Why tank gallons do not translate

Shopping for a tank means comparing gallons of storage. A tankless unit has no storage to speak of, so its rating is a flow rate: how many gallons per minute it can deliver while raising the water temperature by a certain amount. A unit that easily handles two showers in summer may struggle with the same two showers on a cold January morning, because the incoming water is colder and needs a bigger temperature rise.

Manufacturers publish charts that show flow at several temperature rises. A careful plumber reads those charts against your household's real habits rather than picking a model by its maximum burner rating.

How sizing is usually worked out

  1. List what runs at the same time.Think about your busiest ten minutes: perhaps two showers, a kitchen sink and the washer. That combination, not every fixture in the house, sets the peak demand.
  2. Estimate each fixture's flow.Shower heads, faucets and appliances each have a flow rating, often printed on the fixture or in its manual. Low-flow heads lower the total.
  3. Set the target temperature.Decide the outlet temperature you want at the heater. Many households set it to a moderate level for safety and efficiency.
  4. Find the coldest incoming water.Temperature rise is the target temperature minus the incoming water temperature. Size for the coldest time of year, not an average day.
  5. Read the manufacturer's chart.Find the flow the unit delivers at that temperature rise and compare it with your peak demand, leaving some margin.

Terms you will see on spec sheets

GPM
Gallons per minute. The flow a unit can heat at a stated temperature rise.
Temperature rise
How many degrees the unit adds to incoming water. Larger rises mean lower available flow.
Minimum activation flow
The smallest flow that makes the unit fire. Below it, the tap stays cold.
BTU input
The burner's gas input rating. A higher input supports more flow but also needs more gas supply.
Modulation
The unit's ability to turn its burner down for small demands, which affects comfort at low flow.

When one unit may not be the answer

  • IfA new primary suite is far from the existing heater locationThenAsk whether a second, smaller unit near that bath would serve it better than one oversized unit.
  • IfPeak demand only happens a few minutes a dayThenSmall changes such as low-flow shower heads may let a smaller unit cover it.
  • IfThe gas line cannot be enlarged without major workThenCompare that cost with a high-capacity tank before committing to tankless.

Ask the plumber

  1. What peak flow did you assume, and which fixtures make it up?
  2. What incoming water temperature did you size for?
  3. What flow does this model deliver at that temperature rise?
  4. Does our gas supply support this unit at full fire?

Common questions

Is there a standard size for a three-bedroom Fullerton house?

No. Two houses with the same bedrooms can have very different peak demand. Sizing should start from your fixtures and habits.

What happens if the unit is undersized?

During peak use the water may cool, flow may drop, or the unit may struggle to hold the set temperature.

Do low-flow fixtures help?

Yes. Lower-flow shower heads and faucets reduce peak demand, which can let a unit serve more fixtures at once.

General information for Fullerton, not a quote or professional advice. In an emergency, call 911.