BTU vs GPM: Sizing a Tankless Water Heater
What is the difference between BTU and GPM for tankless water heaters?
BTU and GPM measure two different things in a tankless water heater, and understanding both makes sizing much easier. BTU (British Thermal Units) describes how much heat the unit can produce in an hour. GPM (gallons per minute) describes how much hot water the unit can deliver at a given temperature rise.
BTU: heating power (how hard the heater can “work”)
A higher BTU rating generally means the heater can add more heat to incoming cold water. That matters most in colder climates or when the incoming water temperature is low, because the unit must create a larger “temperature rise” to reach your desired output temperature (often around 120°F). If the BTU is too low for your conditions, the heater may not keep up and the delivered hot water temperature can drop when demand increases.
GPM: usable hot-water flow (how much you can run at once)
GPM is the practical, day-to-day number people feel. It reflects how many fixtures you can supply simultaneously—like a shower plus a sink—without running out of heat. However, GPM is not a fixed value across all situations: the same heater will have a higher GPM at a small temperature rise and a lower GPM at a large temperature rise.
How BTU and GPM connect
BTU is the “engine size,” while GPM is the “speed” you can maintain under a specific load. The load is the temperature rise: the bigger the rise, the more BTUs are required per gallon, and the lower the achievable GPM. That’s why product listings often advertise a maximum GPM, but the fine print or performance chart tells you what flow to expect at different temperature rises.
Which number should matter most when buying?
Use BTU to judge whether the heater can handle your climate and target temperature. Use GPM to match your household’s simultaneous usage. For a deeper look at how a 9 GPM, 180,000 BTU indoor LP tankless unit is positioned and what to consider before choosing one, see this guide: https://lirete.com/guide-9-gpm-180000-btu-indoor-lp-tankless-heater-guide/.
FAQ
What size tankless water heater do I need for two showers?
Plan for the combined flow rate of both showers at the temperature rise in your area. Many homes need roughly 4–6+ GPM for two showers, but the required BTU increases significantly in colder regions where incoming water is much colder.
Leave a comment