Tankless Water Heater Sizing
Tankless Water Heater Sizing: GPM, BTU and Temperature Rise
Illustrative image — sizing a tankless water heater around real shower demand: GPM and temperature rise.
Most tankless complaints we hear in Leesburg trace back to one root: the unit was sized for summer. Sizing a tankless water heater is three numbers done honestly, and this guide walks all three so you can read any estimate with confidence.
Number One: Simultaneous Demand in GPM
List everything that might run at once on your worst morning. Typical residential fixture flows look like this:
| Fixture | Typical Flow | Notes |
|---|---|---|
| Shower | Around 2.0 to 2.5 GPM | Water-saving heads run lower |
| Bathroom sink | Around 0.5 to 1.5 GPM | Short bursts, often overlapped with showers |
| Kitchen faucet | Around 1.5 to 2.2 GPM | Dish-filling runs longer |
| Dishwasher | Roughly 1 to 2 GPM | Intermittent draws |
| Washing machine | Roughly 1.5 to 3 GPM | Fill cycles, then idle |
A Leesburg household where two showers and the dishwasher share a morning window carries a peak demand near five to six GPM. One shower plus a sink sits closer to three. The fixture census, not the house size, is what counts.
Number Two: Temperature Rise
Tankless capacity is quoted against a specific temperature rise: the difference between the incoming water temperature and your target delivery temperature. The relationship is simple and unforgiving:
Rise = Target Temperature − Inlet Temperature
Target 120°F with 60°F incoming water is a 60°F rise. The same target against 45°F winter inlet water is a 75°F rise, and the unit delivers measurably less flow at the higher rise. This is why a system that felt endless in June disappoints in January, and why Northern Virginia's cold months belong in every sizing conversation. We size against winter, then enjoy the surplus in summer.
Number Three: BTU Input and Fuel
Gas tankless capacity comes from BTU input: the burner's firepower determines how much flow the unit can push through the rise you ask of it. Larger rise, larger BTU demand, and the gas line feeding the unit must match. That is why meter capacity and line sizing get verified before any gas model is quoted; a starved burner cannot deliver the plate rating regardless of what the box says.
Electric Tankless: kW Does the Same Job
Electric models exchange BTUs for kilowatts, and the arithmetic lands the same way: demand and rise set the kW class. The difference is the supply side. Whole-house units draw serious amperage, so panel headroom and dedicated circuits decide feasibility before economics does. Our electric tankless page covers the panel math, and the gas tankless page covers the combustion side.
Worked Example
Consider a four-person Leesburg home: two bathrooms, morning showers overlapping, dishwasher after breakfast. Peak demand lands near 5 GPM. Winter inlet around 45°F, target 120°F, so the rise is 75°F. A model that delivers 5 GPM at a 45°F rise will not deliver 5 GPM at 75°F; the winter-rated flow might land near 3. The correct pick is the next capacity class up, or a parallel pair, and that conclusion is exactly what the sizing math exists to reveal.
When the Answer Is an Upgrade, Not a Repair
If a healthy, clean, well-maintained unit still folds under peak demand, the capacity is wrong, not the hardware. Chasing a sizing problem with repairs wastes money; the real paths are a capacity upgrade or a reconfigured layout. A technician who measures before recommending will tell you which.
Getting the Numbers for Your House
You can approximate all three numbers from this guide. The confident version comes from a survey: fixtures counted, inlet conditions checked, gas or electrical supply measured, and the calculation shown in a written estimate. That is standard on every tankless installation we quote in Leesburg.
The takeaway: Size against the coldest month and the busiest minute, never the brochure. Add your simultaneous fixture flows, apply the winter temperature rise, and the right GPM class falls out of the arithmetic.