Water Heater Sizing


Trying to figure out how large of a water heater you will need?
Want to make sure that two people can take showers in two different areas without a total wipeout of the hot water supply?
Tired of waiting half an hour to run a bath after the dishwasher has run?

You need to get a water heater that is large enough for your family and home. Here is a brief guide to help you pick the right size for your residence:

Traditional Tank Heaters

No matter what size you buy, don’t forget that gas heaters bounce back more quickly than electric heaters after someone has used a lot of hot water. That’s why the numbers below will differ a bit when sizing for gas or electric heaters. That said, a heater that is large enough for your needs should greatly diminish the waiting time for hot water as the heater recovers.

Here is a quick set of numbers to help you pick a heater large enough for your clan:

  • 1-2 family members = 30-gallon tank
  • 2-3 family members = 40-gallon tank
  • 3-4 members = 50-gallon electric tank OR 40-gallon gas or propane tank
  • 5+ members = 80-gallon electric tank OR 50+-gallon gas/propane tank

Tankless Heaters


Because tankless heaters come in different models based on the maximum temperature rise possible at an established flow rate, you first need to figure out how much of a flow rate that you will need and how great of a temperature rise.
Here is a smart way to do that:

  • List the number of devices that will need hot water at one time and add up the expected flow rate for these devices (gallons per minute).
    For instance, a faucet (.75 gallons/minute) + a shower head (2.5 gallons/minute) would necessitate a tankless heater that can deliver at least 3.25 gallons/minute of heat.
  • Now, figure out the temperature rise that you want. Subtract the temperature of the incoming water from the temperature you want.
    The average incoming water arrives at 50°F, and most people like water at 120°F when hot, minimal. Thus, you will need a tankless heater that can deliver a temperature rise of 70°F. Dishwashers sometimes require temperatures of 140°F, so check your appliances and make your proper calculation.

Now, you are ready to shop.
A typical gas tankless heater can give a 70°F temperature rise at a flow rate of 5 gallons/minute. Electric tankless heaters average about 2 gallons/minute at that rise rate.

Solar Heaters

You need to determine your total collector area and storage volume. A good contractor will help you to figure out the proper size for your solar heating system. Most recommend about 20 square feet of collector area per two family members, roughly, but this depends on where you live and how much sun your region gets.

Tank sizes are similar to those used for gas and electric heaters.

Heat Pump Water Heaters

You need to figure out your peak hour demand and then find a heater that has a “first hour” rating at about the same mark. This rating reflects how many gallons of hot water that the heater can deliver in its first hour of use as it empties. Heat pump heaters have this number listed on their labels and online.