What size tankless water heater do I need? Here’s all you need to know

different sizes in tankless water heaters
Image Credits: Wikimedia Commons

Tankless water heaters are useful appliances which enable you to get a hot water supply on demand. These devices do not need to store water like traditional tank storage water heaters but rather give you install supply when you turn the hot water tap. They are usually rated by the max temperature rise possible at a given rate of flow.  Thus if you wish to size a demand water heater then you must find out the rate of flow as well as the rise in temperature for the application. If you want to know which size tankless water heater do I need then the following given information will prove to be of help.


The first thing that you need to do is to find out the number of devices that you wish to run as well as the rate of flow you need. See the number of bathrooms there are in your house and how many of them you actually use during the cold weather months. Also notice how many bathrooms would need the heater at the same time. Now add the rate of flow of every device and this is the desired flow rate that you would want from your tankless water heater.

different sizes of tankless water heaters
Image Credits: tankthetank.com

Step 2:

Now the next step is to find out the required temperature rise.  This can be calculated by subtracting the incoming temperature of the water from the desired output temperature.

Required Temperature Rise= Desired output temperature-incoming water temperature

Basically a tankless water heater is able to produce warm or hot water based on the temperature differential.

For example – A tankless water heater with a temperature differential of 40 degrees at 2GPM will produce 120 degrees in a southern part of a country whereas the same appliance will produce only 80 degrees in a region much north to the previous mentioned.  This is so because tankless water heaters heat water ‘by’ a specific temperature and not ‘to’ a specific temperature.

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It is important to remember that gas tankless water heaters are capable of producing a higher temperature rise per GPM (gallons per minute or GPM is an important factor related to sizing of the water heater) as compared to the electric models. GPM along with the required temperature difference allows you to calculate for the required kilowatts. You must know that most of the demand water heaters are rated for a varied number of inlet temperatures.

gallons per minute in tankless water heaters
Image Credits:nnifire.com

Step 3:

Another factor which is required to determine the size of the tankless water heater is your geographical location.  This is important as it makes a difference to the incoming water temperature of the area.

Image Credits: Scrumstart
Image Credits: Scrumstart

Step 4:

Most companies that produce tankless water heaters make these appliances in flow rates ranging between 4 GPM and 8 GPM.  The smaller 4 GPM models are suitable for small one bathroom homes or studio apartments or other areas which are application specific. On the other hand, the homes which have 2 bathrooms or more may need 6 to 8 gpm models.  But houses with multiple bathrooms and high requirement of hot water may need multiple tankless units.

Now that you have the answer of ‘what size tankless water heater do I need’ you can easily buy a unit that suits your needs. These appliances are easily available these days. But for their installation, you may need to hire someone with prior experience of the same. You can contact Mr Right for tankless water heater installation and other geyser installation related services.

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    The households with two or more bathrooms may need models with flow rates of 6 to 8 gallons per minute. Homes with a lot of driving directions showers and sinks may need more than one tankless water heater.

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