What size Tankless Water Heater do I need?

Three Factors to Consider When Choosing a Tankless Water Heater

If you’ve fixed your mind on purchasing the more modern alternative to a tank-based water heating system – the tankless water heating system or unit – that’s fantastic. You’ll not only end up saving money, but also a lot of space.

You know that a tankless water heater only uses energy when it is absolutely necessary. It won’t unleash any gas or fire up any heating elements until you turn on your hot water tap. This not only saves you money on hot water, but also money on the gas/electricity required to heat that water.

Although, this is all well and good – but what about the size?

I mean, if you’ve gone to a store or looked around online, you’ve likely seen literally hundreds of thousands of options. Every size claims to be the best, and no website will provide you with the real information you’re looking for.

If you’re an average consumer, you simply won’t be told. You’ll always be pushed to by the biggest, most expensive tankless water heating system or unit, no matter your actual needs. I suppose this is understandable. The most powerful heating units do tend to satisfy consumers most, but you won’t always need the biggest and the best. Especially if you don’t live with many people.

The size that your home requires depends upon many factors.

For example, there’s your timetabling; when people shower; when people wash dishes; what time the car is cleaned, how often people was their hands, etc.

It also depends on the actual number of people in your home, how many bathrooms you have, how often you shower, and a myriad of other things.

With all this said, you’re likely going to struggle to select the correct heater without the correct guidance. For this reason, we’ve compiled this simple guide, to help you decide what tankless water heater size you and your household requires.

This section will cover all the factors you need to consider, and all the questions you need to ask yourself to help you run through the options.

1. How Big is Your House?

This factor will determine many, many things. If you have a small home, you likely won’t need the biggest tankless water heating system or unit on the market.

Always consider the size of your home. This will prevent you from getting sold something you really don’t need. If the salesman at the store doesn’t request this information, you can be sure you’re about to get a bad deal.

The size of your home matters because from this we can estimate the amount of water you use, and the amount of water you’re going to need to heat. (The next section covers this in greater detail.)

With a traditional, tank-based water heating solution or system or unit, you’ll find that once you’re out of hot water, you’re out. And you’re going to have to wait a long time for the whole tank to re-fill, and then heat up.

With a tankless water heater, you won’t be having that problem at all. You can heat as much or as little water as you like, and only pay for what you use.

This is what makes these machines so much more superior to tank-based alternatives. Their efficiency means that running costs are lower. Your monthly fuel bills will decrease and your bank balance will be healthier.

With a small home, you won’t be needing a large water heater. In fact, if your home is very small, such as a studio flat or the like, you might only need a tiny, one-person water heater. This means that you won’t at all need to bother with those huge ones in the store, as a small one should meet your needs. It’ll be fine for showering and washing the dishes.

Therefore, if you’ve got a small home, shoot for a small tankless water heating system or unit.

2. Calculating the Rate of Flow of Water

Do you know what ‘rate of flow of water’ means? And if so, do you know your rate of flow of water in your household?

For our less mathematical readers, we’ll plonk some simple math in here to explain the rate of flow of water.

The rate of flow of water is a simple concept underlining the average volume of water that flows from any given open valve (a faucet or shower for example) during any stated time period.

So, if you open your tap, and leave it open for exactly 1 hour, and 500 gallons of water pour out of that tap within that one hour, your rate of flow of water is 500 gallons per hour. It’s almost like the speed in your car, but just with water and volumes instead of distance.

But why does the rate of flow of water in your household affect what size tankless water heating system or unit you need to buy?

Well, each tankless water heating system or unit has a specific limit of water that it can flow, and this is usually given in Gallons per Minute (GPM).

If your household only has one tankless water unit as it’s water heating system, and uses more gallons of hot water per minute than the tankless water heater can output, you will find that you’ll simply get cold water.

This happens because the output capacity of your heater has been exceeded by demand, and the heater can no longer keep up.

For this reason, you should always know your rate of flow of water before reaching any final decisions on the size of the water heater you want to use.

But guessing won’t do the job, you’ll have to actually measure it out. We’re not just talking about the rate of flow of water of one tap or of one shower, or even just one dishwasher. We’re talking about the total usage of water in your entire household, if all of your outlets are turned on at once.

However, you won’t usually find yourself actually using every single outlet at once, so take this as a sort of “what we need at most” indicator.

How to Do a Simple Gallons per Minute Calculation

This can be measured by going into your bathroom or your kitchen with a stopwatch, either traditional or on your mobile telephone, and with a large measuring bucket, turning on the water for that specific outlet for exactly a minute.

If, from a bathroom tap, you find that 1.6 gallons of water have flown out in that one minute, then one of your taps has a rate of flow of 1.6 GPM (Gallons Per Minute).

This can be used as an estimate for all of your taps. Similarly, you can repeat this measuring process with your shower.

If you find that your measuring bucket overflows during the minute due to a high rate of flow, you can simply time how much water comes out in 15 seconds, and then multiply that by four to find out how much water would come out in a whole minute, giving you a measurement of Gallons Per Minute.

Once these measurements are conducted, find the sum of all of the rates of flows of every outlet in your household.

For example, and a very simplified example, if your household only has two taps, each with a rate of flow of 1.5 Gallons Per Minute Each, you would find the sum of these two, and your maximum rate of flow at any given time in your household would be as follows:

1.5GPM + 1.5GPM = 3GPM.

Therefore, you would need (at most) a 3GPM water heater. Although remember, this is a simplified example, and it’s very rare for anybody ever to have used all of their water outlets at home at the same time, especially all hot.

You can use these measurements as an indication of which size tankless water heater you should shoot for.

TIP: A low rate of flow (below 10GPM) indicates you can probably just go for the smallest, cheapest option, provided the build quality looks suitable.

If you’re still not sure however, call up a local plumber or the people that constructed your home and inquire about the total rate of flow in your home when all the outlets are open.

3. Once you have identified the rate of flow, you must now identify how much heating you actually want your tankless water heating system or unit to do.

When you’ve finally got your rate of flow in your home, calculate how much heating the heater will have to do. Take your desired water temperature, and subtract from this the temperature of the water going into your home through the mains.

This is the temperature change required. If this value is very high, you may need to invest in a greater, more bulky tankless water heating system or unit, as otherwise you may struggle to get the temperature you truly want.

Final Words

It entirely depends on you and your household. If you live with many, go big. If not, go small.

About the author

Steve Bates

Hi, I'm Steve and I currently reside in Chicago, Illinois. Although for most of the last 20 years I was a partner in a HVAC company down in Austin, Texas.

I understand the frustration involved with dealing with water heater repairs and replacing or upgrading to a new, more efficient model.

My aim is offer some basic advice on fixing home hot water systems. And if a repair isn't possible, then to make the process of selecting a new water heater – be it gas or electric - as simple as possible.

So that you can save money and have a safe and reliable hot water system in your home for many years to come.