What Size Tankless Water Heater Should You Buy: Sizing For Your Family Needs

Tankless water heaters are the future of hot water use in residential homes and commercial properties. Tankless water heaters offer a compact design that produces hot water on-demand. However, on-demand does not necessarily mean continuous. Tankless water heaters, both electric and gas, have a hot water production capacity per minute or flow rate. If your family’s demand exceeds the water heater’s production capacity, it can lead to no hot water.

When buying a tankless water heater, most people don’t know what size tankless water heater they should I get.

Tankless water heaters are sized by their flow rate or Gallons Per Minute (GPM). A family of 4 should look for a tankless water heater that can produce 7 gallons of hot water per minute. Most families use about 6.5 GPM during peak times. To determine the tankless water heater size, you’ll need to know how many appliances your family will use simultaneously. A tankless water heater that produces 4 GPM can supply one shower and one appliance operating simultaneously.

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Sizing on tankless water heaters is important because of the upfront installation and operating costs. When a tankless water heater is undersized, it won’t produce enough hot water to keep up with your family’s demand. For example, Taking 2 showers and running a dishwasher simultaneously can use up to 6.5GPM. A 4GPM tankless water heater can’t produce enough hot water to meet this demand.

There’s no advantage to installing an oversized tankless water heater. An oversized tankless water heater carries higher installation costs and operation costs. There’s no advantage to installing an 8 GPM model when your family needs a 4 GPM model. A 4 GPM tankless water heater costs between $400 to $700 plus installation, while an 8+ GPM tankless water heater costs $900 to $2000 plus installation costs.

According to Fixr.com, the average cost of a whole-house tankless water heater is $2,810 plus installation costs of $45 to $200 per hour with an estimated installation time of 2 to 8 hours.

To help you choose the right tankless water heater for your family, we’ve created this comprehensive buying guide. We’re going to walk through many different factors you need to account for when looking at tankless water heaters. 

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How Many GPM Do You Need For a Tankless Water Heater?

The first thing you’ll need to consider is how much hot water you’ll need your tankless heater to produce. We typically refer to this process as “sizing” the tankless water heater, but you will not use dimensional measurements to figure this out. Instead, you’ll need to look at two distinct metrics: flow rate and temperature rise. 

What is a Tankless Water Heaters GPM Flow Rate

Tankless water heaters are sized by the “flow rate” measured in gallons per minute (GPM). Most tankless water heaters can produce between 2 and 10 gallons per minute. To determine your flow rate requirement, you’ll need to add up the flow rates of all the fixtures and appliances in your house that might use hot water simultaneously. 

You can only know your exact flow rate requirement by checking your hot water appliance documentation. This list of typical flow rates for common fixtures and appliances should help you make an estimate, though:

Fixture or ApplianceUsage: Gallons per MinuteBathtub4 GPMRainhead shower head5 GPMStandard shower head2.5 GPMStandard dishwasher2.5 GPMHigh-efficiency dishwasher1.0 GPMStandard clothes washer2.5 GPMHigh efficiency clothes washer1.0 GPMSink faucet1.5 GPMBased on the estimated GPM output. Some fixtures and appliances may have a higher output

What is a Tankless Water Heater’s Temperature Rise

Once you’ve calculated the maximum flow rate your tankless unit will need, you need to figure out the required “temperature rise.” The temperature rise is the number of degrees your water heater will need to increase the incoming ground water temperature to provide sufficient hot water to your home. 

For example, say the temperature of incoming water is 52°F, you’ll need a temperature rise of 58°F to achieve 110°F hot water.  So if 110° were the highest water temperature required by any of your hot water devices, you would need a tankless water heater that can produce a 58° rise in temperature.

Putting flow rate and temperature rise together 

Water heaters are rated in terms of both flow rate and temperature rise. To properly size your water heater, you need to combine your required flow rate with your required temperature rise. 

Ground Water TemperatureExpected Gallons per Minute Output
37 F3.9 GPM
42 F4.7 GPM
47 F5.5 GPM
52 F6.3 GPM
57 F7.1 GPM
62 F7.8 GPM
67 F8.6 GPM
72 F9.4 GPM
Based on a tankless water heater with a 9.5 GPM output on a 35 degree Temperature Rise

Here’s another example. Say you have four devices that might simultaneously require hot water: 

  • Sink: 1.5 GPM / 110°
  • Dishwasher: 1.5 GPM / 110°
  • Washing machine: 2.0 GPM / 120°

Let’s also assume your incoming water temperature is 50°. To run all of these devices simultaneously, you would need a tankless water heater with a flow rate of 5.0 GPM and a temperature rise of 70°. The 5.0 GPM flow rate is the sum of all device flow rates together. The 70° temperature rise comes from subtracting the incoming water temperature (50°) from the highest device water temperature (120°). 

