Size of Tankless Water Heaters

How to Size A Tankless Water Heater

Should you conclude that it’s time to switch to tankless water heaters (what took you so long anyway?), there are some aspects you need to consider first. Not only that, you have to check the market for the best model for your home and money, but you also need to make sure that you get a unit that gives precisely what you need: hot water endlessly.

How large does a tankless water heater need to be?

You don’t just go ahead and buy the very first model you run into. There are some measurements to take, some numbers to add, and some thought to give so that you identify the right size of your tankless water heater.

Without any further ado, let’s see what you need to consider when figuring out the size of your tankless water heater:

  • Where are you going to mount it?

Even if tankless water heaters are small and compact, requiring only 1/3 of the space of 10-gallon tanks, you still need to know where you’ll mount it. Electric models are typical 10in high and 7in wide (give or take), whereas the gas-fired models are larger (30in high and 20in wide). Briefly put, the electric units only need a little space for the mount, whereas the gas models require a more generous space for mounting. Don’t forget that you also need to take the venting for the gas units. When you don’t have the space for running the vent pipes, an electric model may be the better option.

  • What can you notice about the cold water temperature?

Grab a thermometer for finding out the cold water temperature. The temperature of the incoming water is where you begin. It’s the water that the unit will heat at the wanted temp, and it should provide the wanted temperature at a maximum flow rate. For instance, you want hot water in the kitchen to be 110F degrees. When the cold water flow is 50F degrees, the tankless water heater will have to be 60degrees higher so that the water in the kitchen remains at 110F degrees.

  • Check the groundwater temperature

In far northern areas of the country, the water temperature can get as low as 37F degrees, whereas, in far southern regions, it can reach as high as 70F degrees. In Midwest regions, the water temperature ranges somewhere between 50 and 60F degrees. The higher the heat of the incoming water, the less energy the heater will use.

  • What’s the water flow rate?

Unfortunately, the performance of tankless water heaters depends a lot on the water flow rates inside the house. For instance, a low-flow showerhead uses 1.5GPM, whereas a sink may run 1.5GPM through the faucet when you’re washing the dishes. Any running bathtub uses 4GPM. When you cannot identify the essential GPM measurement, you can put a gallon jug right under the faucet, to see how much time it needs for filling it. When a jar only needs to fill in 20 seconds, then three jars need 1minute; therefore, 3times 1 gives 3 for GPM.

  • Run all the numbers for identifying the water heater size

You should begin with the incoming water temperature (let’s say it’s 50F degrees). Let’s say one member of your family has a shower which is 1.5GPM and 105F degrees for temp), another one is washing dishes (which is also 1.5GPM, but the temp should be 110F degrees). Let’s not forget to add the washer (2GPM) with 120F degrees for the temperature. You obtain a 5GPM for that moment, and for getting the desired temperature for all of the appliances, the tankless water heater should be able to raise the temp 70F at 5GPM. It’s a typical performance for gas units, but they’re rated at a specific temperature increase over a maximum amount of GPM.

How big does an electric tankless water heater need to be?

Let’s start by highlighting that every reputed manufacturer is going to provide you all the instruction you need for figuring out the size you need for an electric tankless water heater.

Typically, you would have to use the map provided by the manufacturer to identify the approximate groundwater temperature where you live. Once you identified the area, you have to check the column on a table with the groundwater temp to find out about the number of fixtures that the model can sustain simultaneously. You have to use the maximum flow rate for identifying the best size for your home. For example, if you have 1.5GPM low flow for the showerheads, a 3GPM unit can sustain two showers simultaneously. It can also give hot water for one shower and one sink.

Don’t also forget to consider the voltage when selecting the electric tankless water heater.

How big does a gas tankless water heater need to be?

Let’s start by reminding you that it takes a lot of BTUs for a tankless water heater to heat some cold water almost instantly. Once a heater’s BTU output cannot satisfy the demands, it’s going to reduce the flow and even deliver lukewarm water.

In order to find out the size of your gas tankless water heater, a professional plumber should check three things: the temperature of the incoming water, the peak demand for hot water in GPM, and the heater’s efficiency (it’s in the technical specs).

The plumber will identify the Btus per gallon that the heater would have to increase the incoming water to 120degrees. The peak demand is the sum of the flow rates for all the fixtures and appliances that may use hot water simultaneously. The total should be reduced with 20% since we’re not washing/bathing in 120-degree water. You can actually lower the peak demand by upgrading to low-flow fixtures and water-saving fixtures. You may also postpone some washing if the shower is being used.

You get the total BTU output by plugging the BTUs per gallon and peak-demand into the formula. If two models satisfy the needs,  it’s safer to go with the one with a higher BTU rating. When the output goes over 198,000BTUs (which is the maximum for residential applications), it’s wiser to use two units running together.

For getting a clearer picture, here are some numbers:

  • 1 bathroom- 140,000BTUs
  • 2 bathrooms (2-3people)- 190,000BTUs
  • 3 bathrooms (3-5 people)- 380,000 BTUs

How big does a tankless water heater need to be for a whole house?

You should begin by listing all the fixtures that use hot water from the water heater, adding up the flow rates. Go back to the documentation of the installations to find out the flow rate. If you don’t have it anymore, scroll up for the details.

If you have old fixtures, go with the higher numbers and go with the smaller amounts for the new installations. It’s better to select the higher number so that you don’t get an undersized unit. Don’t forget that you can also use a point-of-use heater for some fixtures if you want hot water for a faucet or anything similar.

Take the temperature of incoming water into consideration and check the maps online for U.S groundwater temperature. It’s a rough guide, and you should get in touch with the water provider for better accuracy.

Once you have all the numbers, you have to find out the temperature rise, with most households requiring 120F degree water and unit heating up to 160F degrees. When the incoming water is 50F degrees, the temperature increase should be 70F degrees.

More often than not, a respected manufacturer will provide you with spec sheets with a maximum flow rate at different temperature rises.  You need a unit that ensures the required flow rate at the specific temperature rise. The higher the flow rate, the lower the temperature increase.

How big does a tankless water heater need to be for an apartment?

Anyone looking for tankless water heater for an apartment should make sure that the unit checks the following boxes:

  • It has a small shape- Most tankless water heaters are small, but they have to be tiny when installing in an apartment. They should be small and compact, saving storage space. Some models may be installed under the sink.
  • Low flow rate- look for models that don’t get a high flow rate, as they only have to sustain one bathroom and one sink simultaneously. A small flow rate saves energy, cutting down the energy bill.
  • Point of use models- They’re the best choice for apartments as they allow installation near the application fixtures.
  • Short waiting time- Unlike the large homes, tankless water heaters for apartments have a shorter waiting time. The pipe is short, needing only a couple of seconds for hot water to travel to the destination.
Scroll to Top