How to Size a Tankless Water Heater

Tankless Water Heaters
Wikimedia Commons

Tankless water heaters are great appliances but are often oversold in terms of their capabilities. Like many great ideas, the marketing hype can often get ahead of the technology and sales people over-commit the capabilities of the product to the consumer. The result is an unsatisfied customer, wasted money and a poorly performing appliance.

Factors That Effect Tankless Water Tank Performance

Take a look at any tankless water heater ad and you'll probably see a promoted feature like "Provides up to 4GPM." Well, that rating is likely the BEST CASE situation for that product and may not be the level of performance you can expect.

Why? Because sizing a tankless water heater is based on 3 things:

  • Temperature of water entering the unit (ground water temp),
  • Desired temperature of water exiting the unit (together these 2 items determine the temperature rise required of the tankless water heater) and;

    The required hot water flow rate (based on shower heads and faucets).

Let's look at an example using a colder climate like Boston, MA. Ground water temperature in Boston is about 47 degrees F. If your want a normal 105-degree shower, that means you have a 58-degree temperature rise (105-47=58). As a result, you need to look at the tankless water heater's GPM flow rating based on a 58-degree temperature rise. Let's continue our example with a popular product, the Bosch 1600P-NG tankless water heater. This unit advertises a flow rate of about 4 gallons per minute (GPM). But if you're still using an older style shower head the older shower head's flow rate can exceed the capacity of that tankless water heater.

An older style shower head (pre-1992) can demand from 6 to 8 gallons per minute (GPM) of flow rate. A newer shower head (after 1992) usually uses 2.2 GPM.

But let's assume that the Bosch 1600P-NG tankless water heater is being used only as a supplemental water heater. What about its 4 GPM claim? Well, this unit only provides 105-degree hot water at a 45 degree F temperature rise as found in warm southern climates.

At a 58 degree rise required in Boston, MA the unit is rated at about 3.3 GPM. The only way this Bosch unit will work as a tankless water heater is if it only provides hot water to the shower or the bathroom where the shower is located in, and a low flow 2.2 GPM shower head (post-1992) is being used.

Also, if this unit is being used as your home's main water heater and is expected to provide hot water to more than the one bathroom, it will be undersized and cannot work as a whole house tankless water heater with only a 3 or 4 GPM rating. A whole house application requires a larger unit or ganged multiple units depending on your home's size and location.