How Big of a Tankless Water Heater Do I Need?

When you are researching tankless water heaters for your home, you’ll often hear the word “sizing” emphasized, and it is an important term. Sizing is defined as matching the needs of a homeowner to the hot water source’s capacity. When it comes to sizing tankless water heaters, the hot water flow rate is the component you need to pay the most attention to.

The temperature of the incoming water is another criterion that must be given due consideration. This temperature will vary by season and region. For example, a tankless water heater in a northern climate will require a higher BTU input during the cold months of winter for it to raise incoming water to a given temperature than it will during the summer months.

Before you begin shopping for a new tankless water heater, here are a few questions you need ask yourself:

  • How many people in my home take showers? What time of day do they do it most often? Is there a “rush” to take showers during the morning or the evening?
  • Keep in mind the following information: a standard showerhead has an average flow rate of 2.5 gallons per minute; although some newer showerheads have lower flow rates than this. Most adults prefer to shower in water that is somewhere between 102 degrees and 106 degrees Fahrenheit.
  • How many bathtubs do I have in my home? Are any of them soaker or whirlpool tubs? What is the capacity of each one in terms of gallons? How often do the members of my household take baths?
  • What major appliances in my home will the water heater need to provide for? For example, will the washing machine and/or dishwasher be running at the same time someone is taking a shower?
  • Compared to European countries, the amount of hot water that most North American homes use is astronomical, so it is not uncommon for the average American home to need their hot water heater to supply multiple major appliances at once. What time of day are these appliances most often run?

Use these questions to determine when the peak demand for hot water is in your home. The next step is to calculate how many gallons per hour (gph) or gallons per minute (gpm) your home will require from its hot water heater during these peak times. Then, you can narrow down your search for a tankless water heater to models that fall within this gph or gpm range.

One of the most commonly given benefits of tankless water heaters is that they provide an endless stream of hot water. While this is true, if your tankless heater is not sized properly, the water’s flow rate will have an adverse effect on the overall performance of the unit. For example, the temperature of the water in your shower will remain consistent, but the flow of the water could slow down to a mere trickle. Unfortunately, it is not uncommon for homeowners to undersize their home’s water flow rate in an attempt to save money by buying a smaller water heater.

The first step in properly sizing a tankless water heater for your home is to carefully add up the flow rates for each showerhead, major appliance, and water faucet in your home that is likely to be in use at the same time.

The second step in this process is to evaluate the temperature of the water that flows into your home. If you live in a climate where the inlet water temperature averages 30 or 40 degrees, then you will need to choose a tankless water heater with a higher BTU input. Generally, inlet water temperatures average:

  • 40 degrees for the northern most states in the U.S.
  • 50 degrees for the uppermost Southern states
  • 60 degrees for the Gulf Coast states, the Southwest, and Southern California

If you live in a large home or have a big family, then you may need to consider installing more than one tankless water heater in your home. Tankless heaters can be installed separately (generally point of use) or in tandem with one another so that they operate as a single unit.

Whether you choose a gas or propane powered tankless water heater or an electric one will also influence the size of model you should choose. Tankless heaters that are powered by natural gas or propane are more capable of producing a greater temperature rise (in gallons per minute) than an electric model is. Most on demand water heaters are rated for a number of inlet temperatures. For example, a 70 degree rise in water temperature can be achieved at a flow rate of 5 gpm for a gas powered tankless water heater and at 2 gpm for an electric model. Cooler inlet temperatures and quicker flow rates will, on occasion, reduce the hot water temperature of the most distant faucet.

Although it can sound confusing at first, properly sizing a tankless water heater for your home is not difficult. Take your time, do your homework, and always err on the side of caution. It’s always better to be safe than sorry.

Fred Rader

About the Author

Fred Rader

My name is Fred and I'm the owner of Household Water Systems. I've built this website to help homeowners such as yourself improve their water systems. I struggled to find accurate, honest reviews when I was searching for a new hot water heater, so I decided to build this website.

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