What Size Water Heater Do I Need?

What size water heater do I need?

Consumers often ask “What size water heater do I need?”, when it comes to purchasing a new residential water heater. You want to be sure you have a water heater that will regularly meet your daily requirements. However, you don’t want to spend more than necessary on a water heater that is not the proper size. Therefore, we have put together a few tips on how to determine what size water heater is best for your home.

What Size Water Heater Do I Need?

Similar to sizing a heater or air condition for your home, sizing aims for a “sweet spot” of a system that’s large enough to meet demand. However, not so large that it is inefficient. Therefore, no professional plumber will recommend you to impulse buy your own water heater without further examination of your home’s water use.

The First Hour Rating

For a tank or heat pump water heater, the first hour rating is the number of gallons of hot water the heater can supply per hour, if it begins with a full tank of hot water. Our professional plumber at Mena Plumbing will look at this rating first, since it combines numerous other factors such as the following:

  • tank capacity
  • the energy source (either gas burners or electric heating elements)
  • how much heating power it has

Peak Hour Demand

The peak hour demand is the hour during the day when you and your family utilize the most hot water. It is also how much hot water demand there is during this time.

Our professional licensed plumber will work with you to determine calculations based on how many people are in your house. In addition, we will determine calculations depending on what time of day you use the most hot water. For most homes, peak hot water use is in the morning when people are showering. You can use this number to determine the best first hour rating. Therefore, if your peak hour demand is 40 gallons, the best match for a water heater would have a first hour rating of 38 to 42 gallons.

Maximum Temperature Rise at Given Flow Rate

If you want to install a tankless (“on-demand”) water heater, the calculations are different. This is because these water heaters do not store water but heat it up as it’s needed.

We will determine the flow rate and the temperature rise necessary for the whole house. This means determining the number of hot water appliances/taps that might be in use at one time, then adding up how many gallons per minute you use, which will determine the flow rate. You can determine the maximum temperature rise by subtracting the incoming water temperature (the temperature of the water from the municipal supply) from desired output temperature. Once we determine these numbers, we can accurately size a tankless water heater for your home.


We hope this answers your question, “What size water heater do I need?” Our professional licensed plumber in Bryan/College Station has experience handling water heater installations. Contact us with the link below for more information or to schedule your professional water heater installation today!

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