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Properly Sizing a Tankless Water Heater

Tankless hot water heaters are an excellent choice for homeowners who want to save money on their energy bills while still enjoying a constant supply of hot water. However, choosing the right size tankless water heater can be challenging. In this tutorial, we'll walk you through the steps you need to take to size a tankless hot water heater for your home.

[Step 1: Determine Your Hot Water Needs] The first step in sizing a tankless hot water heater is to determine your hot water needs. Start by calculating the maximum flow rate of hot water that your household will require. This will depend on the number of fixtures that will be running at the same time, such as showers, faucets, and appliances. To determine the flow rate, you can use an online calculator or consult with a professional plumber.  Another way to get a fast estimate is use 2.5 gallons of water for showers and appliances, and 2 gallons for faucets.  This method may give you a higher GPM usage, but with a wide variance of water efficient faucets around the country, you will get a general calculation.  


[Step 2: Calculate Your Temperature Rise] Next, you'll need to calculate your temperature rise. This is the difference between the temperature of the incoming cold water and the desired temperature of the hot water. Most homes require a temperature rise of around 70 degrees Fahrenheit.  If you live in a colder climate, you will generally have colder water coming in to your home, this requires a higher temperature rise, and you will in turn get fewer gallons per minute.  Sizing up your heater to a higher BTU model may be the right call to insure adequate hot water levels.  In the opposite is true for warmer climates, your warmer ground water temps will allow to water heater to create 120 degree water quicker and faster.  See the chart provided by Rinnai for average ground water temps across the country.  Keep in mind they are showing there largest heater, the Sensei 199K BTU models on the "Simultaneous Shower Use" section.  


Sizing a Tankless Water Heater
Sizing a Tankless Water Heater

[Step 3: Determine the Unit's Flow Rate] Once you know your hot water needs and temperature rise, you can determine the flow rate of the tankless hot water heater you'll need. Look for a tankless hot water heater with a flow rate that's equal to or greater than the maximum flow rate you calculated in step 1.


[Step 4: Check the Unit's Efficiency] It's also essential to consider the efficiency of the tankless hot water heater you're considering. Look for a unit with a high efficiency rating to help save energy and reduce your energy bills.  The premium Condensing units run between 93-97% efficient, while the Non Condensing models are generally around 80-82% efficient.  Don't forget that most condensing models can use schedule 40 PVC to vent them, creating a savings in material cost.  The Non Condensing models require specialized venting sold by the manufacturer.  Don't worry, we break down the differences in venting in another blog "How to Vent a Tankless Water Heater".


[Step 5: Consider Your Household Size] Finally, it's important to consider the size of your household. A larger household with more people will require a larger tankless hot water heater to meet their needs. As a rule of thumb, a unit with a flow rate of 5 gallons per minute is typically sufficient for a household of two to three people. A household of four or more may require a unit with a higher flow rate.

Choosing the right size tankless hot water heater requires careful consideration of your household's hot water needs, temperature rise, unit flow rate, efficiency rating, and household size. By following these steps, you can make an informed decision and enjoy the benefits of a constant supply of hot water while saving money on your energy bills.

If you have any questions or need further assistance, please send us an email or give us a call 855-424-4376.  


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