What size tank less water heater should you get?

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The best way to determine what size tankless water heater is ideal for your household is probably to just look at your needs specifically. It can be potentially very inaccurate to say that water heaters of a certain size will cater to the needs of all types of households, even if the households lie in the same vicinity. This is due to the households having a different number of people with different routines and requirements, which means that the need for water is quite variable across the board. A household that is only sometimes full usually doesn’t have needs as high as a more lively household.

Something to know about what size you should be getting is that the actual dimensions of the heater don’t decide everything. Naturally, a heater with a wider surface area for water heating will have higher heating capabilities, but bigger isn’t necessarily better. Crafty designing can fit pipes of large surface areas in smaller bodies rather than fitting them in bulky bodies.

So let’s look at the things that come into play when you’re deciding on what size tankless water heater you should be getting.


Flow rate requirement:

Flow rate is generally described as the number of gallons or liters of water your house consumes in a minute. It’s measured in Gallons per minute (GPM ) or Liters per minute (LPM). The average flow rate for a house is supposed to be between 6-10 GPM or about 20-37 LPM. Calculating how much flow of water your household needs is a relatively straight forward process. To calculate your own house’s flow rate, you can use pre-determined tables like the one below to determine how much outflow the taps and fixtures in your house generate.

Application Flow rate(GPM)
Shower 1.5-3
Kitchen Faucet 2-3
Dishwasher 2-4
Washing machine 3-5

Simply add the flow rates of the different areas of use that you think are generally used simultaneously.

There are laws in place that limit modern manufacturers in how much outflow some fixtures can provide. For example, a shower fixture should not give an outflow bigger than around 2.5 GPM. Though shower fixtures from the 80s or the 90s may have flow rates as large as 3.5 GPM.

While some applications require quite variable flow rates as in washing machines. All these factors have to be considered in order to arrive at a more accurate calculation of the flow rate. The reason why flow rates are so important while considering which water heater you’re going to buy is that if the actual flow rate exceeds the water heater manufacturer’s stated capacity, the result is probably going to be cold water.


Heating rate:

This is a measure of exactly how strong the water heater is. How much heat it can produce in a given amount of time. A water heater capable of producing a larger temperature change can raise the heat content of water faster than a heater with a smaller heating capability. Meaning that if you suddenly increase the flow rate, the stronger heater is more likely to keep up with the change of flow rate. If the heater lags too far behind i.e. it doesn’t heat the incoming water quickly enough, the result will be cold water again.

To see what kind of heating capabilities are required by your household, the two things that need to be known are the temperature of the water going in, and the required outlet temperature. A location where the input water is colder will naturally have heaters giving outflow at lower temperatures whereas consumers in areas where the temperature is more favorable will only require heaters with moderate heating capacity.

So take a look at the water temperature map before deciding how strong a heater you need.

You will also need to decide whether that requirement is more suitable for a tankless gas or a tankless electric heater. Electric heaters generally have smaller heating rates as it takes time for heating elements to produce heat just like an iron takes a minute to fully warm up, while gas heaters gain heat directly and instantly through combustion.

If you don’t know the temperature of the incoming water, you can assume it to be around 50℉ (10 C). For normal use, the temperature at the outlet should be between 90 to 120℉ (35-50 C). If these conditions hold true for you, you would need a heater capable of producing a 40℉ change to reach 90℉ lower limit and 70℉ change if you want the water to be around 120℉ or 50 C. Similarly, in order to reach temperatures higher than this, you will need a stronger heater or a lower flow rate.

The change of temperature, of course, has to be brought in a large amount of water. Different companies have different units to measure heating capacities. Some measure heat change capability with respect to a flow rate of 1.5 GPM while others have other units for measurement. The conversion from one unit to the other is very simple.

For example, if a heater produces 90℉ change in 1.5 GPM, it can be said that the heater will produce a 30℉ change in water at a flow rate of 6 GPM.

It should be noted that a high heating capability does not necessarily mean that you will get scolding water every time you turn the heater on. It depends on the temperature that you’re asking for from your heater. Some heaters have control units to handle overheating while some go as far as installing AI chips to learn from the requirement routine in order to tackle the various needs of the day.

Parallel heating:

Something to keep in mind for consumers in really cold areas like Alaska or other colder states is that you don’t have to be limited to just one heater. You can connect multiple heaters in parallel in order to achieve lower output latency (or getting hot water quicker). This will not only save you gas compared to what you would be spending on multiple storage heaters but will also ensure that you don’t eventually run out of hot water which is a natural shortcoming of tanked water heaters.


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