How To Cover Furnace Exhaust Pipe

Disclosure: We may get commissions for purchases made through links in this post.

Most homeowners prepare for the winter by double-checking their insulation and other home barriers to ensure that they're able to stay warm during the cold seasons. This may also include covering the furnace exhaust pipe. But how exactly do you cover a furnace exhaust pipe? We have researched the best method for this project, and in this post, we will go over it.

The best and most common way to cover a furnace exhaust pipe (also known as a "flue pipe") is to apply a vent screen cover. Here are the steps to do it:

  1. Clean the flue pipe
  2. Measure the pipe
  3. Prepare the vent screen
  4. Apply sealant (optional)

The great thing about these covers is that they're fairly inexpensive and pretty easy to install. However, caution should be taken when installing these caps, as incorrect installation can lead to issues with your flue pipe. Covering your furnace exhaust pipe can be a crucial step in preventing venting issues during the winter season. Continue reading to learn how to cover your flue pipe with one.

A furnace exhaust vent pipe blowing steam to the cold winter air, How To Cover Furnace Exhaust Pipe

Steps To Cover Furnace Exhaust Pipe

You can purchase a vent cover online or at a hardware store and install them yourself. These covers are typically made of PVC with a metal mesh center that prevents debris, wildlife, and other objects from getting inside of the pipe. They are also known as "vent caps" and typically come in sizes anywhere from 2 to 6 inches in diameter.

A typical chimney pipe with a rain cap

Things you'll need:

  • Vent screen
  • Cleaning cloth or paper towel
  • Screwdriver
  • Roofing ladder

1. Clean the flue pipe

Before installing the cover, it's imperative that you clean the flue pipe. This will help the cover to adhere properly. Dampen a paper towel and wipe down the interior of the pipe. If there is debris or dead bugs inside the pipe, be sure to remove it all so that it has no obstructions.

Read more: How To Clean Furnace Vents In 7 Steps

2. Measure the pipe

Next, use your measuring tape to measure the width of the flue pipe. Most vent pipes will be around 2 to 6 inches in width, so it should be easy to find vent covers to fit these sizes.

3. Prepare the vent screen

Most vent covers are designed to fit directly inside of the flue pipe. You may need to wiggle it a bit, but you should be able to slide the screen inside the top of the flue pipe. If the screen doesn't fit, chances are that you have the wrong size.

Some vent screens will be attached to a cap that will fit on the outside of the flue pipe. And if this is the case, you'll need to slide the vent cap over the tip of the flue pipe.

4. Apply sealant (optional)

This step is optional, as it's not necessarily needed to install the cover. If you're attaching a vent cap to the exterior of the flue pipe, you can apply sealant on the exterior of the cap to insulate the seam. It's best to use high-temperature sealant for this application, as the fumes coming from the vent will be pretty hot as they pass through.

Find this high-temp sealant on Amazon.

Want a visual? See how it's done here:

Should a furnace exhaust pipe be covered?

If you ask more than one HVAC professional whether your furnace exhaust pipe should be covered, you may find arguments both for and against it.

Many specialists believe that a vent screen can obstruct the natural airflow of the pipe and cause issues with the furnace. And other specialists argue that the covers aren't efficient at keeping out insects and rodents.

However, many roofers and HVAC technicians will also tell you that these vent covers can help to prevent most problems as they relate to wildlife and debris entering the flue.

It's best to make sure that you have the right vent screen product, as all products are not created equal. For example, if you're worried about rodent prevention, it's best to use a vent that has a sizable checkered grid installed at an angle of 45 degrees. This diamond-shaped pattern will make it way more difficult for birds and rodents to grip the screen to open it.

When shopping for covers, you'll also want to know the airflow rate, as it measures the amount of air that can pass through the screen in comparison to the pipe being fully open. For example, most checkered screens have an airflow rate of about 85% to 90%.

While tightly closed vent screens have an airflow rate of about 70%, it's best to use screens with a higher airflow rate in the fall and winter months when protection from small insects and bugs isn't a big issue.

Can mice enter through furnace exhaust?

Furnace vent ducting connecting to the furnace

Yes, rodents have the ability to squeeze their bodies and contort into sizes that are 1/4-inch thick. This allows them to enter your flue pipes, door cracks, and other small openings of your home relatively easily.

Also, as outside temperatures drop during the fall and winter months, rodents will naturally gravitate indoors, meaning that they present a bigger problem during this time of the year. The best way to prevent them from entering the pipe is to cover it with a vent cover that has a mesh grill.

What comes out of my furnace exhaust?

An exhaust vent connected to the furnace vent

Furnace exhaust pipes emit toxic fumes created during the combustion of gas. For the most part, these pipes emit carbon monoxide, which is dangerous if inhaled. It's also worth noting that local and state building codes regulate the length and location of furnace exhaust pipes.

Suppose you're concerned about carbon monoxide leaks or your flue pipe potentially becoming detached. In that case, it's best to have a carbon monoxide detector near your furnace and somewhere on the exterior of your home.

What happens if furnace exhaust is blocked?

A dented furnace exhaust vent

A blocked furnace exhaust pipe can be very dangerous. If you notice that there is condensation on the windows to your attic or basement, chances are that the furnace's vent is blocked.

When there are issues with the exhaust, the fumes emitted will cause condensation, resulting in increased humidity levels in the room where the furnace is located.

Also, if you notice rust stains on the grill of the vent cover, it could also be a sign that the vent has an obstruction. Here are a few problems that can occur if the vent is blocked.

Issues with backdrafts

Backdrafts occur when a furnace doesn't receive enough air. Typically this is a result of bad ventilation either due to a blocked exhaust, furnace malfunction, or other component issues.

If the furnace doesn't get the airflow that it needs to function, it will start sucking air down its flue pipe. When this happens, the exhaust fumes travel up and out the flue to the outside. Once there, they will be trapped in your attic or basement.

Exposure to carbon monoxide

A blocked exhaust can lead to an accumulation of carbon monoxide and other combustion byproducts. These harmful gases can lead to many health effects including nausea, dizziness, fatigue, and even fatal conditions.

Again, this is why it's worth getting a quality carbon monoxide detector. These devices are fairly inexpensive, and some can be placed in your electrical socket to detect even the faintest hint of carbon monoxide.

See this carbon monoxide detector on Amazon.

Problems with furnace blower motor

A blockage with your furnace exhaust can also cause issues with the motor itself. If the furnace doesn't have the ability to expel the combustion gases, the blower motor may overwork, which can quickly lead to burnout.

If you notice strange sounds such as clanking or loud thumps when the furnace is operating, chances are that the issue is with the blower motor.

When troubleshooting the furnace, be sure to also check the flue pipe for obstructions if possible. This issue is more common during the colder months of the year when outdoor pests such as rodents and birds attempt to enter the vent pipe.

If you can't reach it, consider having a roofer come out to inspect it for obstructions such as wildlife or leaves.

Wrapping Things Up

A furnace exhaust vent pipe blowing steam to the cold winter air

To sum things up, furnace vent covers can be essential to protecting your flue pipe from infestation from wildlife, pest infestation, as well as debris. We hope this post has explained how to cover a furnace vent pipe with these covers.

And remember, if you're apprehensive about performing this project yourself, it's best to contact a licensed roofer or HVAC technician to install the vent screen for you, as safety should always come first.

Before you go, be sure to check out our other posts:

What Does Furnace Service Typically Include?

Do Furnaces Have SEER Ratings?

Leave a Reply