

Water still travels from the heater to the tap. A long pipe route can produce a wait even when the heater has sufficient output. The point-of-use guide addresses a local solution for one fixture. Replacing a central unit with a larger one does not by itself shorten the pipe.

If a proposal includes recirculation, keep its approved layout and controls. Recirculation moves water through a return path. It changes the pipe and operating questions.

Not every model supports every return setup. Nor does a circulation feature alone promise instant water at all taps. Read the exact system instructions for those limits.

For an outside location, check capacity separately from freeze protection. Read the outdoor guide for model approval and exposed connections. A supported winter curve shows water-heating performance. It cannot prove that external pipes stay protected during a cold-weather outage. Keep that separate protection plan with the installation record.

Keep a whole-house comparison worksheet

Record What to retain
Peak routine Fixtures that overlap and why
Flow Measured or rated value with units; low and high bounds
Temperatures Inlet range; setpoint; mixed-water targets
Candidate US model with fuel or voltage; document revision
Capacity Flow evidence at each relevant rise
Low draw Minimum activation and operating limits
Installation Supply and exhaust; drains and access; permits
Ownership Water-quality rules and service schedule

Compare candidates against the same record. If one uses a warmer inlet or a lower shower flow, it is a different case. Do not rank that as a stronger capacity result without disclosing the change. Equal-task comparisons make limitations easier to see.

Keep installed costs separate from equipment prices. Leave unknown supply upgrades unpriced. Do the same for missing labor or permit costs.

An unpriced item is not included at no charge. If the written work lists differ, resolve those differences first. Only then can their totals help you compare the systems.

The tankless-versus-tank comparison helps when the flow needs too much supply work. The heat-pump comparison describes another storage option. It has different air and capacity needs. Choose from the supported task and real location. A whole-house label does not finish those checks or guarantee every tap can run together.

Frequently asked questions

Does a four-bedroom house need a particular GPM?

Bedrooms do not show the flow that runs together. Use the fixtures, routines and winter inlet evidence. A house with staggered showers creates a different task from one with simultaneous draws. Future use can be a second case, but it should remain explicit rather than become an invented bedroom multiplier.

Should I add every fixture's maximum flow?

Only if that simultaneous task is one the system must support. Otherwise describe the real overlapping routine and retain a separate larger case if needed. Do not enable every fixture by default or omit regular overlaps. The sizing record should show the household choice instead of hiding it inside an unexplained total.

Can I improve a small heater by raising the setpoint?

Do not raise the setting solely to force a sizing result. Higher outlet temperature changes mixing and increases scald risk. Capacity still needs checking under valid conditions, and household temperature controls must follow the appliance instructions. The CPSC scald guidance makes clear that hot water requires care even at a familiar setting.

What if no candidate fits the peak?

Confirm uncertain measurements and the intended overlap first. Then compare a supported larger capacity or another system arrangement. A deliberately staggered routine can be considered, but it is a changed task. Do not recommend the nearest undersized unit or silently lower the demand to turn an unsupported choice into a match.

Whole house tankless water heater verdict

Select a whole house tankless water heater only when the peak task, low-flow use and fit in the home have supporting records. Keep each case with its inputs. A product title or maximum-GPM badge cannot replace the winter curve. Use the calculator to prepare the demand sheet, then return to the technology comparison if the required supply or scope is impractical.

Primary sources checked October 14, 2026: the linked flow table, Tempra installation manual, ENERGY STAR rating definitions, and CPSC scald guidance. The methodology records the derived examples and limits.