How to Size Tankless Water Heaters

Many homes today are switching over to tankless water heaters. The reason they are doing this is to save money. Tankless over tank water heating is much more cost efficient. But how do you decide on what size tankless heater to put in? It is not that hard to figure it out.

• Count the number of hot water devices you will be using at the same time. You will have to know the flow rate and temperature rise in order to know what size you will need. Many times when installing tankless water heaters they are installed in different areas of the home at the same time such as one in the kitchen and one in each bathroom.


• Figure the flow rate (gallons per minute) you expect to use at the same time. Say you and your wife use the bathroom and one is showering and one is shaving with the faucet on. If those devices were using three gallons per minute combined that would be the flow rate needed for that tankless heater.


• Calculate the temperature rise by subtracting the incoming water temperature from the temperature you want your hot water to be. Based on normal situations a rise of 70 degrees with gas heat is possible to get five gallons per minute and with electric heat about two gallons per minute. So using the example listed above you would need a flow rate of three gallons per minute from the tankless heater in that situation.


• Keep in mind that with tankless water heaters they do not heat the water until you actually have a demand for it. So unlike your tank water heater that keeps the water hot whether you are home or not, tankless heaters only use power when you actually are using hot water.