How big of a tankless hot water heater do I need?

Konrad showing of water heater

When shopping for tankless hot water heaters, whether to replace an existing tank-style water heater or if you are expanding your home, you should make sure that you purchase the correct size for your home and family. But how do you know what that perfect size is?

It’s the age old question – “How big of a tankless water heater DO I need?!” Okay, so maybe it’s not an age old question, but it is a question that can be answered.

What do we mean by size?

To begin we need to explain what we mean by the size of a tankless. A tankless water heater heats the water directly on demand – it doesn’t hold it in a storage tank for later use. Once the cold water is heated to the desired temperature, typically 120 degrees, it maintains that temperature indefinitely without you having to worry about the gallon capacity of the tank you have.

Because tankless water heaters don’t store water, it’s important to understand that the “size” we are talking about here is the water flow rate, or how much water can be heated at once. Water flow rate is measured in gallons per minute (GPM) that the water heater can heat, not the physical dimensions of the water heater. We should note, it doesn’t matter if you are looking at tankless gas water heater or tankless electric water heater, they all kind of work the same way, just the heating function varies.


Know your needs before you shop

There are generally two considerations in selecting the right “size” of tankless water heater for your home – temperature rise and flow rate.

The temperature rise is the difference between the incoming groundwater temperature – that is the temperature of the water coming out of the pipes in the ground that come into the home, and the desired temperature of the hot water we want (typically 110-120 degrees)

The greater the difference, the fewer fixtures that the tankless water heater will be able to supply. Fortunately in South Carolina, our groundwater temperature is pretty warm compared to the rest of the country. This means we have less temperature to add to our water so we can get more of it faster, since we have to add less heat to our water to get it to the temperature we want.

That lower temperature rise means that you can get away with a smaller GPM tankless heater as compared to homes in the midwest and northeast parts of the country.

To get the flow rate, you should add up the flow of all the plumbing fixtures in your home that you might run simultaneously. This will help you select the right size of tankless water heater.

Keep in mind, only the water fixtures that are connected to a hot water line should be considered. Toilets, bidets, hoses, and refrigerators with icemakers shouldn’t be counted.

Here is a basic breakdown of the GPM of each fixture – please note some newer fixtures can use less than these amounts and some older fixtures use much more!

If you do have older fixtures, upgrading them before you replace your water heater with a tankless unit can help you save water and possibly energy too.

  • Shower (per head) – 2 GPM
  • Bathroom Sink – 1GPM
  • Washing machine – 4 GPM
  • Dish Washer – 1.5 GPM
  • Kitchen Sink – 1.5 GPM

Let’s say you run a shower, a kitchen sink, a dishwasher and a washing machine at the same time. This would give you a total of 9 GPM which we would use to size our hot water heater.

Hire a professional if you need help!

There are a range of different water heater sizes, in GPM ratings, that are available. It’s important that you get a size that is right.

If the tankless water heater you select is too small, you won’t get enough hot water if multiple fixtures are running. If it’s too big, you’ll end up paying a lot more for something you can’t get the full use out of, though this can be okay if you plan to add on to your home later.

While looking at the temperature rise and adding up the water flow is a good way to get a rough idea of how big of a tankless hot water heater you need, it’s always a good idea to consult with the licensed and insured plumbers at Kay Plumbing to make sure that you get exactly the right tankless water heater for your home and family.


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