Step 1: Add up what runs at the same time
List the hot-water fixtures likely to run at once on a busy morning, not every fixture in the house, then add their flow rates in gallons per minute (GPM). Typical ranges:
| Fixture | Typical GPM |
|---|---|
| Shower | 1.8 – 2.5 |
| Bathroom faucet | 1.0 – 1.5 |
| Kitchen faucet | 1.5 – 2.2 |
| Dishwasher | 1.0 – 1.5 |
| Clothes washer (hot fill) | 1.5 – 2.0 |
| Tub filler | 4.0 – 6.0 |
Two showers and a kitchen faucet at once is about 5–7 GPM. Check the flow rate printed on your own showerheads and faucets; low-flow fixtures bring the number down a lot.
Step 2: Work out your temperature rise
Temperature rise is how much the heater has to warm the incoming water: your target (usually about 120°F) minus the groundwater temperature. In New Hampshire and Vermont, winter groundwater runs roughly 40–45°F, so plan on a 75–80°F rise. That’s much harder work than the 50–60°F rise in southern states.
Every tankless heater has a flow chart showing GPM at each temperature rise. Read the row for your rise, not the headline “up to” GPM, which assumes warm groundwater.
Gas vs. electric output
As a rough rule, heating water takes about 500 BTU per hour for each GPM and degree of rise. At a 77°F rise:
- A 199,000 BTU condensing gas unit delivers roughly 4.5–5 GPM, enough for two showers at once in most homes.
- A 27 kW whole-house electric unit delivers roughly 2–2.5 GPM, about one shower in winter. It also needs a large electrical service, typically several dedicated 40-amp, 240-volt circuits.
- Small electric point-of-use heaters suit a single sink far from the main heater, not a whole house.
If the math comes out close, size up. An undersized unit shows up as a shower that turns lukewarm when someone runs the dishwasher.
Venting, gas line, and installation
Gas tankless heaters draw far more gas while running than a tank heater, so the existing gas line often needs upsizing. Condensing units vent with PVC; non-condensing units need stainless venting. Local code covers all of it, so have a licensed plumber or gas fitter confirm the gas line, venting route, and electrical before you order.