What Size Tankless Water Heater Do I Need?

what size tankless water heater do i need

Deciding to switch from a regular water heater to a tankless water heater is a big step, they do have a lot of benefits. So once you have decided that tankless water heaters are the right thing for you, you certainly need to think very deeply about a few things.

One of these things would be what size tankless water heater do you need. These are a big investment, so you certainly wouldn’t want to miss on the size, right?

So, let’s take a look and see what size of tankless water heater would be perfect for your family.

What Size Tankless Water Heater Do I Need?

Should I Choose Gas or Electric For My Household?

The first thing that you should know is that there are two main types of tankless water heaters. There are gas water heaters and electric water heaters.

Warming up the water

Normally, those gas powered tankless water heaters can heat up the water much faster and are more efficient and economical from those electric ones. However, they do have much higher initial cost because of far more complicated plumbing and installation procedure.

On the other hand, those electric ones are far less complicated to install, but they can’t heat up that great amount of water in short periods of time.

These can be enough for one or two people, but for more, we would strongly suggest going with gas powered ones.

Just to give you an insight, an electric powered tankless water heater can give 70 degrees heat rise to 2 gallons of water per minute, while a gas powered one can rise as much 5 gallons per minute.


When we are talking about size, we don’t mean only the amount of water a heater can heat at a given time. You also have to think about where you are going to place it.

While tankless water heaters are much smaller than those heaters that require a storage tank, these two main types also differ in size.

Electric powered water heaters are around 10 inches high and 7 inches wide, which allows them to be easily mounted on a wall.

On the other hand, those gas powered ones are a bit bigger with 30 inches in height and 20 inches in width.

All in all, those electric powered ones need much less space, while the gas powered tankless water heaters need proper spacing and place to vent the fumes.

If there is no place for proper vent pipes installation, your only choice would sadly be the electric powered water heater.

Other Important Considerations

The main thing for sizing your tankless water heater is the maximum rise of temperature that you will be needing at the specific flow rate.

So, the first thing that you will have to do determine the size of your „on demand“ water heater to be is to know the flow rate and rise of temperature. The size of your tankless water heater will vary drastically depending on where you will be using it – bathroom only, all rooms in the house, or something else.

It is important to say that you should never try to save more money on tankless water heaters by going with a size smaller and believing that it will do the trick just fine. You might end up with not enough water, or wrong temperature, so this is not something to play around with.

Step 1 – Determining the Number of Devices

As mentioned before, the maximum number of devices that will be attached to your tankless water heater can gravely influence the needed size.

Therefore, you will have to count the exact number of devices and determine their total flow rate. Now, simply add up the flow rates shown in gallons per minute (or GPM) and you will get the flow rate for your „on demand“ water heater.

Let’s look at an example:

  1. You are planning to use hot water in your kitchen, and the faucet has a flow rate of 1 gallon per minute.
  2. Now, you will also be using your bathroom shower that has a flow rate of about 3 gallons per minute.
  3. Once added up, that sums for 4 gallons per minute that your water heater will have to have (at least) for you to be able to function properly with it. And get enough hot water, of course.

If you believe that the flow rate of some of your devices is too high, you can simply install low-flow water fixtures on them. This will, naturally, decrease the total flow rate of your devices, and the necessary size of your „on demand“ water heater with it.

Even though we already suggested that you should not try to save some amount of money on smaller water heaters, if you still intend to do so, you can agree with your household members on not running multiple sources at the same time.

Step 2 – Define the temperature rise you need

To do this, you will simply have to subtract the temperature of incoming water from the temperature you would like your output water to have.

You can find out the temperature of incoming water by simply using a thermometer. This is the starting point, and it will be heated to what you want by your tankless water heater.

If you do not own a thermometer, it would be best if you just assumed that the starting point temperature is way lower than it would normally be. In this case, there would be no fear of undersizing your water heater.

The colder the water – the bigger the heater that you will need to install.

For example, you can take it that the temperature of incoming water is 50 degrees F. Of course if you live in very warm climate, the initial temperature of your water will be much higher than that.

In most of the cases, temperatures between 105 and 115 degrees F would be more than enough for normal functioning.

Now, let’s take it that the starting point was 50 degrees F. With that in mind, you will need your water heater to uplift and maintain the temperature of the incoming water by 60 degrees.

What’s the right size?

And finally, how to choose the right tankless water heater size?

Now, here is a little table to help you with the average flow rate and temperature that each of the devices requires:

Demand Type Flow Rate Temperature
Bathroom Sink 0.5 GPM 110°F
Standard Shower 2.5 GPM 104°F
Low-Flow Shower 1.5 GPM 104°F
Dishwasher 1.0 GPM 110°F
Washing Machine 1.5 GPM 120°F
Kitchen Sink 0.5 GPM 110°F
Laundry Sink 1.0 GPM 110°F

1. First, start with the temperature of the incoming water.

In our example, that temperature was 50 degrees F.

2. Next, let’s see what is the maximum flow rate during use. For example, let’s take it that:

  • One person is taking a shower, which is 2.5 GPM
  • The other person is doing laundry, and that is 1.5 GPM
  • And one more person is doing the dishes, which is 1.0 GPM

That all sums up for 5 gallons per minute.

3. Now, if we have the starting point of 50 degrees F and the highest temperature of 120 degrees F, that means that our heater would have to raise the temperature of water to 70 degrees at 5 gallons per minute.

Most gas powered water heaters can do that without a struggle, but they are also rated at a specific temperature rise over a maximum amount of GPM.

For example, a 200,000 BTU gas powered tankless heater can easily raise the water temperature 50 degrees F at a maximum of 7.4 GPM.

Additionally, every gallon per minute that you take from the flow lets your heater increase the water by additional 10 degrees.

In the situation mentioned above, a water heater of 200,000 BTU could easily heat 6.4 gallons per minute to 110 degrees F, or 5.4 gallons per minute to 120 degrees F.

In this case, a 200,000 BTU tankless water heater would just be too much.

Final Word

Switching from a regular water heater to a tankless one is a big decision which requires a lot of investment. Therefore, choosing the right one for you requires some time and thinking.

Today, we explained how to choose the right size for your tankless water heater to be.

The most important things to take into consideration when choosing are the flow rate, starting temperature, and the temperature that you want to get.

Hopefully, this article helped you choose the right size and you will enjoy hot water in no time.

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