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What size Rheem water heater do you need?

Sizing is the technique that matches the capacity of the hot-water source to the needs of the homeowners.

  • For tank water heaters, the key criterion is hot water storage capacity
  • For tankless water heaters, the key criterion is hot water flow rate

Incoming water temperature is a critical consideration, which varies by region and season. That is, a water heater in the North - either tank or tankless - will need a higher BTU input in the winter than the summer to heat and deliver water to a given temperature.

Regardless of which type of water heater is used, you should start with a lifestyle audit of your client's typical usage:

  • How many people are showering and when? Is there a "shower rush hour" in the morning or night?
  • Do they have a deep soaking tub or whirlpool? What is its fill capacity in gallons? And how do they use their tub; e.g. do they take a long shower first and then get into a full tub?
  • When are major appliances in use? Are the dishwasher and washing machine needed at the same time family members are showering? Most Americans are accustomed to staggering hot water use, so it is atypical to find a home where multiple hot water appliances are needed at the same time.
  • How much hot water is needed to deliver the experience clients want in their bathroom remodel? For example, is there enough hot water to fill a deep soaking whirlpool or to operate a vertical spa-type shower for any length of time?


 Establish peak demand, measured in gallons per hour (gph). Then evaluate tank water heaters on the same gph basis to determine how many gallons of storage are needed to meet this demand.

While tankless water heaters do not run out of hot water, if not sized correctly, the flow rate of that water can be adversely impacted. The temperature of the shower will remain the same, but flow could slow to a trickle. So the first step in sizing tankless water heaters is to add up all the flow rates of showerheads, faucets and appliances that are likely to be in use at the same time.

Step two is to consider the incoming water temperature. When inlet water temperatures dip down into the 30s and 40s, larger BTU inputs will be needed. In certain high-volume applications, you may want to specify more than one tankless water heater unit, either installed separately or connected together to operate as a single tankless system. The Rheem EZ-Link technology will facilitate this applicati
on.