Operation and ownership
What Size Tankless Water Heater Do I Need
Pair overlapping hot-side flow with winter temperature rise. Check the exact model curve and supply before choosing a tankless water heater size.
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What size tankless water heater do I need for my home? Use the flow that may run at once and the rise from cold inlet water to heater output. Then check a model curve at that rise. Bathroom count and a headline GPM rating cannot answer the question on their own.
This guide turns measured demand into a model-screening worksheet.
Add the hot-side flow of fixtures that overlap. Record a cold-season inlet range and the heater setting. Calculate temperature rise, then check the selected model at that rise and its real fuel or voltage. The sizing result is a heat task. It cannot approve the gas line, electrical service or setup site. Keep those as separate checks before buying.
What size tankless water heater do I need: measure demand
Write down which taps may run together. Do not sum every fixture if they never overlap. Do not silently stagger uses if the household expects them together. Both choices change the task.
Keep the agreed use pattern with the result so a model is not chosen for an easier case than the home needs.
Measure flow with a safe collection method where practical. US gallons collected divided by minutes gives GPM. A mixed shower includes cold water that bypasses the heater. Label whether each figure is total mixed flow or hot-only flow.
The main calculator lets you enter those as distinct rows.
Record inlet water and temperature rise
Use a cold-season inlet reading or a stated planning range. Avoid treating water warmed in an indoor pipe as proof of the winter supply. Note the date and method. Keep uncertain values visible.
They are inputs to investigate, rather than a reason to invent a climate rule.
Rise equals heater outlet target minus inlet temperature. An assumed 120°F outlet and 50°F inlet give a 70°F rise. These are math inputs, not a setting instruction. Use the real permitted setting and household scald precautions.
Increasing the setpoint is not a way to force a small heater to pass.
Keep the curve and units together
| Worksheet field | What to record |
|---|---|
| Overlapping use | Fixtures that actually run together |
| Flow | Hot-only or mixed GPM, clearly labelled |
| Inlet | Measured or assumed range with date |
| Outlet | Heater target and any mixed-use target |
| Model evidence | Flow at the needed rise and real voltage or fuel |
| Separate limits | Supply, location and service rules |
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Read the curve for the exact model. A maximum GPM at a warm inlet may not cover winter. Electric output can change with voltage. Some units reduce flow to hold temperature.
That response may not meet the requested full stream. Keep the delivered flow as part of the acceptance question.
Use the electric tool for a power estimate when comparing electric units. Total current is not a breaker size. A home load assessment and the model circuit table are still needed. Gas models need their own fuel-input and piping checks.
Frequently asked questions
Can I size by bedrooms or bathrooms?
Those counts do not state overlap or flow. Two homes with the same room count can use water differently. Measure the task that matters. Keep an alternate peak case if visitors or another routine create a draw the normal worksheet misses.
Is extra capacity always needed?
Do not add an unexplained margin. State a separate higher-demand case if you need one. Check that case against the model curve. A transparent change in inputs is more useful than quietly rounding demand to the next appliance size.
What size tankless water heater do I need: verdict
To answer what size tankless water heater do I need, keep flow and rise paired. Use the calculator, then the model curve. Return to the use guide to compare system trade-offs. Evidence: Tempra flow table and DOE sizing and retrofit guide.
Neither replaces the exact model manual.