How To Seal HVAC Vent Into Chimney?

Installing a new high-efficiency, wall-mounted chimney vent with integrated heat and air exchanger is an excellent way to help meet the stringent guidelines of the city code.

But what if you have an existing chimney and are not ready for a major renovation? What options do you have for upgrading and sealing your old flue without tearing out the entire wall and replacing it with something new?

You can seal HVAC vent into chimney by installing a metal liner along the chimney and applying heat-resistant caulk where vent meets liner. Energy Star recommends a 1-inch (2.54-cm) clearance from flues and a 2-inch (5.08-cm) clearance from masonry chimneys to prevent fire hazards.

Besides, if you don’t seal up the vents correctly, hot or cold air can escape from the ductwork and undo your cooling unit’s hard work. In this article, I will go over several different tips for how to seal an HVAC vent into a chimney.

1. Clean the Chimney

Clean the chimney and the vent channels and remove any loose debris, spiderwebs, and soot. This will make the sealing process much easier. Here are a few tips to help you get started if you have a little bit of DIY background:

  1. Put on protective gear (a hat, glasses, and gloves) and turn off the main gas supply to your fireplace.
  2. Next, inspect the chimney’s exterior for cracks, holes, or other signs of damage. If you see anything concerning, halt the cleaning and call a professional to inspect it.
  3. Now, start sweeping. It’s best to use a push-style (or vacuum) brush to clear out soot, ash, and other debris. 

If it helps, use a special flue liner brush to access the upper parts of the chimney. This will make it easy for you to work on them. Use a mask while working with the sealant. The soot can be toxic in large quantities.

Read: HVAC Inspection – How Often? What To Expect? Cost?

2. Use Silicone Rubber Seal

A silicone rubber seal will provide a tight fit and prevent air leaks, keeping your home warmer and reducing your energy bills. To get the best results, you should always use the recommended type of silicone sealant for your particular project. When applied correctly, silicone sealants can provide years of durable, maintenance-free protection for your home.

Silicone rubber is a popular seal material due to its flexibility and durability. As the temperature changes every time you switch the furnace ON/OFF, silicone rubber will expand and contract to prevent cracks and holes from forming in the seal. When shopping for a silicone rubber seal, look for one that’s flexible.

A flexible seal will conform to the shape of the opening. It won’t crack or break easily and will last for years without needing to be replaced.

Read: Commercial HVAC – Everything You Need To Know About

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3. Fire-Rated, Heat-Resistant Caulk

HVAC vents are typically metal and can become very hot when in use. They are often placed against a non-combustible material such as brick or stone, but if the vent is metal, you will want to caulk it to the chimney to prevent heat from reaching the insulation and other combustible parts of the vent.

While there are many types of heat-resistant caulk, we recommend fire-rated, heat-resistant caulk for sealing an HVAC vent into a chimney. The brand doesn’t matter as long as it meets the necessary standards of quality (high-quality caulk does its job better and lasts longer). 

Fire-rated, heat-resistant caulk is designed to withstand high temperatures and withstand wear and tear. But keep an eye on the caulking as it ages, and be sure to replace it when necessary. 

Read: Are HVAC Service Plans Worth It?

4. Maintain Some Clearance From Combustibles (Insulation and PVC Vents)

When sealing an HVAC vent into a chimney, you need to ensure that you maintain some clearance from combustibles. If you seal the vent too close to the insulation or too close to the PVC vents, the heat from the gases rising inside the chimney can cause damage to your home.

Avoid contact between your flue liner and nearby combustibles as much as possible. If you have to place your liner in close contact with combustible material, make sure to leave enough space between the two materials to prevent any direct contact. PVC vent pipes and some types of insulation are flammable. Too much heat from the flue liner could turn combustibles into a fire hazard.

5. Seal With Lightweight Aluminum Flashing

An HVAC vent can be easily vented through a chimney by installing a piece of aluminum flashing around it. This flashing can be found at most hardware stores, and it’s usually less than $100.

Aluminum flashing is a lightweight yet strong metal sheet that is used to cover an opening in a structure. You can use it to seal an HVAC vent into a chimney by bending it around the edge of the vent and securing it with a metal clamp or tight zip ties. Even better, you can use super strength glue.

Read: How To Do Air Balancing HVAC In A House?

6. Use Heat-Resistant Inflatable Seal

An HVAC vent that runs through a fireplace or chimney can be tricky to seal especially when all your movements must be made in a cramped space. You need something that won’t catch fire but also isn’t too big or bulky. Inflatable seals are an awesome solution for these types of jobs because they’re compact and lightweight enough to fit in confined spaces but still have the strength to create an airtight seal.

A heat-resistant inflatable seal is a kind of thermal insulation material that is widely used in the field of heat insulation and sealing. Unlike heat-resistant caulk, it expands to fill in the tiny gaps at the point of contact between the HVAC vent and chimney.

Why Seal HVAC Vent Into Chimney?

The air in your home needs an escape route when things get hot. In the winter, that means heating vents send warm air up through the chimney. But what about the summer? How do you keep all that heat from seeping into your house and spoiling your cool indoor spaces?

Sealing HVAC vents into chimneys prevents energy-wasting drafts. An HVAC vent that is not sealed will allow warm air to escape through the cracks and holes and cold air through the same cracks and vice versa. This will result in your heater or AC unit working harder and for an extended period of time to maintain the desired temperature inside. A sealed HVAC vent will also protect your system from outside contaminants such as leaves, dust, and other debris. 

Sealing your HVAC unit to your chimney is essential for preventing unnecessary energy loss, decreasing maintenance costs, and preventing the transfer of excess heat into your home. The HVAC system in your home is designed to provide a comfortable environment for you and your family. However, it can’t do its job effectively if the intake and exhaust vents are not sealed properly to your home’s exterior. 

Keep in mind that the construction of new homes and buildings can require a lot of specialized knowledge. In many cases, skilled contractors must complete these projects in the most efficient way possible to save time and money. 

Read: How To Increase HVAC Airflow Upstairs?


The HVAC vent inside your house is critical to your indoor air quality. It allows the warm air from your AC unit to flow into your home. When it comes time to seal an HVAC vent into a chimney, you need to take your time and make sure you seal it properly, or better yet, hire a professional to do it for you.