What Size of Water Heater Do I Need?

If you are confused as to what size water heater you need, then to answer that question you’ll need to carefully consider your hot water usage.

This quick and easy to follow guide will show you how to estimate your hot water usage so that you can then determine what size of water heater best suits your needs.

Simply just replacing your old water heater with one that looks like it will do the job is definitely not the way to choose a new water heater, you will most likely find out to your cost that it isn’t up to the job.

Tankless water heaters are sized/rated by the number of gallons per minute of hot water that they can supply and we’ll be looking at those after we’ve looked at the traditional tank water heaters.

Tank Water Heater

This is an easy one to estimate because you can get a pretty good idea of water usage by the number of people in the household.

Household SizeElectric Tank Size Gas Tank Size
1 to 2 people20 – 35 gallon20 – 35 gallon
2 to 4 people35 – 45 gallon35 – 45 gallon
3 to 5 people45 – 60 gallon40 – 50 gallon
5 or more people60 gallons and over50 gallons and over

Adding Up All That Water Usage

So let’s see if the above table is a good guide.

Let’s try an example and take a look at a typical first hour demand or peak hour demand in a 3 person household.

First hour demand or peak hour demand is usually first thing in the morning when the alarm clock goes off and everyone jumps out of bed (ok, maybe not jumps out of bed) and starts to get ready for the day ahead.

Water Usage In Peak Hour DemandEstimate of Water Used In GallonsUsage In 1 hourTotal Gallons Used
Shower12336
Shave21 2
Dishwashing Machine51 5
Washing Hands 21 2
Total45 Gallons

From the above example we get a total of 45 gallons of hot water used/needed in the first hour or peak hour demand.

If you now compare the total of 45 gallons with the first table estimate, you can see that it’s pretty much bang on for a 3 person household.

If you think you would be using more than the average estimated total then definitely go for the next tank size up.

Perhaps you could delay putting the dishwasher on until later in the day and that would save around 5 gallons of hot water during the first hour peak demand period, but most of us just usually load it up after breakfast and set it going.

What’s FHR?

FHR stands for first hour rating and this is a very important piece of product information to look out for when you are looking to buy a water heater tank.

All water heater tanks should have the FHR (first hour rating) information clearly visible for you to see when you are sizing up a water heater tank to buy.

Why Is FHR Important?

This information is extremely important because it tells you how much hot water the tank can produce in the first hour of peak demand. That’s why you need to know how much water you will be using in that first hour of peak demand.

Let’s get back to our example of a 3 person household that had a peak hour demand of 45 gallons, in this case, you need a water heater tank that has an FHR (first hour rating) of at least 45 gallons to ensure that you won’t run out of hot water in that first hour when demand is high.

Tankless Water Heater

Tankless water heaters are a great way of saving money when it comes to hot water usage. Heating only the water that is being called for at that time, tankless water heaters are ideal for small, medium, and large family households.

Tankless water heaters are rated in gallons per minute, they don’t store any water in a tank, (hence the name) they heat the water instantly as and when it is required.

To make sure that a tankless water heater is efficient you need to size the tankless water heater correctly, this will ensure that the system can cope with the hot water demand required of it.

Adding Up Hot Water Usage Per Minute

So we need to have an idea of how much hot water will be required at the busiest time of the day. This is usually first thing in the morning when we are getting ready for the day ahead, there may be, for example, a shower running as well as a dishwasher and/or sink faucet being used too.

Research has shown that the average American shower has a water flow rate of approximately 2.1 gallons per minute whereas a dishwasher uses around 1.5 gallons per minute depending upon which part of the washing cycle it is on, and a sink faucet can use on average 1 gallon per minute of hot water when used for hand washing or cleaning dishes.

This example gives us an estimated hot water use of 4.6 gallons per minute at a peak usage time, but this also depends on other factors too.

Anything Else To Take Into Consideration?

Yes, there is and this is also an important consideration when you are looking to size and buy a tankless water heater.

Ground temperature is an important factor to be aware of as this has a great influence on the tankless water heaters time it takes to heat the water.

If you live in a cold region or you have very cold winters then the ground temp will really chill the cold water supply that feeds the tankless water heater, which in turn can mean longer water heating up time.

So when you have determined the water usage and have an idea of ground temperature, then it’s just a case of selecting a powerful enough tankless water heater to cope with your requirements.



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