Sizing a Tankless Water Heater

Reviews  | Submit a Review

Sizing a tankless water heater is different from sizing the tank-type heating appliances. The rule is still the same; you want your tankless model to provide sufficient hot water at a minimum temperature of 120 F during the peak time of the day.

Like with the tank-type heaters, you don't want to buy an oversized or undersized tankless unit, because it brings unnecessary issues, rising the operating costs; affecting your budget and time.

To determine which tankless water heater model is right for you and what is the correct size or capacity you should buy, there are several factors that have to be taken into account.

Things to consider

How much water will be required during the busiest usage period or peak time can be determined based on the following factors:

  • Size of the home and family.

  • How many bathrooms, tubs, dishwasher and washers are going to be used? Are you planning any additions in the future?

  • Do you have kids or teenagers as the hot water usage is increasing with their needs?

  • What is the pattern of the usage?

  • Quantity and type of fixtures.

Here are the consumption rates as an indication of how much water is required by the certain application. Use it as a reference when sizing a tankless water heater:

  • For shower or bath you need around 20 gallons.

  • Showers with energy efficient shower head - 2.5-3 gal/min.

  • Automatic dishwasher needs around 14 gallons.

  • Washing machine needs 32 gallons to wash the clothes.

  • For shaving you need 2 gallons.

How to calculate the size of a tankless water heater

To calculate what the peak demand for hot water is, list the number of devices you are going to use at one time and add up the flow rates. If the tankless unit is used to supply a shower and two sinks at the same time and if we assume that the peak demand for one shower is 2.5 gpm and 0.75 gpm for each sink, your total peak demand is 4.0 gpm.

Whole house heaters will usually provide 2 gpm to 4 gpm at 75 F temperature rise. Choose the tankless water heater model closest to the calculated flow rate and temperature rise needs.

Note: Standby heat loss or energy wasted when the hot water is not being used, was very important factor when calculating the tank size; but in a case of tankless water heaters and lack of storage tanks, standby heat loss can be completely eliminated. Tankless models also don't have the First hour rating, as it works instantaneous.

emergency plumber

Important parameters to consider

If the largest on-demand heater available cannot meet all your residential hot water needs, several tankless (up to 24 with Noritz multi-system) units can be linked into one system to provide enough hot water for every application and any house demand.

As the cold water passes through the heat exchanger the flow must have the right rate to ensure the proper heat transfer. If it is lower than required by the manufacturer, too much heat from the gas burner can affect the proper heat exchanger performance, deform it or if the safety element works properly, shut the unit off.

Many professionals consider the flow rate as the most important factor when sizing a tankless water heater. The flow rate depends upon the temperature of the incoming water. This is why it is very helpful to determine the climate area where you live (use a map below if you live in North America) so you can find the average ground water temperature.

Rinnai manufacturer of tankless heaters recommends that if the ground water temperature is between 65 and 75 F, for three applications, you can use their model R50LSi (or R94LSi for four applications) while during the winter where the incoming water temperature is between 45 and 55 F, you will use the same model but for two applications instead (R94LSi - three applications).

The flow rate can also be determined by calculating the temperature rise. The temperature rise is basically how much you need to heat the water to reach the desired temperature setting (120 F is the recommended).

For example, if you set the heater temperature to 120 F and your area's ground water temperature is 60 F, the difference is 60 F in temperature rise. Recommendation is to choose a residential tankless heater based on the colder seasons so that you will get the peak demand you need for hot water.

Gas input and heat power from heating elements tankless units can provide and energy factor EF, are also important factors when sizing a tankless water heater. Residential models for example, Energy Star qualified with EF 0.87 or higher have the maximum gas input of 199,900 BTU/hr while many condensing units are usually above this value.

Ground water temperature


Since the incoming water varies throughout the North America and it fluctuates, there are three climate zones that you have to consider when sizing a tankless water heater:

  • Northern zone with the average ground temperature between
    37 F and 51 F.

  • Central zone with the average ground temperature between
    52 F and 61 F.

  • Southern zone with the average ground temperature between

    62 F and 77 F.


As the tankless unit works on-demand and continuously supplying hot water, the tank size is irrelevant when sizing a tankless water heater.

If the tankless unit is not sized correctly you will have the output temperature the same, but the flow will be affected. If the flow is higher the temperature is lower. Water should be heated of at least 120 F where the minimum flow rate is usually 2 gallons per minute.

Suggestion is to calculate all the flow rates from the applications you will use and take into account the incoming water temperature. Lower temperature, more BTU you need. If one unit cannot supply enough hot water, two or more models can be installed by using the multi-system option.

If thinking between gas tankless heaters and electric, it is good to know that gas units can provide hot water at the higher flow rates than electric models. Be sure that flow rates will meet the peak demand at the temperature rise you need.

(Photo credit to Amazon)