Furnace Exhaust Pipes – Here’s What You Need to Know

As far as safety is concerned, the exhaust pipe is one of the most important parts of a furnace. Why is that?

The exhaust pipe is responsible for removing poisonous flue gases from your furnace. If the exhaust pipe is blocked or broken, harmful gases can get into your home’s air.

In this article, I’ll explain what a furnace’s exhaust pipe is for. I’ll also discuss what factors you need to consider when venting a furnace, and how to identify a furnace by its exhaust pipe.

How does the furnace exhaust pipe work?

When your furnace burns gas, it creates heat, water vapor, and carbon dioxide. As part of the combustion process, by-products are produced: Nitrogen oxides, sulfur oxides, and most notably—carbon monoxide (CO).

The exhaust pipe’s job is to remove these harmful gases from your home and vent them outside.

How does the exhaust pipe remove flue gases from your furnace?

The exhaust pipe is attached to the inducer fan. The inducer fan creates a draft through your furnace’s combustion chamber and heat exchanger. As the gas undergoes combustion, the flue gases flow through the heat exchanger and exhaust pipe. The flue gases exit the exhaust pipe and get dumped outside your home.

Where is the furnace exhaust vent located?

Your furnace’s exhaust vent is located on the outside of your home. 

If you have a standard-efficiency furnace, then your exhaust vent should be located on the roof.

If you have a high-efficiency furnace, then your exhaust vent should be located on the roof or on the side of your home.

Why does the location of the exhaust vent differ between standard and high-efficiency furnaces?

It’s due to the contents of the furnace exhaust.

In standard-efficiency furnaces, the exhaust contains hot gases. Because of the high temperature of the exhaust, the gases naturally draft up. This is why the exhaust vent for a standard-efficiency furnace is located on the roof of the home.

In high-efficiency furnaces, the exhaust contains mostly water and carbon dioxide. The exhaust from a high-efficiency furnace is not hot enough to rise up on its own. Instead, an inducer fan is used to push the exhaust out of the house through a venting pipe.

High efficiency (condensing) furnace venting

Venting for high-efficiency condensing furnaces is a complicated topic since there are many factors at play. Especially if you’re retrofitting a high-efficiency furnace into an older home.

There are two types of high-efficiency furnace venting setups: single-pipe and two-pipe. Both have their pros and cons. The one that is best for your home depends on a myriad of factors:

  • Availability of existing venting systems to use as a chase
  • Local government codes and homeowner association rules
  • Location of the furnace in your home
  • Venting setup of other appliances
  • Airtightness of home

For more information about venting a high-efficiency furnace, check out my article below:

What if you have PVC furnace intake and exhaust pipes?

If you have PVC intake and exhaust pipes for your furnace, then you have a high-efficiency furnace.

Why are PVC pipes needed for high-efficiency furnaces?

PVC pipes are needed for high-efficiency furnaces due to the contents of the furnace’s exhaust.

Exhaust from high-efficiency furnaces contains acidic water. The acidic water will corrode metal exhaust pipes quickly. PVC does not corrode, so PVC pipes are used instead.

Furnace PVC exhaust pipe condensation

If you have a high-efficiency furnace, then its PVC exhaust pipes will need to be sloped towards the furnace.
Specifically, most installations require a minimum of ¼” of slope per foot of pipe. This allows the furnace condensate to flow back towards the furnace’s drain pipe. This keeps the furnace’s condensate contained inside your furnace’s drainage system.

Why can’t the furnace condensate be drained outside?

There are two reasons why furnace condensate shouldn’t be drained outside:

  1. If the condensate flows outside, then it may freeze if outdoor temperatures get too low. Frozen condensate can break your PVC exhaust piping.
  2. Some areas don’t allow you to drain furnace condensate onto the ground because its acidity may pollute the groundwater.

What if you have a metal furnace exhaust pipe?

If your furnace’s exhaust pipe is made out of metal, then you have a standard-efficiency furnace.

Why are metal exhaust pipes needed for standard efficiency furnaces?

Metal pipes are needed for standard efficiency furnaces due to the high temperature of the exhaust.

How hot is gas furnace exhaust?

Exhaust temperatures from standard efficiency furnaces are usually in the 300-400° F range. The exhaust gases from a standard-efficiency furnace are hot enough to melt a PVC pipe. That’s why standard-efficiency furnaces use metal exhaust pipes.

Do you need a gas furnace exhaust fan?

Almost all furnace exhaust systems use an exhaust fan to push the flue gases outside. An exhaust fan for furnace flue gases is called an inducer. The inducer is built into the furnace and turns on whenever the furnace heats your home.

