Water Heater Sizing Guide

Water heater sizing depends on the type of system being selected. Tankless sizing is about flow under the required temperature rise. Tank sizing is about stored capacity and first hour delivery. Those approaches should not be blended into a single shortcut.

Start Tankless Sizing With Temperature Rise

Navien defines temperature rise as the desired setting minus the inlet temperature. The manufacturer's example is 120F minus 55F, which equals a 65F rise. That calculation matters because a tankless heater has to raise incoming water to the target temperature as water moves through the unit.

The same tankless model can feel different in different inlet-water conditions because the required rise changes. Navien's example does not provide a national map or local inlet temperatures beyond the example, so the useful point is the formula itself: desired setting minus inlet temperature equals temperature rise.

A useful tankless sizing conversation should therefore include both the desired hot-water setting and the cold-water inlet assumption. If either side is missing, the flow rating may not describe the real demand. The Navien formula gives the sizing conversation a measurable starting point without inventing location-specific numbers.

This is the main reason tankless sizing should not rely on a single advertised flow claim. The flow has to be read beside the rise. Without that pairing, a model can look sufficient on paper while the sizing question remains unresolved.

Size Tankless Flow to Simultaneous Demand

Navien gives practical tankless flow estimates tied to the number of fixtures running at the same time: about 3.5 GPM for 1-2 fixtures, 5 GPM for 2-3 fixtures, and 7 GPM for 3-4 fixtures. The manufacturer also says to choose a unit with a flow rate equal to or greater than the maximum simultaneous hot-water demand, using the coldest-day rise.

That guidance shifts sizing away from vague household labels. The question is not just whether the home is small or large. The question is what hot-water uses may overlap, and what flow the unit can deliver at the required temperature rise.

A proposal should therefore identify expected simultaneous demand and coldest-day rise. If it only lists a brand or a headline GPM value, the sizing logic is incomplete. Navien's sizing rule is equal to or greater than maximum simultaneous demand under the coldest-day rise.

Use Fixture Math to Check Tankless Fit

Bob Vila gives a simple fixture-flow example: a dishwasher at 3 GPM plus a shower at 2.5 GPM means a minimum 5.5 GPM tankless flow. That example is useful because it shows the arithmetic behind simultaneous demand. The model needs to satisfy the combined load, not just the largest single fixture.

The example also shows why sizing can change based on habits. If the dishwasher and shower are expected to overlap, the tankless unit must be evaluated against the combined flow. If they are not expected to overlap, the demand picture may be different. Bob Vila's example does not create a universal household answer, so the safer method is to list likely simultaneous uses and add the fixture flows where they apply.

This math belongs with the temperature-rise check, not apart from it. A tankless unit must meet flow at the required rise. The Bob Vila example supports the demand side of the sizing check, while Navien's temperature-rise formula supports the heat-rise side.

Size Tank Systems by Storage and First Hour Rating

Bob Vila gives household gallon ranges for tank water heaters: 1-2 people at 23-36 gal, 2-4 people at 36-46 gal, 3-5 people at 46-56 gal, and 5+ people at over 56 gal. Those ranges are not tankless flow ratings. They apply to stored-volume sizing for tank systems.

Bob Vila also defines First Hour Rating, or FHR, as the hot water a full tank delivers in one hour. That definition matters because tank sizing is not only about the gallon label on the tank. It is also about how much usable hot water the tank can deliver during a busy period.

A careful tank sizing review should keep those two ideas together. Household size gives a starting gallon range. FHR explains delivery from a full tank across one hour. Household size alone is not enough, and tankless GPM rules should not be used as if they were tank gallons.

The clean comparison is therefore tank storage and FHR for tank systems, versus flow and temperature rise for tankless systems. Mixing those measures is where many sizing conversations become unclear.