How to Size A Tankless Water Heater | Our Buying Guide

If you are just starting your journey to buy a tankless water heater, you are likely asking yourself, “What size tankless water heater do I need?”

It is probably the most important question when wondering how to choose a tankless water heater.

Before that can be answered, it takes a change of mindset.

People are used to thinking in terms of capacity. Like, a hot water tank for 5 people will require at least a 60-gallon tank.

Since tankless water heaters supply endless hot water, you don’t have to worry about capacity, but instead, think in terms of water flow rate. Gallons Per Minute (GPM) is the key when it comes to sizing tankless water heaters.

The lowest price you can get for the highest GPM is easily the Takagi T-H3-DV-N that will give you a whopping 10 GPM. You may not need that big of a tankless water heater so make sure you read the whole article.

How many gallons per minute (GPM) do I need?

The flow rate of the number of fixtures running at the same time is your Gallons Per Minute (GPM). So, you don’t really have to think of how many gallons you use over the course of the day. Only the gallons used at the same time.

For example, let’s take the morning as that is when most people are taking showers or doing laundry.

Let’s say that you have a two bathroom house with 4 people. In the morning you have two showers running and somebody put in a load of laundry at the same time.

You have to figure out how many gallons each of those fixtures uses per minute and add it up.

How many GPM is a shower? How many GPM is a dishwasher? Or washing machine?

You can check your specification sheet on your manual for the fixtures. Or, you can take a look at this handy cheat sheet below to get an idea.

Take a week or so and write down the times that you have multiple fixtures running at the same time and which ones they were. At the end of the week, you will see some patterns and understand exactly when your peak water demand is and how many GPM you will need from your on demand water heater to cover that.

Tankless water heater sizing calculator

Before we get to how to use the calculator to size a tankless water heater, let’s talk about the factors that go into understanding what size inline water heater you need.

We already covered GPM.

Now you need to understand what temperature rise means.

Temperature rise is how you will figure out what the actual GPM that the tankless water heater is.

When you see the GPM rating of a tankless, it is the best case scenario. In certain circumstances, you won’t get that water flow and still have the water come out hot.

What happens when the GPM you need is higher than what the tankless water heater is capable of is you end up with endless lukewarm water.

This is why sizing a tankless water heater is so important.

To determine your temperature rise, you need to figure out the temperature of the water coming into your home.

Take a look at this groundwater temperature map to get general idea of what temperatures you can expect depending on where you live. If you need real time temperature data, then consult this interactive map from the USGS site.

As you can see, the temperatures vary wildly by location.

Now here is the fun part. In the sizing calculator below, enter the temperature that you expect your incoming water to be.

Then in the next field enter how hot you want your water setting to be for outgoing hot water.

Hit the button to calculate it and there is your water rise temperature.

Once you have this temperature rise number, you can look at the specification sheet of the tankless water heater you want to buy and see what the GPM is rated for based on that number.

How to determine how big of a tankless water heater I need?

Let’s recap.

What size tankless water heater depends on your GPM at peak hot water demand. And your temperature rise which is determined by your incoming water temperature subtracted from your hot water setting.

As an example, I will show you what came out for me. I live in Boston so my groundwater temperature is around 47°F.

I have small kids at home so I keep my water set around 105°F. This is a safe temperature and it also will keep my gas bill down. My temperature rise then is 58°.

Our peak water demand is a shower, washing machine and faucet running at the same time so I calculated my GPM as 6.

Since I live in Boston I should be looking at a gas tankless water heater since they work best in cold climates. I wrote a really detailed article reviewing the best gas tankless water heaters for different sized households that you should read by clicking here. 

So, I need to know what size tankless water heater is best for a family of 4.

Looks like the Rinnai RUC98iN from Amazon is going to be best to suit my needs. It is listed as being 9.8GPM, but since my factors are not the best case scenario, it drops down to 6.7 GPM. This will probably rise in the summer as my incoming groundwater temperature will also be a bit warmer.

If you live in a cold climate that is going to be a huge factor in which tankless water heater to go with. Read up on my full article on how to use a tankless water heater in a cold climate with 8 simple tips.

How to choose a tankless water heater

There are two kinds of tankless water heaters. Electric and gas. Each has its pros and cons, of course.

There are a couple of rules of thumb over which kind would be better.

I you live in a Northern, colder area, then you should definitely look into a tankless gas heater.

They are generally more powerful than their electric peers and can heat the water more quickly. This is especially true if you want a whole house tankless water heater for larger families.

Electric tankless water heaters are more geared to areas with warmer ground water so that they can maintain a higher GPM flow rate. They are also small and can be used as point of use water heaters. They are ideal for people in apartments, smaller homes or in a vacation home.

Once your needs are established it may become clear that a single tankless water heater will not get the job done.

In this case, look for an inline water heater that can be synced with another and buy two for the whole house.

Or, you can buy several smaller electric ones and use them around the house where they are needed as point of use water heaters.

Are tankless water heaters worth it?

After sizing a tankless water heater and evaluating your needs, it may become clear that a tankless water heater is not the way forward for you. They are not for everybody after all.

In the right circumstances, a tankless water heater is the best way to go. If you have heard negative reviews, then you probably are dealing with somebody who didn’t properly evaluate their needs and got one that didn’t offer the GPM necessary. If they had sized their tankless heater well, then they would have gotten the one with a higher GPM and more than likely would be very satisfied.

I hope that this article on how to size a tankless water heater was helpful and will put you onto the best one for your needs.

Any questions? Drop a comment below and I’ll do my best to help you out!