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First Hour Rating | Water Heater Pros | Sacramento, Ca

First Hour Rating

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Water Heater First-Hour Rating Explained


Understanding everything about residential water heaters can be a confusing mess. There are lots of technical terms you should be familiar with, in order to fully understand the typical water heater. One of those unfamiliar topics is the water heater’s first-hour rating.


The first hour rating of your water heater is important 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 is about the amount of 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 if it’s known to you or not, this article will explain what this rating is all about. 


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 full capacity and recovery rate. When you buy a water heater, Attached you will see a first hour rating label, as mandated by the government.


To find out a water heater’s first-hour rating, you need to multiply the tank’s full 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 amount of hot water the heater is capable of delivering at peak hour.


Why is only 70% of a water heater full capacity used when we calculate the total 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 amount of 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 ready to be heater by the operation of the water heater.


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


However, a typical water heater is set to turn on and heat up water as the hot water leaves the tank, and the cold one comes in. This is because the 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% of hot water to be the same temperature that you desired.


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

How to calculate the first-hour rating


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

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 as shown; 50 gallons X 0.70 + 40 gallons per hour (GPH) = 75 (GPH) first-hour rating. This means that you can get 75 gallons of heated water in the first hour of use.


With this information, you will get a nice estimate on your daily hot water usage. You can also use this reference when buying a new heater for your family.



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