How To Determine What Size Water Heater Is Needed For A Home

sizing a water heaterInstalling the right size of water heater for your needs ensures you’ll always have plenty of hot water without wasting energy. Sizing a new water heater isn’t particularly complicated, but it does take a little calculation.

Sizing a Storage Tank Model

For a conventional tank water heater, sizing is relatively simple. Start by determining which hour of the day your household uses the most water. Calculate how much water you’ll be using during this hour. This is your peak hour demand. If your use includes two showers at 10 gallons of water each and 4 gallons for handwashing dishes, your peak hour demand is 24 gallons. Look for a new water heater with a first hour rating that’s within a gallon or two of your peak hour demand. As a general rule for sizing a new water heater, a 40- to 50-gallon tank is enough for a household of 3 or 4 people.

Sizing an On-Demand Model

To size an on-demand (tankless) water heater, you’ll need to determine both the flow rate and temperature rise that will meet your needs. Start by writing down the number of hot water applications (faucets, appliances, etc.) you expect to use at any one time. Then, add together their flow rates in gallons per minute (gpm).

If your water heater will need to simultaneously supply a shower with a 2.75 gpm flow rate and a faucet with a 1.50 gpm flow rate, you’ll need a system with a flow rate of at least 4.25 gpm.

To determine your required temperature rise, subtract the incoming ground water temperature from your desired output temperature. Around the Los Angeles area, it’s safe to assume your incoming water temperature is around 60 degrees year round. Typically, you’ll want an output temperature of 120 degrees, so that’s a 60 degree temperature rise.

Sizing a new water heater is important not just for your comfort but also for your energy savings, so if you’re unsure what size is right for you, consult a plumber.