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Determining Proper Size for Tankless Water Heater

If you’re considering installing a tankless water heater in your home, congratulations on making a prudent move to improve your home’s water heating and efficiency capabilities. Tankless water heaters are the wave of the future, with numerous homeowners replacing older models with these modern choices that come with longer lifespans, speedier heating and more efficient processes from top to bottom.

At Monkey Wrench Plumbing, Heating & Air, we’re proud to offer a wide variety of tankless water heater installation and repair services to those looking to make this leap. Perhaps the most common question we get from clients who have made this decision: What size does my new tankless water heater need to be to properly supply my home? Today’s blog will go over precisely how you make this decision.

Water Flow Rate

The first important metric you’ll need to be aware of here is flow rate, which describes how much hot water a given tankless water heater can produce. Flow rate is expressed in gallons per minute (gpm), and every heater you consider for purchase will have its flow rate listed prominently.

To get started, you want to assess your commonly-used hot water fixtures and the kinds of flow rates they tend to come with. Here are a few examples, using average low-flow fixture rates:

  • Tub faucet: 4.0 gpm
  • Showerhead: 2.5 gpm
  • Washing machine: 2.0 gpm
  • Kitchen faucet: 1.5 gpm
  • Dishwasher: 1.5 gpm
  • Bathroom sink: 1.0 gpm

Now, be aware that most tankless water heaters allow for the use of two fixtures simultaneously at most. So you want to be thinking about which combinations of fixtures might be used during peak times in your home.

Groundwater Temperature and Temperature Rise

However, flow rate is not all you need to know about here. The other important factor is the temperature of your water in the ground before it enters your plumbing, otherwise known as groundwater temperature, plus the amount this number must rise when it’s being heated to your desired level. This metric is known as temperature rise.

For temperature rise, it’s vital to know the average groundwater temperature in your region. Maps like the one on this page are a start, and there are more detailed versions available for your city or state.

Combining Metrics

Once you know your groundwater temperature and the flow rate you require, it’s generally very easy to combine these. Most modern tankless water heaters list sizes in terms of a combination of flow rate and temperature rise – a given unit might provide a 7.0 gpm flow rate in a warmer area where required temperature rise is only 20 or 30 degrees, but the same unit could provide a flow rate below 5.0 in a colder area where the required temperature rise is higher.

For more on determining the proper tankless water heater size for your home, or to learn about any of our plumbing or HVAC services, speak to the staff at Monkey Wrench Plumbing, Heating & Air today.