Tankless (on-demand) Water Heater Sizing

Some people try and save money by under-sizing their tankless water heater needs.  Unscrupulous vendors might be tempted to get you to oversize them.  The simple thing is, tankless water heaters are rated by the maximum temperature rise possible at a given flow rate.

In order to properly size a demand water heater, you need to determine the flow rate and the temperature rise required in the home.  It’s also important to recognise your typical daily use.   Sure you might have a 6 bathroom home, each with a tub, separate shower, with his and hers sinks. But will you use all of them at once? Unlikely.

You also need to understand that the water is being heated from cold, so it takes time for the cold water to run from the heater to the outlet, plus the time for the heater to heat the incoming water before you get hot water. (Unless you have a continuous hot loop fitted).

Here’s a simple calculator that will help you approximate the size of the tankless system you could need, and you can check out the units at the manufacturer’s sites below.

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Manufacturer’s sites