What Size Tankless Water Heater Do I Need?

Tankless water heaters are about heating your home’s water supply on demand. They’ll save you money on utilities (both on energy and water bills) and take up less space than traditional storage tank water heaters.

When it comes to tankless models, though:

You really need to get the sizing right for your home.

Storage tank water heaters store hot water for later use, so most households can just think about overall gallon capacity and go from there.

Tankless water heaters are more specific to gallon per minute flow rates, and it’s worth doing the math before making a purchase–that is, if you want to enjoy the full benefits of running one of these systems.

This article will tell you what to consider when sizing your tankless water heater, how to calculate your required water flow rate, and some of the differences between gas and electric tankless heaters on this question.

Sizing in 5 Simple Steps

Household size and how many devices do you plan on running?

The first thing to consider is how many people is your tankless water heater going to be servicing?

And will it be operating locally (say, just in the bathroom) or throughout the entire house. Many modern tankless heaters are capable of heating the water for your entire household, but the smaller sizings just won’t be able to cut it.

For storage tank heaters, the breakdown is a little easier since you can just take the total gallon capacity for each day’s usage. Tankless water heaters don’t store hot water though–they rely on heating the water as it flows into the mechanism, and putting out hot water rapidly on demand. This means we need a different way of calculating for tankless water heaters…

Adding the total flow rates

The way to do this is by taking the total flow rates of all hot water faucets/devices you plan on using at any single time. For example, if you plan to service the entire house with your tankless heater, you can probably envision a time where you’ll need two showers running at the same time.

Let’s split the difference and call the standard shower flow rate a steady 1.5 gallons per minute. That means your tankless heater needs to be able to handle at least 3 gallons per minute in order to meet your household needs.

Add to that someone cleaning vegetables in the kitchen sink, a running dishwasher or maybe a cycle of the laundry machine, and you can see that the total flow rate can add up pretty quickly.

Below I’ve listed some typical flow rates as household guidelines, though be aware that these figures can vary from home to home and within each appliance.

Fixture/Appliance Flow rates

Typical Flow Rates

Bathroom Faucet Flow Rate – 0.5 – 1.5 GPM

Kitchen Faucet Flow Rate- 3.0 – 7.0 GPM

Shower Flow Rate – 1.0 – 2.0 GPM

Dishwasher Flow Rate – 1.0 – 2.5 GPM

Clothes Washer Flow Rate- 1.5 – 3.0 GPM

Calculate the necessary temperature rise

Now that you know how much water you need to heat, the next step is to figure out how much you need to heat it.

This is called the necessary temperature rise, and it’s the difference between the temperature of the water coming into your tankless heater and the temperature of water coming out:

Temp. of Shower (104F) – Temp. of incoming water (40F) = Temperature Rise (64F)

For the above example, we get a figure of 64 degrees Farenheit, which means your water heater needs to be able to heat at least 1.5 gallons per minute, and each gallon by 64 degrees. We will speak more about the different capacities of gas versus electric models below, but it’s important to first know the likely incoming water temperature in your area.

The picture below gives a basic guide to the average groundwater temperatures in the US. You can see that these numbers vary considerably, with the base temperatures of some states being as high as 75 degrees, while others go as low as 35. That makes a big difference to the load on your tankless water heater and how much it needs to do to get your water up to the necessary temperature.

Be aware of these groundwater figures, but it’s always a good idea to shoot lower than what’s listed. If you’re not sure of the incoming water temperature in your area, it’s a good idea to assume 40 degrees as the baseline. This will ensure you don’t underestimate the necessary temperature rise.

Consider Peak Time Usage

My last tip is to make room in your plans for the water-apocalypse.

For 95% of the day, you might only be running the occasional tap, flush a toilet or run the dishwasher. Any tankless water heater can handle each of these tasks individually. It’s the peak hour traffic that you need to plan for. When that day comes (you’re preparing for a wedding, having guests over for dinner, etc.) you don’t want to run out of hot water.

By planning for your maximum possible peak hour traffic, you’ll ensure that disaster never strikes. Besides, by shooting high on this point, your tankless water heater will run more comfortably on regular days that see lighter loads.

Gas versus Electric tankless water heaters capacity

Generally speaking, to heat your water a full 70 degrees Farenheit:

  • Gas tankless heater can handle 5 gpm; and
  • Electric tankless heaters handle 2gpm.

Gallons per minute

We use the 70 degree measurement because this is roughly what it will take to raise your temperature from the standard 40 to the steamy hot dishwasher and kitchen sink status of 110 degrees.

As you can see, gas tanks are still significantly more powerful in this area, although it does vary by manufacturer and model. The specifications should always be listed on your product, so remember to keep an eye out for temperature rise and gallons per minute figures. If you’re still split between the two, read this article on how to pick between gas and electric tankless heaters.

Example Case

Just as an exercise, let’s run through a good peak hour usage case.

It’s Friday night. Your daughter is preparing to go out for a party, while you and your spouse are preparing to have guests over for dinner. Between 8-9pm, you need:

  • 2 showers running (3gpm);
  • One bathroom sink faucet for shaving (0.5-1.5gpm);
  • Downstairs bathroom sink for brushing teeth (0.5-1.5); and
  • The kitchen sink to prepare food (3-7gpm).

If we just take a raw total, you’re looking at somewhere between 7 and 13 gallons per minute for this chaotic night.

That’s a large load for any single tankless water to handle, especially if you happen to live in a cooler climate and need a larger temperature rise.

Assuming you can coordinate to not use the sink during shower times, you can get this figure down below 7gpm, but you can see how peak time usage will test any tankless heater. Even for your standard gas tankless heater, it’s a large load.

This is why keeping your tankless heater to a specific zone in larger households can be useful.

It may also simply mean some planning ahead of time: if you both want to have hot showers, take turns during that 8-9pm window.

Shave earlier in the day. Prepare your food in the kitchen after the showers are finished. If you understand that your tankless water heater isn’t broken, it’s just overloaded, you can plan ahead to ensure you get the most out of it, while saving on energy and water waste.

Summing Up

Hopefully my water-apocalypse situation would have already discouraged you from shooting low on total gallons per minute figures. But just in case, It’s important to note that saving money by choosing a tankless water heater that’s too small for your home is a bad idea.

First of all, in the long run you won’t save money. It will cost you more in water bills as you let gallons of cold water run down the drain while waiting for the stream to heat up. Also, if you’re constantly pushing your tankless water heater beyond its capacity, it’s more likely to break down and require maintenance in any case.

Don’t try to save by shooting low when choosing a tankless water heater: shoot high and invest on a single time purchase upfront, then let it pay for itself in energy bills over the span of several years.

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