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How To Dimension A Tankless Water Heater Appropriately

Low circulate fee, poor temperature management, and sky-high set up prices — selecting the right measurement tankless water heater could be the distinction between you and your loved ones coping with these points.

In the event you’re available in the market for one in every of these tankless heaters however not sure how one can measurement one accurately, you’ve come to the appropriate place!

On this article, we’ll spotlight the components it is advisable think about when sizing your tankless heater. Moreover, we’ll stroll you thru an easy-to-follow, 3-step course of to set you up for fulfillment!


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What Are The Completely different Tankless Water Heater Sizes?

At first, the finest sizing issue for a tankless water heater is circulate fee, measured in gallons per minute (GPM). As such, you don’t measurement these methods as you’ll standard storage tank items.

Tankless heaters include completely different circulate charges; these methods can ship as much as round 10 GPM (at a good temperature rise).

A tankless system’s circulate fee will largely rely on whether or not the unit is suited to whole-house or point-of-use functions.


Whole-house tankless water heater.
Complete-house tankless water heater.
Supply: Dwelling Reworking Prices Information

Complete-house water heater items, also referred to as centralized water heaters, are massive and have excessive gallons per minute (GPM) circulate fee. This elevated capability signifies that they will deal with water stress demand from multiple faucet at a time.

For example, a whole-house water heater can provide scorching water to a operating bathe, dishwasher, and washer, all on the similar time. Naturally, this makes them a dearer choice than their point-of-use counterparts.

Typically, a whole-house system produces between 5 – 10 gallons per minute.


Point-of-use tankless water heater.
Level-of-use tankless water heater.
Supply: Tankless Membership

A degree-of-use (POU) tankless water heater is smaller than a whole-house system. As such, they’ve a decrease circulate fee, to allow them to solely cater to the wants of 1 faucet/outlet.

On the upside, they’re cheaper, and their measurement means which you could place them inside cramped areas, like a closet or cupboard.

This water heater can turn out to be useful for added taps that your present system could not cowl. For example, a POU water heating system can be good for a property with a separate constructing/room.

Typically, a POU unit produces between 0.5 – 2 gallons of scorching water per minute.

Tankless Water Heater Sizing — Elements To Think about

To measurement your water heater accurately, you’ll want to contemplate the next components:

Households Circulation Price

In the event you learn our article, A information to Tankless Water Heaters, you’ll recall that these methods solely warmth water on demand. It is because, not like standard storage water heaters, these tankless methods don’t include a storage element.

What does this should do with circulate fee?

A standard storage system heats its storage tank intermittently to take care of the suitable water temperature whereas a tankless system doesn’t; a tankless heater has to warmth water because it travels by way of its methods, slowing down the circulate fee.

It, due to this fact, stands to motive why circulate fee is seen because the most vital tankless water heater metric.


The next are examples of home equipment circulate charges in Gallons Per Minute (GPM):

Equipment Circulation Price (Gallons Per Minute)
Rest room 2-3
Bathe 1.5 -3.0
Toilet/Kitchen Faucet 2-3
Dishwasher 2-4
Washing Machine 3-5

Temperature Rise

Temperature rise is the distinction between the temperature of groundwater (water flowing in) and the set temperature of your tankless water heater (water flowing out).

Groundwater temperature varies in response to location, so it’s finest that you just consult with a groundwater temperature map (or measure your water temperature) earlier than you buy your water heater. We’ll deal with this additional within the part to come back.

Family Dimension

This encompasses each the literal measurement of your property’s stature (distance between rooms and, due to this fact, pipes) and the variety of occupants residing in your house.

Electrical/Fuel-Fired Tankless Water Heater

Electrical and gas-fired tankless water heaters differ considerably from each other — they don’t have the identical ranges of effectivity.


A gas-fired tankless water heater can meet a 70 °F temperature rise with a circulate fee of 5.0 – 5.5 GPM, whereas a big (36 kW) electrical heater maxes out at somewhat over 3.0 GPM.

How Do You Appropriately Dimension a Tankless Water Heater For Your Dwelling?

