HotWaterMeasure two different kinds of demand
A storage water heater needs to cover water drawn through a busy hour, using the water already stored and the heat it can add during that hour. A tankless heater needs to supply the fixtures running at the same time. These are related questions, but the units and equipment ratings are different.
In the planner, Tank demand uses flow, duration and the number of uses in one hour. Tankless load uses flow and the number of fixtures operating together. Two showers taken one after another change the hourly requirement without necessarily increasing simultaneous demand.
For a storage tank, look beyond the gallon label
A tank’s nominal gallons describe its storage volume. The first-hour rating describes a tested delivery capacity that also reflects recovery. Two heaters with the same nominal tank size can have different first-hour delivery. Heat-pump models can also perform differently across operating modes and surrounding conditions.
Our calculated heater-temperature gallons are a planning reference. They are not a certified first-hour rating and are not a recommended tank size. Ask the installer to compare the estimate with the manufacturer’s ratings, test conditions, operating mode and your actual draw pattern.
For tankless, pair GPM with temperature rise
The flow number on an advertisement is incomplete without the inlet and outlet temperatures. A heater’s manufacturer chart should show the flow available at the required temperature rise. Mixing at a fixture also matters: a shower using both cold and hot water draws less from the hot side than its total showerhead flow.
What to include in a sizing conversation
- How many showers, faucets and hot-fill appliances overlap.
- How long the busiest uses last and how many happen in one hour.
- Measured cold-water temperature during a demanding season.
- Current heater model, capacity, fuel type and the problem you want solved.
- Any planned bathroom, laundry or household changes.
The final selection also depends on available fuel or electrical capacity, installation space and local requirements. Those need an on-site review.
Calculate your household profile
Further reading
See the Department of Energy’s tankless water-heater sizing overview and heat-pump water-heater decision guidance.