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Sizing a Tankless Water Heater

  • December 2013
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Tankless water heaters are a great way to conserve energy and cut utility costs in your Indiana home. After you’ve made the decision to convert to tankless water heating, you may have many questions. The most important thing for you and your family when choosing your water heater is to have hot water, at the desired temperature, when you need it. The best way to make sure you meet these needs is by sizing a tankless water heater correctly.

Many complaints that are often seen about tankless water heaters involve issues that are a result of improper sizing (the water isn’t hot enough when using more than one faucet or appliance, etc). Sizing a tankless water heater correctly will help you avoid these issues and ensure that your family has the correct amount of hot water at any given time of day. In order to properly size your tankless heater, you need to know the needed flow rate and temperature rise for your home.

Maximum Flow Rate for Tankless System

Determining the maximum flow rate expected in your home at peak time is an important first step to choosing the right size tankless system for your home. Follow these steps to determine the maximum flow rate at peak water usage times in your home:

  1. Make a list of all the hot water devices your family uses at once at peak time. Peak time is the time of day that has the highest demand for water. An example of peak time is when the dishwasher is running, the clothes washer is running, and the shower is in use.
  2. Write down the flow rates for each hot water device being used during peak flow time. If you’re unsure what the flow rate is, consult the owner’s manuals for the devices. In the above listed example, assume that your dishwasher has a flow rate of 1 gallon per minute, your clothes washer has a flow rate of 0.25 of a gallon per minute, and your shower head has a flow rate of 2 gallons per minute.
  3. Add together the flow rates of each hot water device to determine the maximum flow rate needed for your tankless system. In the above listed example, your maximum flow rate needed would be 3.25 gallons per minute.

Temperature Rise Helps Determine Tankless Sizing

The next step to determining what size tankless system you should purchase is determining the temperature rise necessary. In order to calculate the temperature rise, you’ll need to know what temperature the water is when it’s coming into your tankless heater and subtract that amount for the desired temperature. You should assume that the water coming into your home is approximately 50 degrees, unless you know otherwise.

For most household purposes, you’ll want your water heater to heat your water to 120 degrees. This is a temperature rise of 70 degrees. If you have a dishwasher that doesn’t have its own heater, you may require your hot water to reach 140 degrees, which means you would require a temperature rise of 90 degrees.

Once you know the flow rate and temperature rise your family needs, you’ll be able to choose the right tankless water heater for your home. Most gas tankless water heaters are able to provide a temperature rise of 70 degrees with a flow rate of 5 gallons per minute. Most electric systems are able to provide the same temperature rise of 70 degrees with a flow rate of 2 gallons per minute.

If you find that you have a higher flow rate in your home than your tankless water heater can achieve, you can take steps to reduce the flow rate necessary at peak time, such as installing low-flow water fixtures, switching to high efficiency appliances when possible, and reducing the demand for water by eliminating one or more demands at peak time.

For more information on sizing a tankless water heater and other ways to save energy throughout the house, contact our professionals at Meyer’s. We’ve been serving Griffith, Munster, Highland, St. John, Schererville and Gary since 1951.