Guide
Tankless water heater size for a family
Sized by what runs at once, not by how many people live there.
Short answer
A family of four usually peaks at two showers at once, 4.2 GPM. That takes a 160,000 BTU condensing gas unit where winter water comes in at 50°F, and 199,000 BTU where it comes in at 40°F. Tankless heaters are sized by flow at your temperature rise, not by the number of people or bathrooms.
Gas unit by household
| Household | Usual peak | Flow | Inlet 50°F | Inlet 40°F |
|---|---|---|---|---|
| 1 to 2 people | One shower and a kitchen tap | 3.6 GPM | 140,000 BTU | 160,000 BTU |
| 3 to 4 people | Two showers | 4.2 GPM | 160,000 BTU | 199,000 BTU |
| 4 to 5 people | Two showers and a kitchen tap | 5.7 GPM | beyond one unit | beyond one unit |
| 5 to 6 people | Two showers and the washing machine | 6.2 GPM | beyond one unit | beyond one unit |
199,000 BTU is the largest residential gas tankless input, set by gas code and venting limits. A peak beyond it needs two units linked together, or a household habit of not running the washing machine during showers.
How to find your household’s peak
Think about the busiest ten minutes of a normal day: usually the morning, with two showers running and someone at the kitchen tap. List what runs at the same moment, add their flows, and size for that. A bathtub fills at around 4 GPM on its own, so a household that fills a large tub while a shower runs needs a much bigger unit than the head count suggests.
A 3 bedroom house
Bedrooms are only a rough stand-in for people. A three-bedroom house with two bathrooms is usually sized for two showers at once, the family-of-four row above. If the second bathroom is rarely used at the same time as the first, the one-shower row applies and the unit can be smaller.