How do you replace a furnace vent cap?

The furnace vent cap is an important part of your HVAC system. It prevents backdrafting, which occurs when air pressure in the vent pipe reverses and pulls dangerous gases back into your home. A cracked or corroded vent cap can compromise your indoor air quality and lead to carbon monoxide poisoning. Replacing a faulty vent cap is a relatively straightforward DIY project that you can complete with basic tools and materials.

When to Replace Your Vent Cap

There are a few signs that indicate it’s time to replace your furnace vent cap:

• The vent cap is visibly rusted, corroded or cracked
• You can see daylight around the edges of the vent cap
• You frequently smell odd or unpleasant odors in your home
• Your carbon monoxide detector frequently goes off

Don’t ignore these warning signs. A compromised vent cap can allow dangerous exhaust gases to enter your living spaces. Replacing the cap takes less than an hour and costs around $20-50.

How to Choose a Replacement Vent Cap

When selecting a new vent cap, you’ll need to match it to the size and type of your existing vent pipe. Most furnace vents are 3″-4″ diameter and made of PVC plastic or galvanized steel. Key things to look for include:

• Material – Match the existing pipe material. PVC caps for PVC pipes, steel caps for steel pipes.
• Diameter – The cap’s diameter must precisely fit your vent diameter.
• Pitch – Pick a cap with at least 1/4″ of rise per foot to allow moisture drainage.
• Seal – Choose a tight-sealing cap design to prevent backdrafting.
• Style – Direct vent caps have an intake and exhaust. Single wall caps just have an exhaust.

Also consider getting a cap with a screen to prevent animals or debris from entering the vent system.

How to Measure Your Vent Pipe

Before shopping, you’ll need to measure the diameter of your existing vent pipe. Follow these steps:

1. Locate the exterior vent pipe outside your home.
2. Use a tape measure to measure the diameter across the very top of the pipe.
3. Write down the measurement. Most vents are 3 or 4 inches.
4. Also note the material – PVC plastic or galvanized steel.

With the pipe diameter and material in hand, you’ll be able to select the proper replacement cap.

Safety Precautions

Anytime you work on gas appliances and venting systems, safety should be the top priority:

• Turn off power to furnace at breaker box
• Have a carbon monoxide detector in working order
• Work outdoors to avoid exhaust backdrafting into home
• Use a tall ladder and have someone steady it
• Seal any unattached vent pipes to avoid CO leakage
• Never reach inside a furnace or vent
• Call a pro if you have any doubts or issues

Venting exhaust systems connect to your furnace’s gas burner and can leak deadly carbon monoxide if disturbed. Take smart precautions.

How to Remove the Old Vent Cap

With your new cap purchased and safety in mind, you’re ready to swap out the old furnace vent cap:

1. Turn off power to the furnace at the breaker box.
2. Go outside to where the vent pipe exits your home.
3. Use a flathead screwdriver to pry any screws or fasteners from the base of the old cap.
4. Inspect the top of the vent pipe for any remaining screws.
5. Wearing work gloves, twist the old cap counterclockwise to unthread it.
6. Pull straight up to remove the old cap.
7. Seal the open vent pipe to prevent exhaust escaping.

In most cases, the vent cap is attached with simple screw or clamp mechanisms that can be undone by hand. Wear thick work gloves when gripping the cap, as the edges may be sharp.

How to Install the New Vent Cap

Installing the new furnace vent cap takes a few simple steps:

1. Apply plumber’s tape to the threads of the new cap.
2. Align the new cap over the vent pipe.
3. Slowly twist it clockwise until hand-tight.
4. Secure the base with screws if your cap has them.
5. Make sure the cap is oriented to allow moisture drainage.
6. Remove any rags or seals from the vent pipe.
7. Turn furnace power back on and test for proper airflow.

The key things are to get a tight seal, secure the base, and orient for drainage. This prevents backdrafting and moisture buildup.

How to Test Proper Venting

To ensure proper venting once the new cap is installed:

1. Turn on the furnace and let it run for 5 minutes.
2. Feel around the edges of the cap to check for exhaust leaks.
3. Light a flame in front of the cap and observe the airflow direction.
4. Turn on some hot water taps and smell for unusual odors.
5. Monitor your carbon monoxide detector.
6. Call a technician if any issues are found.

Normal airflow should be steady and move outward. If you observe leaks, reversed airflow, odd smells, or CO detector alarms, shut down the unit and call for service.

Conclusion

Replacing a corroded or compromised furnace vent cap is an essential maintenance task to keep your HVAC system operating safely and efficiently. With the right tools and safety precautions, it can be completed by handy homeowners in about an hour. Be sure to select a quality cap designed specifically for your existing vent pipe material and dimensions. Test thoroughly after installing to catch any improper venting issues. With a secure, leak-free vent cap in place, your furnace can continue providing warm comfort and healthy indoor air.