What Size Gas Line Do I Need For Furnace?

Adding furnaces is very important in houses, especially if you live in a very cold climatic region. But buying one for your house is not the end. You need to get the right gas line size for the unit. This article will help you know how to choose the right size of gas line for a furnace. 

As a general rule, you can use ½ inch-¾ inch gas line for the furnace, depending on the size of the furnace. To determine the right gas line size, you must find your furnace’s BTU and the gas line’s length and pressure. After that, you can receive a proper gas line size estimation. 

A few factors require consideration while selecting the right gas line size. This article has everything you need regarding the gas line size for the furnace. We shall also share a few tips for installing the gas line in the furnace. 

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What size of gas line is needed for a furnace?

Installing furnaces in the houses is very important as it keeps your house warm and comfortable in the winter. 

It is more important in colder climates, as the winters are very harsh. 

When buying a gas fuel furnace, you must consider the gas line. 

The gas line is plumbing that runs throughout the house, and individual appliances use branch lines. 

Selecting the right gas line size for a furnace will help in efficiently passing the gas through the furnace. 

Determining the right gas line size for any furnace can be confusing because there are furnaces with multiple sizes. 

The BTU capacity determines the furnace sizes. 

The BTU capacity will affect the gas line size. 

Therefore, you will require different sizes of gas lines for furnaces with different BTUs. 

In most cases, ½ or ¾ inch of lines are used. But, there is no exact measurement. 

There exists an instruction book informing about the different gas line sizes based on the BTU capacity on the furnaces. 

It should tell you what gas line size to use with the furnace. 

You can also specify it yourself by some factors or methods, which I will discuss in the coming sections. 

How to determine the right size of gas line for a furnace?

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The gas line has to be in the proper size to handle and support the BTU capacity of the furnace. 

Otherwise, you may end up having serious issues. 

The furnaces come in various sizes with BTU values, like 20,000, 30,000, 60,00, and more. 

So, your system must carry the BTU capacity of the furnace, and you must measure a few things for that. 

You must know the BTU of the furnace, the length of the gas line, and the gas line pressure to determine the gas line size. 

You also need to check whether the place of the furnace is attached or detached from your house. 

Once you consider everything, you will receive the right gas line size. 

Let’s talk about these factors in detail. 

BTU value of the furnace

Before you start deciding the gas line size for your furnace, you must know the BTU capacity of the furnace. 

BTU is a unit that calculates the thermal energy of any unit. 

The BTU determines the size of the furnace, and the gas line size highly depends on this BTU.

The BTU capacity can range anywhere. It can start with 20,000 or less or end with 100,000 or even more. 

So, based on this BTU capacity, you must get the perfect gas line size for the furnace. 

For example, if you have a furnace below 35,000 BTU, you will need a gas line with ½ inch diameter. 

If the BTU is more than 35,000, you will require a gas line with ¾ inch diameter. This gas line size can handle furnaces with a BTU capacity of up to 80,000.

To receive the best from your furnace, you must have a good ceiling and insulation. R-13 would be a good choice. 

The location of the furnace

The furnace can be placed in any room. 

You must consider the location of the furnace, whether it is an attached room or a detached one.

For example, you have a detached garage and want to install a furnace. 

In that case, you will require a longer gas line than normal. 

Things can be easier if the garage is attached. 

Gas line length

Like the other factors, measuring the length of the gas line is also vital. 

It will help you determine the right gas line size for your furnace. 

If the length of the gas line is not too much, a ½ inch gas line size will be fine for your furnace. 

A ½ inch gas line pipe would work well for a 10 or 20 feet gas line length. 

The size will change if the length is too short or too long. 

The length can increase if you wish to install the furnace in any detached area, like a garage or workshop. 

In that case, the length of your gas line has to be 100 feet long, and you need a gas pipe measuring ¾ inch in diameter to handle such long lengths. 

Even if the BTU is high and the distance is long, a ¾ inch gas line would be enough.

Gas line pressure

The length can also affect the water pressure. 

For example, if you use a ⅜ inch pipe for 10 feet, it will exert a water pressure of 95. 

The same size of gas line used at 80 feet will decrease the pressure to 31. 

You need 11 inches of water column for the gas to flow properly. 

The same gas line size can support up to 0.6 PSI of pressure every 100 feet. 

Most gas lines have a pressure ranging between 5 and 10 PSI.

Number of appliances

Some appliances in the houses use natural gas. 

You need to know which of the devices in your house uses natural gas to operate. 

You will need a more extensive gas line for your furnace if you have many such devices. 

Each appliance uses a certain amount of gas to operate. 

So, this can affect the gas line size to add this usage. 

Natural gas used daily 

You need to find out the amount of natural gas to use daily in cubic feet. 

It will let you know the amount of natural gas your house needs, which further helps get a significant gas line for the furnace. 

If you use less gas, you may use a smaller gas line. 

Otherwise, you require an extensive one.

For example, you need a ½ inch gas line for a furnace having 30,000 BTU of energy capacity. 

But, you will require a gas line sized ⅜ inch if your house consumes 1,000 to 1,200 cubic feet of natural gas daily. 

