What Size Gas Line Does A Tankless Water Heater Use?

Beginner Info, Kitchen, Tankless Water Heater

What Size Gas Line Does A Tankless Water Heater Use
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Well, in this post, I would be throwing light on how to use a perfect gas line for your tankless water heater.

Moreover, I would also be responding to the queries related to gas line dimensions in order to help the readers of izzysmarthomeguide.com make an informed decision.

So, without any further ado, let’s begin…

What Size Gas Line Does A Tankless Water Heater Use?

A typical tankless water heater would be needing a ¾-inch gas line.

Moreover, the gas line size to a large extent depends upon the BTU rating of the tankless hot water heater, the other gas appliances, and most importantly, where they are installed on each branch from the meter and regulator.

In general, two methods are used for determining the required pipe size, that is, the longest length method, and the branch length method.

In the longest length method, you need to determine the pipe size of each section by using the longest length of piping from the point of delivery, the gas meter, or even regulator to the most remote outlet and the load of the section.

On the other hand, in the brand length method, the pipe size of each section of the longest pipe run from the point of delivery to the most remote outlet MUST be determined by the longest run of piping and the load of the section.

Just for your information, branch length sizing is the most common method.

Proper pipe sizing is very important for the optimum performance of your tankless water heater.

In fact, proper sizing helps the system in maintaining the required minimum pressure drop.

Gas Pipe System Sizing

Line SegmentLine SegmentApplianceBTU RequiredLine LengthMinimum Pipe Size Required For 0.3 W.C DropMinimum Pipe Size Required For 3.0 W.C Drop
Branch 1ETankless Water Heater 199,900 20 ft 1” ½”
Branch 1GGas Furnace 75,000 15 ft ½” ½”
Branch 1DBranch Main Line 274.900 30 ft 1” ¾”
Branch 2JGas Dryer 25,000 20 ft ½” ½”
Branch 2HGas Range 55,000 30 ft ½” ½”
Branch 2KGas Logs 40,000 10 ft ½” ½”
Branch 2B + CBranch Main Line 120,000 45 ft ¾” ½”
Main TrunkA Main Trunk Line 394,900 20 ft 1 ¼” ¾”

From the aforementioned chart, it is clearly visible that a tankless water heater with a capacity of 199,900 BTU will require a 1-inch pipe size for a 20 ft branch length based on the 0.3 in w.c. pressure drop.

Furthermore, the same appliance would be required just a ½” pipe size based on the 3.0 in w.c. pressure drop.

Many of you would be inquisitive to know what actually a branch line is?

Well, a branch line is nothing but a pipe off the mainline that feeds a group of appliances.

The pipe size of the main pipe on the branch must be sized based on the total BTU of all the appliances on that branch line and pipe length.

The trunk line pipe is the main pipe from the meter/regulator that feeds the different branches. The trunk line must be sized based on the total BTU from each branch-line system or the sum of the total BTU of all the appliances on the system and pipe length.

However, exceptions are bound to be there.

For example, if the pressure is adequate and the run length is short enough, a ½-inch line may do the trick for certain models such as Noritz EZ Series.

Pipe Sizing Formula and Factors

What Size Gas Line Does A Tankless Water Heater Use

You can use the aforementioned formula for pipe sizing.

This formula is from the National Fuel Gas Code.

In order to find out Q, you need to divide the BTU capacity of the appliances by 1024.

Furthermore, to determine the allowable pressure drop, find the system static input gas pressure using a Manometer.

Afterward, you need to find the highest minimum gas pressure from all the appliances.

Then, subtract the highest minimum gas pressure from the static input gas pressure to get the difference.

Will a Tankless Water Heater Work On a ½-inch Gas Line?

I know very well quite a few would be inquisitive to know whether a tankless water heater would perform appropriately on a ½-inch gas line or NOT.

Frankly speaking, the short and sweet answer would be it DEPENDS, but I know very well, you’re NOT here for this silly answer, right?

Let’s dive deeper to find the CORRECT information that can help you to make an informed purchase decision.

In general, a typical residential gas system is low-pressure which simply means that the home is supplied with a gas pressure of around 7 in. w.c. (inches of water column).

So, the piping needs to be sized in such a way that the pressure drop is half an inch of the water column or less when all the gas appliances are in the ON state.

In layman’s terms, this can be a limiting factor in case you’re planning to upgrade from a conventional tank-styled water heater to a tankless counterpart.

You don’t have an option but to upgrade your gas piping system so that it can support the requirement of a tankless water heater, that is, the volume of fuel that is required.

For example, 200,000 BTU gas appliances will require not less than a ¾-inch gas supply line.

I hope we’re on the same page.

So, now let’s come straight to the question of whether a tankless water heater will work on a ½-inch gas line or NOT?

To be honest, in specific conditions a ½-inch gas line may be used.

In the 2012 National Fuel Gas Code (NFPA54. ANSI Z223.1), a 3.0 in. w.c. pressure drop chart was added for certain conditions.

This chart allows a 200,000 BTU gas appliance to be installed on a ½-inch gas line up to 40 ft. in length.

However, the following conditions must be met:

The minimum static gas pressure must be 8 in. w.c.or greater.

The calculated dropped pressure (the static pressure minus the 3.0 in. pressure drop), must be greater than the highest minimum gas pressure required by any of the gas appliances on the system.


In this section, I would be responding to the queries that we’ve received from our clients as well as readers of izzysmarthomeguide.com.

And, in case, I’ve not covered your question then please feel FREE to reach out to us via the contact form.

What happens if a gas line is too small?

If a gas line is too small then it will adversely impact the performance of your gas tankless water heater as well as other gas appliances.

In simple words, if a gas meter is undersized then it will lead to starving off the attached gas appliances for gas especially when the major appliances are running at the same time.

What size is the residential gas line?

In general, homes are equipped with gas lines that are 3/4 inch (“) in diameter.

However, quite a few homes do have a combination of 1-inch, 1/2-inch, and 3/4-inch gas lines.

What size gas line do I need for a gas stove?

Usually, the gas line coming into your kitchen will be 1/2-in. black threaded pipe and the connection to the stove will be either a male (external threads) or female (internal threads) 1/2-in. fitting.

What type of pipe is used for natural gas?

The most popular type of pipe that is used for natural gas is black steel.

However, some other available options are galvanized steel, copper, and brass.

Just for your information, in some areas use of copper gas pipe is prohibited so if you’re planning to use it then make sure that in your areas it is allowed to use.

How much pressure is in a residential gas line?

In general, the natural gas pressure of the residential gas line falls in the range of 1/4 psi to 60 psi.

The pressure in the residential gas line to a large extent depends on the number of homes or businesses served by the line.

On the other hand, gas lines that are used to move the gas from the wellfields to local utilities comprise pressure of up to 1,500 psi.

Wrapping Up

First and foremost, I would like to thank you for being with us till the end.

Secondly, I would like to bring to your notice that I along with my core team have put loads and loads of effort into coming up with this idiosyncratic superlative piece of information.

And, in case, you liked our effort and want to appreciate us then please do share this post with like-minded people and the ones who are in search of it.

After all, sharing is caring, isn’t it?

That’s all, as of now :):)

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