Some older furnaces use a natural draft system to expel flue gases from the home. In a natural draft furnace, there is no inducer fan. Instead, the hot flue gases naturally flow upwards out of the home through a flue pipe or chimney.

If you have an older natural draft furnace, you probably don’t need an exhaust fan. Natural draft furnaces are installed in such a way that does not require an exhaust fan for the flue gases.

If you have a newer furnace then your furnace will have an inducer fan built into it. So there’s no need to install a separate exhaust fan.

Hi, my name is Trey Lewis and I’m the founder and chief editor at HVAC Training Shop. My goal for this website is to help homeowners troubleshoot and maintain their home’s HVAC systems. Whether it’s changing an air filter, troubleshooting a blower motor, or just buying a new humidifier, I want to make sure that you’re covered.

14 thoughts on “Furnace Exhaust Pipes – Here’s What You Need to Know”

  1. I have a standard efficiency furnace with metal exhaust piping and have a condensation problem with water dripping into my fireplace in the winter when it is not raining

    • Hi Pat,

      Standard efficiency furnace condensate can be caused by an improperly sized exhaust pipe or combustion issues. Either way, it’s a serious issue and you should turn off your furnace and get it checked out by an HVAC professional ASAP.


  2. I have a high effiency furnace using black pvc pipe and had my furnace tech installing a digital thermostat told me it should be white pvc because the black one will crack over time ,is this true

    • Hi Dwayne,

      The color of the pipe does not matter.

      However, there’s a good chance that the “black PVC pipe” installed on your furnace is actually ABS pipe. Certain types of ABS pipe can be used for furnace venting in some areas. It depends on your local codes.

      Some older types of ABS pipes were known for cracking. However, the newer types of ABS don’t have the same issues, as long as they are made for venting purposes.

      Hope this helps,

  3. I have a high efficiency propane furnace (not natural gas) and I have two pipes coming out of the roof. The fresh air intake pipe is about 1 1/2 feet shorter than the exhaust pipe and has an elbow on top facing away from the exhaust pipe. For the second time, I have smelled furnace exhaust in my house. I have had to open the doors to clear the air. I have kept the snow away from the pipes. (our snow is measured in feet). Is it possible that, depending on the barometric pressure, the heavier exhaust gas is floating down and getting sucked into the fresh air intake pipe? My furnace is about three years old and this has only occurred twice.

    • Hi Sharon,

      I’m not sure if your “fresh air intake pipe” is for fresh outside air for your home or furnace combustion air. Either way, you should not be smelling furnace exhaust in your home. I recommend getting an HVAC professional to take a look at your system.

      Hope this helps,

  4. I had a new high efficiency furnace with a concentric pic exhaust pipe/air intake set up installed last winter. We had no problems last winter. This year we are hearing loud knocking noises where within 1 foot of where the pipe leaves the side of the house, We’ve been told by several heating contractors that they have never heard of such a thing, but could be the pic pipe expanding and contracting. So far no one has come up with a solution to get rid of the knocking. Any thoughts.

    • Hi Linda,

      My guess is that the PVC pipe is expanding into the side of your house when it heats up.
      If there isn’t enough clearance between the pipe and the hole that it goes through, the pipe will sometimes make popping noises as it expands and rubs up against the side of the hole.

      My recommendation would be to ensure that the pipe moves freely through the hole and has a gap around it to account for thermal expansion.

      Hope this helps,

  5. Hello
    I have a 3 yrs old house with high efficiency furnace (with a concentric pic exhaust pipe/air intake set up). Staring last year,
    drifting snow and/or ice formation block the exhaust or intake pipe or both and cause the furnace to go into safety lockout mode and shut down automatically. Usually I clean (couple of times, in the middle of the night) the exhaust/intake pipe/vents outside & the furnace starts to work again. My questions why this happens? is this normal problem? is the issue with the way the pipes installed or the fan inside the furnace or what?
    Thank you

    • Hi Ephrem,

      Your issue seems to be the location of the intake/exhaust vent. I recommend getting an HVAC professional to check out your system to see what can be done to fix it.

      Hope this helps,

    • Hi Al,

      Great question. The roof vent cap is needed to protect the interior of the exhaust pipe and prevent debris from getting inside of the furnace’s exhaust venting system.

      Hope this helps,

  6. I have a HE Trane gas furnace with two long pipes (about 40 feet) vented through the side of the house. My furnace is shutting down after a few minutes and I have isolated the trigger to the flame rollout sensor. I’ve read that an exhaust obstruction could lead to improper combustion and cause this sensor to trip. The PVC exhaust piping is all tightly glued together and clamped down. Do I need to start cutting to gain access to clean it? I tried suction with a large shop vacuum at the exhaust end but I don’t think it provided enough vacuum to overcome the long run of slightly upward tilting pipe.


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