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What Size Tankless Water Heater Do I Need?

A tankless water heater can be a great investment in your home heating system that has the potential to save you money. If the idea of a tankless water heater is new to you, you probably have questions, such as:

  • What size tankless water heater do I need?
  • How can I figure out whether a tankless water heater is right for me?
  • What are the advantages of tankless water heaters?

Green Energy Mechanical Inc., Canton’s water heater repair experts, frequently introduce customers to tankless water heaters. They are often pleasantly surprised by the ease of installation, efficiency, and low maintenance cost of their new water heaters. Whether you’re ready to look for a tankless water heater or still exploring your options, the time you spend learning about tankless water heater technology will help you make the best choice for yourself and your family.

How Are Tankless Water Heaters Different from Conventional Water Heaters?

Unlike conventional water heaters, tankless water heaters do not store hot water. Instead, the heater draws cold water from the water supply and passes it through heating elements on demand to increase the temperature before it reaches your taps or appliances.

Determining the Heating Capacity Your Tankless Water Heater Needs

The heating capacity of your tankless water heater indicates how much water it can heat in a given length of time. In the United States, we measure heating capacity in gallons per minute. 

Your hot water demands, which influence what size tankless water heater you need, depend on the following two factors:

Peak Hot Water Usage

You don’t have to consider the total amount of hot water you need over the course of the day because the tankless water heater has a limitless supply of water. The crucial limiting factor for a tankless system is how much water you will need on short notice. For example, if you are taking a shower, you can measure the flow rate of your shower head to determine how much hot water you might need.

Appliances such as dishwashers and washing machines will generally indicate the gallons per load, which can help you determine whether your heater has the capacity to run either or both of the machines simultaneously with the shower, faucets, and other water fixtures. You can ignore the toilet unless you have a model that provides hot or warm water. 

You can add up the flow rates of all water sources in your home to get a maximum flow rate in gallons per minute. Whether you will need that full capacity depends on your lifestyle. If you have a large family, host large parties, or have frequent guests, you might need more capacity than just a single person. 

Water usage is only half the story when determining what size tankless water heater you need.

The Temperature of Your Water Supply

In order for a water heater to meet your needs, it must heat the water from its current temperature when it enters your home to the desired temperature. The difference between the incoming water temperature and the desired temperature is the temperature rise. 

What size water heater you need depends on both aspects of the temperature rise. If you don’t need particularly hot water, the temperature rise will be less, and you will not need a larger water heater. If you live in a warm climate, your water is likely already warm, so your water heater won’t have to heat it very much. 

You can find maps of the United States that show the water temperature and allow you to determine the typical temperature rises for each part of the country.

Choosing the Right Tankless Water Heater

Once you know the temperature rise and the flow rate, the next step is to find a model matching your specifications. At this point, you can also consider price, energy costs, and other factors. Your professional installation technician will help you evaluate the options.

If you cannot find a tankless unit that meets your needs, consider reducing your usage requirements; for example, by buying efficient appliances or using cold water for laundry.

Talk to the Water Heater Experts at Green Energy Mechanical Inc.

For a personalized, in-depth answer to the question, “What size tankless water heater do I need?” talk to the heating specialists at Green Energy Mechanical Inc. Our team understands the operation, repair, maintenance, and installation of modern tankless water heaters. Call us at (781) 236-3421 to explore your options in Canton, MA, today.

Author Bio:

Jonathan Neves

Worked in the HVAC industry since he was 17. He started as an HVAC service technician for commercial buildings and eventually came into the residential market because of his love of working with people. He believes that what really sets Green Energy Mechanical apart is how long they stand behind their work and how compassionate his employees are for their clients.


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I could not be more pleased with the service provided. We have upgraded our water heater to a tank less. We are also getting our heat and ac installed by them. They care for your home like you would
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