Tankless Water Heater Sizing

If you are in the market for a new tankless water heater, a few essential considerations must cross your mind. You must ask yourself how big of a tankless water heater do I need? Another way to put the question is, how many GPMs of hot water do I need? We'll cover all that so you know what to look for when buying a new unit. When somebody talks about the size of a tankless water heater, they don't usually mean dimensions. What they are probably asking is, what is the BTU output?

What Is a BTU?

shower control valve depicting tankless water heater usage

A British thermal unit is a measurement of how much heat it requires to raise the temperature of a pound of water by just one degree. This is used as a standard for tankless water heater sizing and AC units, and other machines.

How Many BTUs Do I Need?

That depends on a few different factors. Instead of BTUs, it's easier to think about your usage in terms of heated gallons of water per minute. A traditional water heater can guarantee a minimum temperature (120 degrees) until the tank is empty.

Tankless water heaters are limited in how many gallons they can heat per minute. There are various factors here, but it primarily boils down to the inlet water temperature how many BTUs. Let's look at some approximate numbers:

  • 160,000 BTU's yields about +40 degrees at 8 GPM 
  • Or, about +75 degrees at 4.2 GPM
  • 199,000 BTU's yields about +40 degrees at 6 GPM
  • Or, about +75 degrees at 5.2 GPM

Look at each manufacturer's information, and you will see the models vary from the estimates above. Those estimates are just a shortcut. If a 40-degree temperature rise suits you, then you can take the GPM you need and then multiply it by 20,000 for an approximation of how many BTUs you need, and then you'll know the range you should be looking at. That begs the question:

How Many GPM's of Hot Water Do I Use?

tankless water heater sizing and usage capacity

Think about how many people live in your house and all the faucets and appliances. Lavatory and kitchen faucets use about 1.5-2.2 gallons per minute. Your showerhead might use 2 to 3 gallons per minute, and be sure to factor in body sprays if you have them. The tub faucet probably produces 3-4 gallons per minute. Then you have the dishwasher and the washing machine, both about 2-3 GPM. 

So How Big of a Water Heater Do I Need?

Add up the GPM of all your rooms with faucets or appliances to get a total. Once you have that total, start working backward. Consider your habits and what will be used simultaneously. As an example, for a home with two bathrooms needing a 40-degree rise in temperature, the math would need to look something like this:

(2 showers x 2 GPM) + (1 kitchen faucet x 2.2 GPM) + (1 washer x 2 GPM) + (1 dishwasher x 2 GPM) = 10.2 GPM

Now, buying a tankless water heater that heats 10.2 GPM would likely be overkill for this homeowner. The odds that they're going to need to run all those faucets and appliances simultaneously are not very good. It is pretty reasonable to assume there will be scenarios where they'll have two people showering while one person cooks, and that would require a minimum of 6.2 GPM in this household. This hypothetical homeowner should probably go with a unit that has an output of about 140,000 BTUs.


Now that you have a better idea about tankless water heater sizing and what you're looking for, you should shop around and look at tankless units here at PlumbersStock. We carry quality brands, like Takagi, Bradford White, and Noritz. If you have any questions remaining, please do not hesitate to contact us.

Related resources:
Do Tankless Water Heaters Save Money?
How Does Tankless Technology Work?
On-Demand vs. Tank Water Heater (Infographic)
How Long Do Tankless Water Heaters Last?
Tankless Water Heater Venting

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