Unlike other areas of the plumbing trade, water heater sizing depends mainly on personal preference rather than hard and fast rules. The ideal water heater size is one that provides enough hot water for typical use while minimizing energy/fuel costs.
Water heater sizing is based mainly on the occupancy of a building or residence but should also take other factors into consideration; such as:
- Recovery rate - the amount of water that can be heated per hour. A typical electric residential water heater will recover about 15-20 gallons per hour. This varies by the energy source used, gas and oil fired water heaters generally have higher recovery rates than electric tanks. The water heater's spec sheet will tell you the recovery rate.
- Future family size - as a water heater generally lasts 10 years it may be better to oversize a young couple.
- Number of females - women on average spend 30% longer in the shower than men.
- Number of young children - as any parent knows, kids get every thing around them, including themselves dirty and the hot water usage increases accordingly.
- Specialty fixtures - large soaker tubs and whirlpool baths will require more hot water to fill than can be provided by small tanks.
- Intended building use - for example a library will have very little need for hot water while a Laundromat or restaurant will have excessive usage.
- Pipe size - if the hot water supply pipe is required to be larger than 3/4" a commercial water heater with a 1" or larger outlet will be required or smaller tanks connected in series.
Residential water heater sizing
The table below is a rough starting point when selecting the proper water heater size for you needs, remembering to take into account the above special factors.
2 people 30-40 gallon tank 3-4 people 40-60 gallon tank 5-6 people 60-80 gallon tank
(recommended minimum size for whirlpool tubs)
Commercial water heater sizing
Commercial water heaters and storage tanks are sized according to the building type and occupancy, using predetermined usage stats. A.O. Smith has prepared several charts to help selecting an appropriate commercial water heater for many types of buildings.