Tankless Water Heater Comparison – Sizing your Tankless Water Heater

Before making a Tankless Water Heater Comparison you need to perform proper sizing of your home’s requirements,

Tankless water heaters are rated by the maximum temperature rise possible at a given flow rate.

In simple words, you will first have to calculate what will be your required water flow rate (based on your application: Single point / Multi point / Whole house), and then you will have to determine the required temperature rise (based on the climate you live in).

These two important numbers are essential for making any tankless water heater comparison.

Determining Flow Rate

Water flow rate is determined in GPM (Gallons Per Minute) or LPM (Liters Per Minute).
According to your application (Single point / Multi point / Whole house), list the number of fixtures that will be attached to the tankless hot water heater, and determine the number of fixtures that will be used simultaneously. Then use the table below to calculate the total maximum required flow rate.

Typical Flow Rates

Please note

1. These numbers may not reflect your appliances or fixtures due to:
2. Differences in water pressure from location to location.
3. Efficiency of appliances and fixtures.
4. Installing low-flow showerheads and faucets may reduce these numbers by 30% – 50%, resulting in a smaller unit.
5. If you have a multi shower head system – you need to find out exactly how much water all the shower heads will consume.

Example 1: A bathroom single point application (only shower)

Depending on your showerhead, you need a flow rate of 1.5-3.0 GPM (5.5–11 LPM)

Example 2: A bathroom multi point application (Bathroom Faucet + Shower + Bathtub)

You can assume that you will not use the shower and the bath simultaneously, so use the higher flow rate fixture (Bathtub).

You may also assume that you may use the Bathtub and the Bathroom Faucet simultaneously, so add the Bathroom Faucet’s rate of flow (0.5 GPM / 2 LPM) to the Bathtub’s flow rate (2.0-5.0 GPM / 7.5-19 LPM).

You need a heater that supports flow rate of 2.5-5.5 GPM / 9.5-21 LPM.

Example 3: A whole house application (2 Bathrooms and a Kitchen)

In this example you have a few options. You can assume simultaneous use of two bathrooms, dish-washer and kitchen sink – resulting in flow rate of 6.5-12.0 GPM (24.5–45.5 LPM). This will require a large unit, which might be to expensive.

Another option would be to assume simultaneous use of either two bathrooms or one bathroom and a kitchen. That will result in a smaller and cheaper unit that needs to support only 4.5-9.0 GPM (17–34 LPM)

Note: You can install low-flow showerheads/faucets or water aerators to reduce the demand flow rate. This will enable you buy a smaller heater.

After sizing it properly, you will need to find the required Temperature Rise for your instant flow water heater.