What size tankless water heater do you need

Our rating of the best tankless water heaters on the market is useless if you don’t know EXACTLY what size of a tankless unit you need for your house. So let me help you with that.

I’ll explain how to choose the right size in 3 easy steps so you don’t overpay for excessive performance.

Step 1: Determine the combined flow rate (in GPM). How many gallons per minute (GPM) do I need for a tankless water heater?

Although it may sound too “professional” it’s very easy to do:

  1. Simply count the maximum number of fixtures and appliances that you want to run simultaneously. Make a list of them.
  2. Now find out what are their flow rates. That is how much hot water they use per minute.
  3. Add up all numbers to get the total amount of hot water per minute.

If you don’t know what are the actual flow rates for your shower or faucets, use the average values:

FixtureAverage flow rate
Lavatory faucet0.5-1 GPM
Kitchen sink1.5-2.2 GPM
Showerhead1-2 GPM
Tub faucets 4-7 GPM
Washing machine2 GPM
Dishwasher1.5 GPM

For example, you have 2 bathrooms in your house. And you want to be able to use both of them simultaneously plus a kitchen sink and a washing machine.

That would be: (2 * 2) + 2.2 + 2 = 8.2 GPM for the whole house.

Now you can choose either one big whole house tankless water heater (like Rinnai RUC98iN) or use a smaller one (like this) and buy additional units for showers or appliances.

Use low flow showerheads to reduce the total flow rate and save money!

If you use low flow showerheads, your hot water demand is going to be lower and you won’t need a huge and expensive tankless water heater. Modern low flow showerheads provide the same water pressure and give you the same shower experience as higher flow models.

Step 2: Determine the temperature rise in your region

Now that you calculated how much water you need to heat per minute, you need to determine how hot the tankless water heater needs to heat the water. In other words, you have to determine the temperature rise.

Temperature rise is the difference between the temperature of the incoming cold water and the outgoing hot water

It’s very important since the ACTUAL flow rate of a specific tankless water heater largely depends on the temperature rise. You can’t get 11 GPM flow rate with Rinnai RUR199iN in Boston in winter. But you surely can in Miami.

The groundwater temp varies in different states throughout the USA. Using our groundwater temperature map you can easily determine the temperature rise for your region (just hover over the map)

If you’re still confused, do this:

  1. Find your state on the map.
  2. Determine the groundwater temperature by the color. Or just hover over your region.
  3. Now subtract the groundwater temp from the temperature of outgoing hot water (it’s usually around 120°F) and you’ll get your number.

For example, if you live somewhere in Kansas, your average incoming cold water temperature is about 52°F. So for 120°F hot water your temperature rise will be 120 – 52 = 68°F.

Now you’re ready to go to the next step:

Step 3: Put it all together

Ok, at this point you know your hot water demand and the required temperature rise.

Your next move is to go and choose a tankless water heater that meets the requirements.

Remember, the maximum flow rate that manufacturers mention in the description is not what you’ll get at your place. The lower the incoming water temperature, the lower flow rate a tankless unit would be able to provide.

Also, keep in mind that it’s always better to oversize the unit than buy the one that can’t supply enough hot water.

You can check the actual flow rate for your temperature rise in the specifications of a model or in the review section on our main page.

What size tankless water heater do you need to replace a 50-gallon water heater?

As it’s been said, it all depends on your hot water demand and temperature rise. Personally I would recommend buying Rinnai RUR160iN if it fits your budget.

If you’d like to look for a cheaper alternative, consider Rinnai V-Series. It shows great performance and it can supply a two-bathroom house with endless hot water.

What size tankless water heater do you need to replace an 80-gallon water heater?

If you have a large house and you want to buy a big whole house tankless water heater, go for Rinnai RUR199. It’s probably the most powerful tankless water heater on the market. And it’s able to supply a large 3 bathroom house even if you are planning to use them all at the same time plus a washing machine.

If this model is too expensive for you, look at Rinnai RUR160 (there are both natural gas and propane options). It’s cheaper but still very powerful.

And if that unit also doesn’t fit into your budget, give a look at Takagi model that’s capable of providing up to 10 gallons of hot water per minute. It’s much cheaper but it also works worse than Rinnai in cold climates.

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