When a water heater fails, the easy move is to buy the same size again. That is often fine, but it is worth a minute to ask whether the old heater actually suited the household. Families grow and shrink, bathrooms get remodeled, and a heater that ran out every morning may have been too small all along.
Sizing looks a little different for tanks and tankless units, but the starting point is the same: how much hot water you use at your busiest time.
Work out your peak demand
- Pick your busiest hour.For most households it is the morning, when showers, a dishwasher or a washing machine run close together.
- List what runs together.Count showers, baths, laundry and dishes that overlap in that hour.
- Note complaints with the old heater.Running out mid-shower or waiting a long time between showers are clues the old size was short.
- Share it with the plumber.Bring this list to the conversation instead of a gallon number alone.
How each type is sized
Tank
- Look at the first-hour rating, not only the gallon capacity
- First-hour rating covers stored water plus what it can reheat in an hour
- Must physically fit the space with room for straps, venting and service access
Tankless
- Sized by flow rate in gallons per minute at your needed temperature rise
- Add up the flow of fixtures you run at the same time
- Gas units must have enough gas supply for their rated input
Where to find sizing numbers
- Tank capacity
- Gallons, on the rating plate
- First-hour rating
- On the label and the product's specifications
- Tankless capacity
- Maximum flow at a stated temperature rise, in the specifications
- Fixture flow
- Often printed on showerheads and faucets, in gallons per minute
Fit and connection checks
- Measure the height, width and door opening of the closet or garage space
- Leave room for seismic straps and service access
- Confirm the vent will suit a different size or design
- Confirm gas supply or electrical capacity if changing type or size significantly
- Ask whether the city requires anything for the new installation
Common questions
- Is a bigger tank always better?
- No. An oversized tank keeps more water hot than you use. Size for your busiest hour and the space you have.
- What is a first-hour rating?
- It is how much hot water a tank can deliver in its first hour of use starting full. It is a better guide to real-world capacity than gallons alone.
- Will a new heater fit where the old one was?
- Often, but not always. Insulation and design changes can make newer models larger. Measure the space and ask the plumber to confirm.
General information for Fullerton, not a quote or professional advice. In an emergency, call 911.