This guide will help you choose which natural gas or propane tankless water heater to buy. has been selling tankless water heaters since 2010. We have answered thousands of questions over the years, and distilled it all down to one page, right here for you. Please comment below with any feedback.

Quick key to abbreviations. (Abbreviations make the article shorter and easier to read.)
TWH = tankless water heater
BTU = British Thermal Unit
GPM = gallons per minute
NTI = needed temperature increase
TI = temperature increase

The 3 Things You Must Know to Choose the Right Gas Tankless “Size” or BTU capacity

Tankless water heaters are 70% smaller than tanks.

Tankless water heaters are 70% smaller than tanks (average)

People shopping for a gas or propane TWH need the following info to know how many BTU to get in a heater.

1. Your coldest water temperature of the year. If you don’t know, then follow these instructions.
1.1.  Is it a really cold week in winter, right now? If so, then run the cold water under a thermometer for the full amount of time you plan to use that point of use. (Why? Because the pipe temperature inside the house/building is affecting the water temperature, and you may or may not get the actual cold water temperature from your water service.)
1.2. If it’s not one of your coldest weeks in winter right now, then the method above might lead you to underestimate the size of TWH you need when that cold weather arrives. Instead, use this table of Cold Water Inlet Temperatures for Selected U.S. Cities. (We do not recommend the commonly shared maps of average groundwater temperature: Your groundwater temperature may be very different from your cold water temperature, as supplied by the public utility.)

2. Sum up flow rates of all the ways that you might use hot water simultaneously. Here are the flow rates for common uses:
Shower head (low flow): 1.5 GPM
Shower head (standard flow): 2 GPM
Shower head (high flow, big showerhead): 2.5 GPM
Kitchen sink faucet: 1.5 to 2 GPM
Bathroom/lavatory sink faucet with aerator: 0.5 to 1 GPM
Bathtub faucet: 4 GPM!
Clothes washer: 2 GPM
Dishwasher: 2 GPM (but you probably should not include this in your calculation, since it is unnecessary to use your valuable, available TWH hot water flow rate)

3. Your needed temperature increase (“NTI” for short). Subtract the hot water temperature you seek from the temperature in question #1. Example: You decide the hottest water you need is 105°F shower temperature. In Texas during the coldest winter weeks, the incoming water temperature might be about 45°F. Subtract 45°F from 105°F. The difference is 60°F. That’s the NTI.
However, you might want hotter water for clothes washing. But note that many current washers have a built-in temperature boost. And almost all dishwashers have that.

Use the Data to Find the Correct TWH “Size”

Look at the specifications of one of the TWH models you saw. Find the table or spec showing the temperature increase (“TI” for short, also called temperature rise, temperature delta, delta T) at a given flow rate. Often it is not shown on Amazon or eBay, but you absolutely must have this info to make the correct choice. All of the product descriptions on have TI vs. flow rate data.

Now, go back to the total simultaneous flow rate that you found in #2 in the previous section above. Find the number on the TWH’s TI table.

Next, we do some simple math to determine whether a TWH will heat water to 105°F.

Let’s say that you are considering a TWH that provides a TI of 39°F at a flow rate of 2.5 GPM. Take your flow rate number in #2 above, and with simple math, convert the TI with that flow rate. Example for one low-flow showerhead using 1.5 GPM:
1.5 ÷ 2.5 = 0.6
39°F ÷ 0.6 = 65°F
Therefore, that heater provides a 65°F TI using one low-flow showerhead alone.

Now for example, let’s say a second person begins to shower, also with a 1.5 GPM low-flow showerhead. Will there still be hot water for both? Simply divide your TI by 2. Now, both showers will have a 32.5°F TI. Big difference, right? This is the way tankless works: A heater will heat water with a lower TI at a higher flow rate, (and a proportionately higher TI at a lower flow rate). All tankless water heaters work on the same principle.

Your maximum NTI and water flow rate makes a big difference in output temperature for each point of hot water use, and thus tankless water heater selection, as you can see.

If a TWH doesn’t have a high enough TI using the flow rate number from #2, then:

  • Reconsider if you realistically will use all those points of hot water use simultaneously. For example, can you run the clothes washer at different time than your shower?
  • Choose a heater with more BTU and/or higher efficiency rating (also referred to as Energy Factor, EF or AFUE). A heater with a higher EF will produce more hot water than one with a lower EF, at the same BTU.

Installing indoors with vent pipe, or installing outdoors?

If you are in a climate where freezing is rare, such as south Florida and many parts of California, then consider an outdoor model to bypass the need for venting. If you need stainless steel venting for a non-condensing heater, GadgetsGo has a full selection. Here are some of our heaters descriptions that have detailed information about venting:
Marey 16L propane venting (scroll down the product description to the venting section)
Marey 16L natural gas venting (scroll down the product description to the venting section)
Rinnai RV53iN venting (scroll down the product description to the venting section)

Condensing or non-condensing?

If you are installing your TWH indoors, then consider the next major decision: a condensing or non-condensing TWH. Here are the main advantages and disadvantages to condensing tankless water heaters.
Condensing pros:
♥ Most condensing models allow for plastic pipe such as PVC, CPVC or polypropylene. This material is inexpensive and readily available locally.
♥ Condensing models are highly energy efficient. Most are more than 92% efficient. Navien TWHs have energy factors as high as 99%, the highest in the industry.
Condensing cons:
Condensing models are 30% to 50% more expensive per BTU than non-condensing.

Here are the main advantages and disadvantages to non-condensing tankless water heaters.

Non-condensing pros:
♥ Non-condensing models are about 20% to 30% less expensive per BTU than non-condensing.
Non-condensing cons:
 Non-condensing models require stainless-steel vent pipe such as Z-Vent. Stainless-steel is an expensive material, about three times more expensive than plastic. is fully stocked on the vent pipe, but it is typically not available locally. 
⇓  Non-condensing models are not as energy efficient as condensing, but still much more efficient than a hot water tank. Energy factors are usually in the 80% to 90% percent range.

Does all that make sense? Do you know which tankless water heater to look for now? If not, you can contact us with your info in #1 through 3 above (or whatever you can determine for #1, 2 and 3). Also decide whether you want indoor or outdoor, condensing or non-condensing. Then, we will give you some free recommendations.

Ready, Set, GadgetsGo

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