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?

Sizing Tank Water Heaters

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 Ruud EZ-Link™ technology will facilitate this application.

Hot Water Usage Audit Questionnaire

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  1. Baths: How many bathrooms are in the house?
  2. Showers: How many showers are in the house and how many showerheads, body sprays and side sprays are in each shower? How much water do they use? Standard showerheads have a flow rate of 2.5 gallons per minute, although new water-efficient showerheads have a lower flow rates. Most people are comfortable showering in water temperatures around 102°F to 106°F.
  3. Bathtubs: How many bathtubs and whirlpools are in the house? How many gallons are needed to fill each to capacity? Note: While small tubs are usually about 40 gallons, deep soaking tubs can hold up to 140 gallons. As with showering, remember, most people bathe at temperatures between 102°F to 106°F.
  4. Schedules: What is the typical bathing and bathroom use schedule in the home? How many occupants are likely to be bathing simultaneously?
  5. Other hot water appliances: Are any other hot water appliances in use at the same time? If so, these need to be calculated also, e.g., dishwasher, hot-water laundry, kitchen use, etc.
  6. Geography: Where is your home? Consider the winter inlet water temperatures in the area to make sure there’s sufficient hot water flow on the coldest days. The rule of thumb is:
    i.    40°F for the northern tier of states.
    ii.    50°F in most parts of the South.
    iii.    60°F year-round in Southern California, the Southwest and Gulf states.
  7. Do the math; select the right unit: Add up your peak demand in gallons per minute and see which size of tank water heater will satisfy this peak requirement at the very coldest time of the year; i.e., when the difference between the inlet and outlet water temperatures will range as high as 75°F if you live in the Northeast or Upper Midwest. For example, if a Minneapolis homeowner picks a system that will handle a Delta T of 75°F in the winter (45°F inlet to 120°F outlet) to meet the needs of a household that runs two showers simultaneously every weekday morning, this consumer need not worry about the summer, when the inlet temp should be 20°F to 25°F warmer.