What Does First Hour Factor Mean When Discussing Water Heaters?

Two important terms are always discussed in deciding upon which water heater to use or buy. The first id energy factor which is a direct indicator of how two or more different heaters perform in respect to efficiency of fuel use. For example a higher energy factor number means the unit is more efficient. It does not mean that you will lower your fuel bill  necessarily. The second factor is called "First Hour Factor". In broad terms this is a way of deciding upon size or water usage requirements and whether a specified heater can do the job or not.

First Hour Factor Water Storage Heaters

This factor is an indication of the quantity of hot water that any domestic hot water heater can deliver during a period of peak usage. You've seen the impact of this when a couple of people have taken a bath within a few minutes of each other ... there is no more hot water for a 3rd bath. The reason was because of the fundamental design of the heater (probably a storage heater in this case) was such that it could not possibly supply the hot water required in such a short space of time (lets say 1 hour). The higher the first hour factor then the more hot water can be created within 1 hour.

Let's consider a storage tank hot water heater first. First hour rating means that starting with the tank full of hot water the number given for a particular unit is the volume of water for which the system can supply water at the right temperature for the first hour of use such as 3 5 minute showers, 1 washing machine load,   1 dishwasher load and so on.

First Hour Factor Water Tankless Heaters

Tankless water heaters often provide insufficient hot water under heavy load conditions so care must be taken to specify such a heater based upon peak water requirements when making a final selection.

For a tankless water heater first hour rating means the flow rate is the maximum flow rate of water than can be heated to the correct temperature. The impact of this factor will vary depending upon where you live. If you live in Orlando, Florida, where water coming into the heater is say 50 degrees compared to Winnipeg in Canada where water feeding the heater is say 35 degrees then if your final temperature requirement is 80 degrees in both cases you will need a bigger first hour factor unit for Winnipeg than for Orlando.

Go back to the basic water heating formula here to check this statement

Let's look at a couple of examples to make this clear ... first of all you need to decide what your probable peak usage of water is. To do this construct a simple table listing all hot water applications and what you think the hot water consumption is and then also note the probable maximum number of times in 1 hour you might use these applications.

Which First Hour Factor Should I Use?

The table below shows a suggested way to determine an average first hour factor but be careful since every house and home is different ... source California Energy Commission

In 2004 I did a survey of most US manufacturers' rating for first hour factors of their individual products and then created this table showing the maximum, minimum and calculated average values for the different types of fuel used for hot water heating in the home. You'll see from the table below that oil fired storage hot water heaters had the largest capacities. In assessing the values for tankless water heaters take note that the units are in gallons per minute.