Can Mice Enter Through the Furnace Exhaust? (5 Fixes

If you’ve spotted mice in your home, you’re probably wondering how they got inside in the first place. These small rodents are really good at finding tiny openings and gaps that lead into your house.

One place you might not think to check is your furnace exhaust. But can mice actually enter through the furnace exhaust?

Yes, mice can enter through your furnace exhaust if it doesn’t have proper screening or if the screening is damaged. Mice only need an opening about the size of a dime (roughly ¼ inch) to squeeze through, and furnace exhausts often have gaps or worn screens that make entry possible.

While it’s not the most common entry point, mice can and will use your furnace exhaust to get inside if they find an opportunity. The warm air coming from the exhaust is actually attractive to them, especially during cold weather when they’re looking for a cozy place to stay.

How Mice Can Get Through Furnace Exhausts

Furnace exhausts are designed to vent gases and heat from your home to the outside. Most modern furnaces have PVC or metal pipes that lead outside, and these pipes should have screens or caps to keep animals out.

However, these protective screens can get damaged over time. Weather, rust, and general wear can create holes or gaps that mice can use to get inside.

Mice are incredibly flexible. They can flatten their bodies and squeeze through openings that look way too small for them. If your furnace exhaust screen has even a small tear or gap, a mouse can probably fit through it.

Also, some older homes don’t have proper screening on their furnace exhausts at all. If your exhaust pipe just opens directly to the outside without any barrier, mice have a clear path right into your heating system.

Why Mice Are Attracted to Furnace Exhausts

Mice aren’t just randomly checking every opening in your house. They’re drawn to furnace exhausts for specific reasons.

First, there’s the warmth. Furnace exhausts release warm air, and mice can feel this heat from the outside. During fall and winter, when temperatures drop, this warm air is like a beacon telling mice there’s a cozy spot nearby.

Second, the exhaust provides shelter from the elements. Rain, snow, and wind can’t reach mice if they’re tucked inside the exhaust pipe or the areas around it.

Third, once a mouse gets into the exhaust system, it often leads to other parts of your home. Mice can follow the ductwork and pipes to find their way into walls, basements, or other areas where they can nest.

Signs Mice Have Entered Through Your Furnace Exhaust

If mice have used your furnace exhaust to get inside, you’ll usually notice some telltale signs.

You might hear scratching or rustling sounds coming from your walls or near your furnace. These sounds are often loudest at night when mice are most active.

Droppings are another clear sign. Mouse droppings are small (about the size of a rice grain), dark, and pellet-shaped. If you see these near your furnace or heating vents, mice have probably been in that area.

You might also notice a strange smell. Mice leave urine trails, and if they’ve nested in or near your furnace system, you might smell a musty or ammonia-like odor.

If you actually see a mouse near your furnace or heating system, that’s obviously a clear sign they’ve found a way in.

The Dangers of Mice in Your Furnace System

Having mice in your furnace exhaust or heating system isn’t just annoying. It can actually be dangerous.

Mice chew constantly because their teeth never stop growing. If they get into your furnace system, they might chew on wires, insulation, or other components. This can damage your heating system and create fire hazards.

Chewed wires can short circuit your furnace or cause electrical fires. Even if the damage doesn’t cause a fire right away, it can make your furnace work less efficiently or break down completely.

Mice also leave droppings and urine everywhere they go. If these waste products get into your ductwork, they can be blown throughout your home every time your heat turns on. This can affect your indoor air quality and potentially cause health problems, especially for people with allergies or respiratory issues.

Also, dead mice in your heating system create terrible odors. If a mouse dies in your ductwork or near your furnace, the smell can spread throughout your entire house.

How to Check Your Furnace Exhaust for Mouse Entry

If you think mice might be getting in through your furnace exhaust, you need to inspect it carefully.

Start by going outside and finding where your furnace exhaust exits your home. It’s usually a PVC pipe (white or gray plastic) or a metal pipe coming out of your house, often near ground level or on a side wall.

Look closely at the end of the pipe. Is there a screen or cap covering it? If not, that’s a problem. Mice can walk right in.

If there is a screen, check it for damage. Look for tears, holes, rust spots, or gaps around the edges. Even a small opening is enough for a mouse to get through.

While you’re outside, look at where the exhaust pipe connects to your house. Are there any gaps between the pipe and the siding? Mice can use these gaps to get into your walls, even if they can’t get directly into the exhaust pipe itself.

You should also check inside near your furnace. Look for droppings, chew marks, or nesting materials (shredded paper, fabric, insulation) that might indicate mice have been there.

How to Keep Mice Out of Your Furnace Exhaust

Once you’ve identified the problem, you need to fix it to keep mice out.

The most important step is to install or replace the screen on your furnace exhaust. You can buy specialized exhaust vent covers at hardware stores. These covers are designed to let air out while keeping animals, including mice, from getting in.

Make sure you get a screen with small enough holes. The openings should be no bigger than ¼ inch. Anything larger and mice can still squeeze through.

When you install the screen or cap, make sure it’s secured tightly. Use screws or clamps (depending on the type of cover) to attach it firmly to the exhaust pipe. A loose screen can be pushed aside by a determined mouse.

You should also seal any gaps around where the exhaust pipe enters your house. Use caulk or expanding foam to fill these spaces. Just make sure you’re using materials that can handle the heat from the exhaust.

Check your exhaust screen regularly, especially before winter when mice are actively looking for warm places to stay. Replace damaged screens right away.

Other Entry Points to Check

While you’re dealing with your furnace exhaust, it’s a good idea to check other potential entry points too.

Mice are opportunistic. They don’t just use one entry point. If they can’t get in through the exhaust, they’ll try somewhere else.

Check around pipes, wires, and cables that enter your home. These often have gaps around them that mice can use.

Look at your foundation for cracks or holes. Even small foundation cracks can be entry points for mice.

Check your attic vents, roof vents, and soffit vents. Make sure they all have proper screening.

Don’t forget to check doors and windows. Worn weatherstripping or gaps under doors can let mice in.

The goal is to make your entire house as mouse-proof as possible, not just fix one entry point.

When to Call a Professional

Sometimes, dealing with mice in your furnace system is something you should leave to the experts.

If you see signs of mice but can’t find how they’re getting in, a pest control professional can do a thorough inspection and find entry points you might have missed.

If mice have already damaged your furnace or ductwork, you might need an HVAC technician to make repairs. Don’t try to fix damaged furnace components yourself if you’re not trained to do so.

If you have a serious mouse infestation (seeing multiple mice regularly, finding droppings everywhere), professional pest control can help you get rid of them more effectively than DIY methods.

Also, if you’re not comfortable climbing ladders to inspect or repair your exhaust, or if your exhaust is in a hard-to-reach spot, it’s safer to hire someone who has the right equipment and experience.

Conclusion

Mice can definitely enter through your furnace exhaust if it’s not properly protected. These small rodents only need a tiny opening to squeeze through, and damaged or missing screens on your exhaust give them an easy way in.

The warm air from your furnace is attractive to mice, especially in cold weather. Once they get into your exhaust system, they can cause damage by chewing wires and insulation, and they can affect your air quality with their droppings and urine.

The good news is that you can prevent this by installing proper screening on your furnace exhaust and checking it regularly for damage. Make sure the screen has small openings (no bigger than ¼ inch) and is securely attached.

Also, remember to check other potential entry points around your home. Mice are persistent, and they’ll keep trying different ways to get inside if one way doesn’t work.

If you catch the problem early and seal up your furnace exhaust properly, you can keep mice out and protect both your home and your heating system.