How To Size A Tankless Water Heater – Forbes Advisor

Editor’s Note: Forbes Advisor may earn a commission on sales made through affiliate links on this page, but this does not affect the opinions or ratings of our editors.

Compare quotes from top rated water heater installers

Free, no-obligation estimates

Instantaneous water heaters continuously supply the kitchen and bathroom with hot water. For many homeowners, installing instant water heaters is a welcome departure from the waste heat wait cycle of instantaneous water heaters.

Dispensing with the tank also means reducing the tolerance limits. Water heaters can afford to heat excess water, but water heaters need to heat the right amount. The correct dimensioning of the instantaneous water heater means that you never have to do without hot water – without having to buy a system that is too large or too expensive.

What a water heater does

Unlike water heaters, traditional tank style water heaters heat 40 to 50 gallons of water using a gas or electric burner. The burner maintains this temperature by occasionally turning on when the water temperature drops. It’s like keeping a pot of water warm on the stove by turning the burner on and off as needed.

Instantaneous water heaters do not keep hot water in reserve. These slim devices, which are usually wall-mounted, have an internal heat exchanger that heats the water as it flows through the heat exchanger. The tubes drag back and forth inside the heat exchanger so that a large length of tube can be packed in a small space. It is similar to how a garden hose filled with water warms the water after a while in the sun.

Most homes use a water heater or two, usually located in a basement, toilet, utility room, or hallway. Instantaneous water heaters can be operated either with gas or electrically. Because of the distance between the water heater and the faucet, users experience the same instantaneous temperature difference as they do with tank heaters. Although the water can run hot all day, it is not instantaneous heat.

Some homeowners choose to install additional heat recirculations that move the water in a continuous cycle between the tap and the heater. It’s the same type of system used in hotels to keep hot water near the tap and minimize water waste. Some instantaneous water heaters have built-in heat recirculation.

How to calculate the correct size of a water heater

Calculate temperature rise

The temperature rise is the difference between the winter or coldest groundwater temperature in your region and the preferred setpoint temperature of your instantaneous water heater.

Consult a map of average winter groundwater temperatures to identify the temperature in your area. Or measure your water temperature in the coldest season in your area with a thermometer, as it comes directly from the ground (usually on an outside tap).

So if your preferred tankless setpoint temperature is 120 degrees and your area’s groundwater temperature is 50 degrees at its coldest, the temperature rise will be 70 degrees.

Determine the peak demand for hot water

The peak hot water requirement is the maximum amount of hot water your home can use. It’s not supposed to be a realistic number; It is designed to ensure that your water heater can meet the theoretical peak demand.


  • Sink: 1 gpm
  • Shower: 2 gpm
  • Tub: 2 gpm


  • Rinse: 1.5 gpm
  • Dishwasher: 2 gpm


Laundry washing machine: 2 gpm

Estimate and add the maximum number of services that can draw hot water at any one time, e.g. B .:

Shower = 2 gpm
Sink = 1.5 gpm
Dishwasher = 2 gpm
Total: 5.5 gpm

Or for a house with many people who use hot water:

Shower = 2 gpm
Tub = 2 gpm
Bathroom sink = 1.5 gpm
Sink = 1.5 gpm
Dishwasher = 2 gpm
Total: 9 gpm

Choose a water heater

Instantaneous water heater manufacturers offer calculation tables or online calculators to help you find a model based on the peak demand for hot water and the temperature rise in your area.

For example, your home may have a peak hot water demand of 6 gpm for the temperature increase around you of 60 degrees. This may point you to a few models that match this description.

However, if the temperature rise changes to 40 degrees, the manufacturer can recommend completely different models. Or if the temperature rise stays the same but the rate of use changes, you need to consider different models of water heater.

You need to contrast the two sets of data to find the right model of water heater for your home.

Advantages and disadvantages of the water heater


  • Small units save space compared to large tank models
  • Continuously hot water so you don’t have to wait for the hot water to pass through
  • There is no standby hot water, so no wasteful heating of the water


  • No hot water reserve in the event that the electricity fails
  • Costs no less than tank models if all factors are taken into account
  • Excessive calcification from very hot burners, therefore more frequent maintenance required

Compare quotes from top rated water heater installers

Free, no-obligation estimates

You might also like

Comments are closed.