Step 1: Decide Most Temperature Rise

As beforehand talked about, that is the distinction between the tankless set temperature and the wintertime chilly water temperature.

To find out your temperature rise, use the next calculation:

Desired Output Temperature – Incoming Water Temperature = Temperature Rise

How To Decide Groundwater Temperature

There are two methods so that you can decide your groundwater temperature:

  • You’ll be able to consult with a groundwater temperature map (as seen under).
  • Alternatively, you may run a thermometer underneath a cold-water faucet that can assist you decide the precise temperature of your groundwater.

The next map illustrates the various groundwater temperatures all through the U.S.

U.S. Ground Water Temperature Map — tankless water heater size.
U.S. Floor Water Temperature Map.
Supply: Rinnai


Let’s use an instance to place the above system into follow.

Our house is positioned on the central coast of California. We’ll estimate that the groundwater temperature is 55 ℉ (12.7 ℃) primarily based on the map.

Subsequent, we want a set temperature from our tankless water heater; let’s assume a set temperature of 110 ℉ (43 ℃).

Primarily based on the above values, our calculation will look as follows:

110 – 55 = 55

Subsequently, our family would require a temperature rise of 55 ℉ (12.7 ℃).

Step 2. Peak Scorching Water Demand Of The Dwelling

Now it is advisable decide what number of scorching water fixtures you’ll be utilizing concurrently (throughout peak use) to calculate your water use in Gallons Per Minute (GPM).

How To Decide Circulation Price Of An Equipment

In the event you don’t know the circulate fee, estimate it by holding a pan or bucket underneath the tap or showerhead and measure the circulate for a minute. Measure the quantity of water and multiply by 60 to get the gallons per minute (or liters per minute).


For our instance, we’ll use the next:

  • Bathe (x2): 2 GPM every
  • Washing Machine: 2 GPM

Whole: 6 GPM

Step 3: Choose The Applicable Unit/s

Lastly, it’s time to pick a unit able to assembly your circulate fee and temperature rise calls for.

How To Decide The Applicable Unit

Most tankless water heater producers present their prospects with product-related spec sheets.

Except for basic specs, these sheets typically include circulate charts (see under).

Circulation chart indicating appropriate tankless water heater fashions for corresponding GPM & temperature rise variables.
Supply: Rinnai


For our instance, we want a tankless heater that may present us with:

  • A temperature rise of 55 ℉ (12.7 ℃)
  • A peak circulate fee of 6 GPM

Wanting on the above chart from Rinnai, we are able to decide that any of the orange or inexperienced color-coded fashions will meet our calls for.

Can A Tankless Water Heater Be Too Large?

Sure, a tankless water heater could be too large for a family.

Whereas a bigger than mandatory tankless heater received’t trigger any injury to your family, it received’t carry any benefits. In truth, there will likely be appreciable cost-related disadvantages.

Tankless water heaters don’t come low-cost. Their set up prices, particularly, are substantial. So it stands to motive that buying a bigger than mandatory heater will include the upper unit, set up, and operation prices.

Sizing A Tankless Water Heater For A Household

Sadly, this quite common query doesn’t have a concrete reply.

Certain, you will have come throughout some makes an attempt at answering it, however in all honesty, there are too many variables to contemplate.

Circulation fee, variety of occupants, groundwater temperature, and the kind of tankless heater (gas-fired or electrical) are simply a number of the components that you just’d want to contemplate.

As such, it’d be unrealistic and even irresponsible for us to offer you actual numbers.

As an alternative, we advise that you just measure your circulate fee and your temperature rise to find out the tankless water heater measurement that most closely fits your circumstances.

Ultimate Ideas

Now, let the sense of aid wash over you — see, that was quite a bit easier than you will have anticipated.

Realizing how one can measurement a tankless water heater doesn’t should be tough. All it is advisable do is decide your temperature rise, calculate your circulate fee, and choose a tankless unit that meets your calls for.

It’s so simple as that!

If, nonetheless, you’re feeling that you just’re nonetheless struggling or have any associated questions, we invite you to be part of our group and get in contact with one in every of our specialists.



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