Now, just because your devices’ combined flow rate adds up to 5.0 GPM doesn’t mean you need a water heater with a flow rate of 5.0 GPM. If you don’t plan on using all of your hot water devices simultaneously, you can get away with buying a less expensive heater with a lower flow rate. 

Are Tankless Water Heaters Worth the Cost

When considering a tankless water heater, people often wonder whether it’s worth the cost.

Unlike tank water heaters, tankless water heaters are worth the cost due to the increased lifespan, reduced maintenance, and continuous hot water on demand.

  • Tankless water heaters can last upwards of 20 years, whereas tank water heaters can fail in as little as 8 years.
  • Tankless water heaters don’t have a tank that can fail and leak, reducing the risk of water damage.
  • Tankless water heaters have no tank to maintain. Tank models require annual flushing maintenance to preserve the tank, which is often overlooked, contributing to the reduced lifespan.
  • Tankless water heaters can provide hot water on demand. A 50-gallon tank water heater can become exhausted in as little as 30 minutes of continuous use and requires a minimum of one hour recovery.

Electric vs. Gas Tankless Water Heaters

Once you’ve determined the required flow rate and temperature rise, you’ll need to decide between electric and gas tankless water heaters.

Electric tankless water heaters cost less to buy and install

One of the primary benefits of buying an electric tankless water heater is that they are less expensive to purchase than their gas-powered counterparts. Whole-house electric models typically cost between $500 and $700, while whole-house gas-powered models cost $1,000 to $1,200+.  

The cost of installing an electric tankless heater is also significantly less than installing a gas-powered tankless heater. Once you’ve confirmed your home’s wiring is compatible with the model you’ve chosen, installing the electric heater is a relatively quick process. 

You should note that electric tankless water heaters may require some electrical upgrades. Some electric water heaters require up to 4 – 220v breakers to operate, significantly raising installation and operation costs.

Installing a gas-powered heater can incur some pretty high costs as well. You’ll need to install a ventilation system to get rid of the vapors your water heater might emit safely, for starters. There are also many safety and inspection costs you need to consider, such as confirming the size of your gas line and ensuring an adequate supply of fresh air. 

Electric tankless water heaters are easier to maintain

One reason electric units cost less than gas-powered units is that they are simpler devices. There are fewer components inside, which means fewer moving parts can break down and require expensive repair.

When a repair is needed, you’ll typically pay less than if you were repairing a gas-powered unit. Most tankless water heaters will beep to notify you when tankless water heater maintenance is needed.

Electric tankless water heaters are more energy-efficient

The best gas-powered tankless heaters tap out at about 85% energy efficiency. An 85% efficiency rating is pretty poor compared to most electric models, averaging 98% energy efficiency. So even if natural gas is cheaper than electricity in your area, you might still end up saving money on energy costs by going with an electric unit. 

Electric heaters are more environmentally friendly. Producing electricity is already less harmful to the environment than burning natural gas is, and the increased energy efficiency of electric models only adds to the difference in impact.

The most significant disadvantage of electric tankless water heaters is the installation. Electric tankless water heaters require one 240v breaker for each burner. Large GPM models require three or four 220v breakers to operate, meaning you may need to add an electrical subpanel or other electrical upgrades.

Gas-powered units support higher flow rates 

Gas-powered tankless water heaters have both natural gas or propane models. Some buyers choose to go with a gas tankless water heater over an electric one because gas models can support higher flow rates. The best electric units max out at 8GPM, while some gas models can go significantly higher than that. If you need a tankless heater for a large household or some industrial purpose, you may have no choice but to go with a gas-powered option. 

Natural gas is cheaper than electricity in some parts of the world

Depending on where you live, the discrepancy in price between natural gas and electricity might make it enticing to go with a gas-powered model. However, even if natural gas is significantly cheaper than electricity, the increase in efficiency with electric models and the forecasted increase in natural gas prices might still make going electric the better option. 

Indoor or outdoor installation

Unlike traditional water heaters, which must always be located inside, tankless water heaters can be installed inside or outside your home or office. 

The main benefit of installing your tankless heater outside is that you don’t need to install a venting system. Electric units don’t require ventilation, so there is no real benefit to installing them outside. 

Mounting your tankless water heater outside is typically reserved for warmer climates where the temperature doesn’t drop below freezing too often. Most outdoor tankless water heaters have built-in freeze protection as low as -30 degrees Fahrenheit.

HomeInspectionInsider.com is owned and operated by Hubert Miles is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com. HomeInspectionInsider.com also participates in affiliate programs with other affiliate sites. Hubert Miles is compensated for referring traffic and business to these companies.

Hubert Miles

I've been conducting home inspections for 17 years. I'm a licensed Home Inspector, Certified Master Inspector (CMI), and FHA 203k Consultant. I started HomeInspectionInsider.com to help people better understand the home inspection process and answer questions about homeownership and home maintenance.

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