How to size a tankless water heater (GPM and temperature rise)
A tankless water heater is sized with two numbers: how many gallons per minute you use at once, and how many degrees the water must be heated. Get both right and you have endless hot water.
Step 1: peak flow in gallons per minute
Add up the fixtures that realistically run at the same time. Typical flows:
- Shower: about 2.0 GPM (1.8 GPM for many newer heads)
- Kitchen or bathroom faucet: 1.0–1.5 GPM
- Dishwasher: about 1.0–1.5 GPM
- Washing machine: about 2.0 GPM
Two showers and a kitchen faucet together come to roughly 5.5 GPM.
Step 2: temperature rise
Temperature rise is the hot-water setpoint (usually 120 °F) minus the temperature of the cold water entering your home. Groundwater is around 70 °F in the southern states and can drop to 40 °F or lower in northern winters. That means a southern home may need a 50 °F rise while a northern home needs 80 °F or more.
Step 3: match the unit
Manufacturers publish flow rates at different temperature rises. A heater rated around 9–10 GPM at a small rise may deliver only 5–6 GPM at a 70–80 °F rise. Always read the flow at your rise, in winter, not the headline number.
Gas or electric
Gas units deliver far more hot water per unit and are the usual choice for whole-house use. Whole-house electric tankless heaters need very large electrical service, often 100 amps or more for the heater alone, so check your panel first.
Installation checks
- Gas line size: tankless units need a high BTU input and often a larger gas line than an old tank.
- Venting: condensing units can often vent with PVC; non-condensing units need stainless venting.
- Water quality: in hard-water areas, plan an annual descaling flush or a softener.
Installation should be done by a licensed plumber in line with local code.