Sizing a Tankless Water Heater for a New Hampshire Winter

Tankless sizing is not a count of bathrooms. It comes down to two numbers, and in New Hampshire the second one changes a lot between summer and winter.

Number one: peak flow

Peak flow is the most hot water your household draws at the same moment, in gallons per minute (GPM). Add up the fixtures that realistically run together at your busiest time: two showers while the dishwasher runs, say, or a shower and a kitchen tap.

Your fixtures' actual flow rates matter. Showerheads, faucets and appliances vary, and older fixtures often use more water than new ones. The U.S. Department of Energy's sizing method starts with exactly this list of what runs at once. ^[1]

Number two: temperature rise

Temperature rise is how many degrees the heater has to warm the water: the temperature you want at the tap minus the temperature of the water entering the house.

The incoming water temperature is the part that changes with the seasons. It is colder in a New Hampshire winter than in summer, so the heater has to add more degrees in January than in July. The Department of Energy's sizing guidance says to assume 50°F incoming water unless you know otherwise. ^[2] If you can measure your own water in winter, that number is better than any assumption.

Why the two numbers fight each other

A tankless heater has a fixed maximum heating capacity. It can spend that capacity on more gallons at a small temperature rise, or fewer gallons at a large rise. It cannot do both.

So every model has a different GPM at every temperature rise. Manufacturers publish that relationship as a flow chart or table. For scale, the Department of Energy gives about 5 gallons per minute at a 70°F rise as typical for gas-fired tankless heaters, and notes that cooler incoming water can reduce the temperature at the most distant faucet. ^[2] The largest GPM figure in a model's marketing is usually at a much smaller temperature rise.

The practical rule: size the heater for your peak flow at your winter temperature rise, using that model's own published chart.

What happens when a unit is undersized

When demand goes above what the heater can deliver at that temperature rise, the water gets cooler, or the flow drops as the heater limits it to hold temperature. It rarely fails completely; it just can't keep up. That is the "it worked fine until winter" complaint.

When one unit isn't enough

If peak flow at winter temperature rise is more than one unit can provide, the usual options are:

• a larger unit, if the gas supply and venting can support it
• two units working together
• lowering peak demand, for example with lower-flow showerheads
• a different kind of water heater altogether (see tankless vs. tank vs. heat pump)

Gas supply is part of sizing

A heater that can deliver the flow on paper still needs enough gas at full output. A large gas tankless heater can need far more gas than an old tank heater, so the gas line, and on some homes the meter or propane regulator, is checked as part of sizing, not after. ^[3]

What we ask about when sizing

• which fixtures and appliances realistically run at the same time
• how many people are home at the busiest times
• the fuel you have, and the size and route of the gas line
• where the heater would go, and how it would vent
• anything you are planning: a new bathroom, a larger shower, a soaking tub

Request an estimate, or call and we will go through it with you.