Using the gas line with the highest capacity for your furnace is always better.

How far can the gas lines run?

    The distance a gas line can run completely depends on the size of the gas line. 

    For example, if you use a ½ inch gas line, you can run it up to 40 feet. 

    You must upgrade the size if you want to use a gas line for further distance. 

    You can use ¾ inch gas lines for more than 40 feet. 

    The distance can differ based on the gas line size. 

    I shared about these two gas line sizes because they are the most commonly used gas lines applicable to all furnace sizes. 

    How to install a gas line for a furnace?

    Once you have determined the right gas line size for the furnace, it is time to know the steps for installation. 

    Here I will share a few tips for installing a gas line for the furnace. 

    However, you can always hire a professional if you feel uncomfortable doing it yourself. 

    • Make sure that the furnace’s power is off before you start any work related to it. 
    • If you are running a new gas line, use a ¾ inch of black steel pipe; for example, schedule 40 or 80. For natural gas, use schedule 40; for propane, use schedule 80.
    • Use the fittings suitable for the gas lines. Do not use improvised fittings, or there could be chances of leakages. 
    • Ensure the tightness of every connection. Use 2 wrenches to tighten the fittings properly so that one twist can hold the fitting and the other can tighten the connection. 
    • Once you have installed the line, turn on the gas and test for leakage. Use soapy water and brushes to check the connection for bubbles. Bubbles mean there has been a leak; otherwise, everything is perfect. Fix the leakages, if any. 

    These steps will ensure that the gas line has been properly and safely installed. 

    Besides the ¾ inch black steel pipe, you also need fittings and proper connectors for the gas lines. 

    You must ensure that the connections are tight enough. 

    Always test for leakages and fix them immediately before starting the furnace and gas lines. 

    Different gas line sizes for furnace sizes.

    There are no sizes of furnaces. 

    It is the BTU value that differentiates the furnace and its size. 

    Since the furnaces have different BTU values, the gas line will differ accordingly. 

    Here is a small table guide about a few furnace and gas line sizes. 

    FurnaceGas line sizes
    30,000 BTU½ inch, and ⅜ inch if the house uses 1,000 to 1,200 cubic feet of natural gas per day
    35,000 BTU½ inch
    38,000 BTU¾ inch
    60,000 BTU½ inch
    80,000 BTU¾ inch and 1 inch if the gas line is longer
    100,000 BTU½ inch
    Furnace size and Gas line size guide

    How much does it cost to install or repair a gas line?

    The gas line installation cost can be around $12 to $25 per linear foot, including labor, material, piping, and permission.

    The separate labor cost to install a gas line can cost around $3 to $11 per foot, based on the pipe’s diameter and job difficulty. 

    Licensed plumbers charge around $45 to $150 per hour and sometimes $100 per hour for new gas lines.

    The repair will cost $6 to $7 per foot. 

    It may cost $355 to $743 per gas line from an existing connection and $500 to $2,000 from the meter. 

    Converting to natural gas or getting a new line from the street to the house can cost around $2,000 or more. 

    Final thoughts

    The size of the gas line for a furnace does not exist. 

    Different furnaces have different BTU values, and the gas line differs depending on the BTU value. 

    Consider certain factors when deciding the gas line size for a furnace. 

    The factors include the BTU value of the furnace, the gas line length, gas line pressure, house size, appliances, and daily gas use. 

    However, despite the different BTU values, ½-inch and ¾-inch gas lines are mostly used for furnaces. 

    It can change based on the length, pressure, and other things. 

    You can follow the table I shared in the article for a proper gas line. 

    However, those are not the exact size but approximate measurements. 

    It may change based on several factors.

    You can consult an expert or follow any instruction book to get the right gas line size for the furnace. 

    Related Articles:


    Can I use a flexible gas line for the house?

    You may hear many things about this, but it is considered safe and functional. You can use it for short runs, for instance, from the stove to the wall. 

    When the cooktop feeds the gas, the clothes, the dryer, or the gas range, the brass appliance connector tubing will be used in the house. 

    Where can I use a flexible gas line?

    CSST is a flexible stainless steel tube that gives natural gas and propane to houses, businesses, and industries. 

    Do not get confused with the appliance connectors. 

    They are flexible pipes connecting the wall or floor to the phone, like an oven and dryer. 

    Can I use PVC for gas lines?

    PVC pipes can adapt to any conditions; thus, you can use them for natural gas and water lines. 

    Due to the high pressure of natural gas, it is better not to use PVC pipes. Instead, you can use a black steel pipe. 

    Reference: Furnace Science Direct, Effects of HVAC on combustion-gas transport in residential structures, HVAC System and Part of Indoor Air Quality.

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    Arthur Smith

    Howdy! I am Arthur Smith, an electrical engineer who is extremely passionate about electronics. I have lived in different parts of the US and currently reside in Wisconsin. I am one of those rare children who knew what he wanted to study and do in life right from the start. I was a curious child who wanted to know how switches work and how the AC works, and I would always observe my dad whenever he would handle the wires and fix things around the house. I currently work as an electrical engineer at a reputed company and write for this blog. And I read loads of books or play video games in my free time.

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