How To Size Water Heaters | Choosing A New Water Heater

Don’t wait until your water heater springs a leak and floods your basement!

Water heaters typically last 8 to 12 years. If your water heater is over 10 years old or if you are upgrading a bathroom, now may be the time to upgrade.

But before you go out and purchase a new water heater, learn how to size it correctly in order to save energy while meeting your household’s water needs.

Then, speak with a qualified contractor to safely and correctly install your new water heater.

How to Size Water Heaters

In order to properly size your storage water heater, you should estimate your household’s peak hour demand. This is the amount of hot water used during the peak hour of use.

  1. The average household member uses around 12 gallons of hot water when taking a shower.
  2. Multiply 12 times the number of people who take a shower in an hour’s timespan. For many households, this is around 3 people. In the case of 3 showers during the peak hour demand, this household would need a water heater with a first hour rating of at least 36 gallons.

The peak hour demand for your home should be a little lower than the water heater’s first hour rating (FHR).

First hour rating measures the amount of hot water the heater can supply per hour (starting with a full tank of hot water). It’s different from the amount of water the tank can hold. The FHR is determined by the size of the tank, the type of fuel, and the size of the burner or element. Keep in mind that gas and oil-powered units usually have higher FHR ratings than electric models.

If your peak hour demand is 36 gallons, look for a water heater with a FHR around 40 gallons.

When shopping for a new storage water heater, find the EnergyGuide Label for information on capacity (first hour rating), estimated yearly operating cost, estimated yearly electricity consumption, and other important information.

How to Size Tankless Water Heaters

Tankless, or on-demand water heaters, require a different method for sizing. It’s also a bit more complicated, so speak with a professional first before deciding on the right unit for your home.

  1. Add up the number of devices you want to simultaneously run and find out their flow rates (gallons per minute).
  2. If you have a shower with a flow rate of 2.9 GPM and want to be able to run the kitchen sink with a glow rate of 1.2 GPM, you will need a tankless water heater with a flow rate of at least 4.1 GPM. You can reduce the flow rates of your plumbing fixtures with low-flow models.
  3. The next step is to determine temperature rise. Simply subtract the temperature of the water that enters the unit from the set output temperature of the unit (usually around 100-120 degrees). If you don’t know the incoming water temperature, it’s generally around 50 degrees.
  4. For this example, you will need a tankless water heater that can produce a temperature rise of around 50-70 degrees at 4.1 GPM. Always round a bit up so you can be sure your new water heater has the ability to meet your home’s demands.

Energy-efficiency is the main reason people become interested in switching to tankless. As the green movement gains momentum, so does the popularity of tankless water heaters. Learn more about tankless water heater options as a potential alternative to traditional tanks.

Professional Water Heater Installation

Energy Efficiency Incentives and Rebates: Certified water heaters qualify for federal tax credits and incentives from local utility companies. Visit,, and speak with your utility provider and water heater installer for more information on rebates and incentives.

Installation Considerations: Despite the common belief that replacing a water heater simply involves replacing the old unit with the new, there are many other factors to take into consideration:

  • Building Codes
  • Water Heater Mount
  • Venting System
  • Drain Pan
  • Supply Pipes

Speak with a professional about the total cost of the water heater installation before any work begins. Don’t forget to factor in warranties and the reputation of the manufacturer and installation company. A reputable company will go over all of the costs with you and provide to-the-cent upfront pricing before any work commences.

Service Champions is known for trustworthy, on-time water heating service throughout the East Bay, South Bay, and Sacramento areas.

If you have any questions or concerns, don’t hesitate to contact us.

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