Running out of hot water in the middle of a shower or while doing the dishes is a major inconvenience. To insure there will be a large enough supply of hot water when needed, the hot water heater must be sized to meet household demand.
Sizing a hot-water heater is a technique that properly balances the capacity of the heater with the requirements of the household. Correctly sizing a hot water heater requires a few calculations in executing a lifestyle audit.
Demand
Determine the habits of the people living in the house. Start by figuring out how many people shower during the morning and/or evening rush hours. Also take into account if anyone uses a bathtub and when a dishwasher and washing machine are used.
Calculations
Calculate peak hot water demand in gallons by adding up all the uses of hot water during peak demand periods.
1. Add up all the people who bath during a typical peak period and multiply that by the number of gallons used per shower and determine the amount of water used to fill a bathtub, if applicable.
2. Add the amount of hot water used by the washing machine and dishwasher.
As an example, if there are three people showering during peak periods, multiply the number of showers by the number of gallons per minute flow rate of the shower heads. Take into account that federal regulation requires new shower-heads to restrict flow to 2 1/2 gallons per minute, so a 10-minute shower will use 25 gallons of water, of which approximately 70 percent will be hot water. If the dishwasher uses 8 gallons of hot water and the washing machine uses 20 gallons plus a bathtub that holds 30 will total 88 gallons total peak usage of hot water (3 X 25 = 75 X .70 = 52.5 + 8 + 20 = 80.5). As a rule, most households do not use all the appliances at the same time, so multiply the final calculation by .70 (80.5 X .70 = 56.35). So for this example a water heater with a 55- to 60-gallon capacity would be required.
Tank or Tankless
Keep in mind that when choosing a tank-type hot water heater, the main criterion is tank storage capacity, but for tankless, or “on demand,” water heaters the rate of water flow is what matters.