What size tankless water heater do I need?

Q: What size water heater do I need for a family of 4?

A family of four typically has a peak-hour demand of about 70–80 gallons, so a 50-gallon gas tank (about 70–90 gallon first-hour rating) or a 50–66 gallon electric tank is the common choice. For tankless, plan on 7–10 GPM at a 70°F rise in cold climates so two showers can run at once. Always size from your actual peak-hour fixture use, not just bedroom count.

Q: Is first-hour rating or recovery rate more important?

First-hour rating (FHR) matters most for households with concentrated peak demand, since it reflects storage plus one hour of heating. Recovery rate matters more for steady, all-day use. The DOE requires the FHR on the EnergyGuide label — match it to your peak-hour demand and you will rarely run out. Verify the final selection against the manufacturer data and local code.

Question

What size tankless water heater do I need?

Accepted Answer

Tankless units are sized by simultaneous flow (GPM) at your required temperature rise, not by gallons of storage. A single shower needs about 2.5 GPM; two showers plus a sink is roughly 6–7 GPM. In northern climates, incoming water near 40°F means a 70–80°F rise, which cuts a unit’s rated GPM roughly in half — so size from the cold-weather rating, not the spec-sheet maximum.

What size tankless water heater do I need?

Answers