What size tankless water heater do I need？

When buying a tankless water heater, most people don’t know what size tankless water heater they need.

If you are looking to install tankless electric water heater and you’re unsure of which size you should buy, here are some factors you should take into account:

- The size of your house

- Flow Rate (GPM): How much hot water you will need at any given time

- Temperature Rise (ΔT): The difference between the incoming cold water temperature and the desired temperature

Your Household Size

If water heater is too small, it will result in it not being able to produce enough hot water for your household demand. For example, you might not be able to run more than one shower at a time, especially in the freezing winter.

On the other hand, water heater is too large, it will end up being more costly than necessary, since you will be paying more upfront for the larger size (although it could be useful if you plan to upsize your home later).

Hence, to work properly, a tankless water heater needs to be just the right size to meet your home's hot water demand.

Rate of Water Flow

Tankless water heaters are often referred to as demand water heaters. On-demand refers to having continuous hot water output for as long as the hot water tap is open. However, having multiple hot water taps open together will reduce the hot water output. Having too many hot water taps open at once will produce lukewarm water if the tankless water heater’s output capacity is exceeded.

To help you estimate the flow rates of most of your faucets, we shared some typical flow rates below. The actual flow rates may be different for your house and different appliances.

Temperature Rise

After you have calculated the flow rate of every faucet in your house, you must now determine how much heat your heater needs to generate to give you the required heat in your water outlets. We’ll take into consideration the temperature of the water that flows into your home and the temperature of the water that you want to flow out of your tap to determine the required temperature rise.

You might still have some questions relating to the size of instant electric water heater you need, please let us know in the comments below.