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What Size Tank Water Heater Do I Need? A New York City Plumber Answers

April 15, 2021


This is a question we often get from NYC area homeowners who are in the market for a new tank water heater. While we can’t provide an accurate answer without an in-home assessment, we can give you the information you need to better understand your tank water heater sizing needs.

The best way to size your tank water heater is based on your peak hour demand and the water heater’s FHR (first hour rating).

We know you might not be familiar with those terms so we’ll explain in more detail below.


Need a professional to perform an in-home estimate to accurately size your new water heater? Ranshaw can help. For 50+ years, we’ve provided professional, long-lasting water heater installations throughout the Queens and NYC area.

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Peak hour demand

If you’re installing a tank water heater in your home, you want to base your water heater size on your “peak hour demand.”

Peak hour demand refers to the amount of hot water (in gallons) your household uses in its busiest hot-water-using hour.

For example, if you know that your family uses the most hot water from 6am to 7am (when everyone in your home is showering), then you’ll want to calculate the maximum gallons of hot water your family might need during that hour.

To help you determine your peak hour demand, we’ve provided the average gallons of hot water per common hot water activity below.

ActivityAvg gallons of hot water used
Shower20 gallons
Hand washing1 gallon
Shaving2 gallons
Running dishwasher4 gallons
Running laundry14 gallons

Use the chart above to add up the gallons of hot water that you assume your household might ever use in an hour. This is your peak hour demand.

For example, if your household takes 2 showers, runs the dishwasher and runs the washing machine all at once and that is the most amount of hot water your household uses in an hour, your total peak hour demand is:

  • Shower: 20 gallons of water x 2 = 40
  • Dishwasher: 4 gallons of water x 1 = 4
  • Washing machine: 14 gallons of water x 1 = 14
  • Total peak hour demand: 58 gallons of water

Using this total peak hour demand, you’ll want to install a tank water heater that has a capacity of at least 58 gallons of water. In general, most residential tank water heaters can hold 30-80 gallons of water

First hour rating

Make sure that you choose a tank water heater that has a “first hour rating” of the same or slightly higher than your peak hour demand.

A water heater’s first hour rating (FHR) is the amount of hot water that the water heater can produce in a single hour. You can find this information on the manufacturer's site.

To ensure comfort, we recommend choosing a water heater that has an FHR of at least 1-2 gallons more than your peak hour demand.

Using the above example for peak hour demand, you’ll want to find a tank water heater that has an FHR of 59-60 gallons of water.

Have more water heater sizing questions? Ask Ranshaw

Our plumbers have the experience and skills to accurately size your new water heater.

We’ll inspect your current tank water heater system and chat with you about your hot water needs to select the water heater size that will keep you comfortable and your water heating bills low.

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