Water Heater First-Hour Rating Explained

 

Understanding everything about residential water heaters can be confusing. You should be familiar with many technical terms to fully understand the typical water heater. One unfamiliar topic is the water heater’s first-hour rating.

 

The first-hour rating of your water heater is essential information. All brands of water heaters will give you this information, usually shown on the rating card and or label attached to your water heater. This information concerns the hot water your water heater can produce within the first hour. 

 

If you already own a water heater, then all the more reason you should know about this rating. If not, then you should learn it. Regardless of whether it’s known, this article will explain this rating. 

 

What is the first-hour rating of a water heater?

 

By definition, the first-hour rating means the capability of your water heater to deliver fully heated water in the first hour. It is usually influenced by your water heater’s total capacity and recovery rate. When you buy a water heater, you will see a first-hour rating label, as mandated by the government.

 

To determine a water heater’s first-hour rating, multiply the tank’s total capacity by 0.70 (70%) and add this number to the heater’s recovery rate. The total will be the first-hour rating, measured by gallons per hour. Why use 70% percent for? 70% is the hot water the heater can deliver at peak hours.

 

Why is only 70% of a water heater's total capacity used when calculating the first-hour rating?  When you demand hot water by turning on a faucet or shower, new cold water will enter the tank, usually from the bottom, to replace the hot water used. This will cause the overall water temperature inside the tank to drop, as it’s being pushed out by the colder water that mixes in; hot water is removed from the top of the tank while cold water is replenished and ready to be heated by the operation of the water heater.

 

As you use more hot water, so does the amount of new cold water that enters the tank; this action will gradually relax the rest of the water in the tank. Once you’ve gone through 70% of your heater’s total hot water capacity, the remaining amount will likely have gone cold.

 

However, a typical water heater is set to turn on and heat water as the hot water leaves the tank and the cold water comes in. This is because the water heater’s thermostat can sense the loss of heat based on your set temperature setting. Due to this intermittent heating, you can’t expect all 70% hot water to be the desired temperature.

 

Most residential water heaters will allow at least a 20-degree drop in temperature from your setting. If you set your tank to heat water at 125 degrees, expect hot water temperatures ranging from 105 to 125 degrees, within the 70% amount.

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How to calculate the first-hour rating

 

The formula for getting the first-hour rating of your unit is as follows:

The tank capacity in gallons X 0.70 (rule of 70%) + recovery rate = first-hour rating in gallons per hour.

 

A 50-gallon water heater and a recovery rate of 40 gallons per hour (GPH). Calculate the formula: 50 gallons X 0.70 + 40 gallons per hour (GPH) = 75 (GPH) first-hour rating. You can get 75 gallons of heated water in the first hour.

 

This information will give you a good estimate of your daily hot water usage. You can also use it as a reference when buying a new heater for your family.