In many cases you should take some care to seal up the slot where your furnace filter slides into.  Sometimes there will be a cover that screws on or clips on, and sometimes there will be some other type of cover, and other times you’ll need to get creative to seal it up.  And here’s why you should seal it up.

Here’s a pretty standard furnace setup, with supply and return ductwork.  And here’s the filter, so that’s the filter slot that you want to seal.  This is one of the wide filters, but it’s the same thing with a standard one inch filter.

And here’s the system’s blower fan.  This is what blows the air through the ducts and into the rooms.  Air is blowing upwards from the fan, and that creates higher air pressures in this part of the duct.  So if there are any holes or leaks in the ducts then air is going to leak out.

Air is being pulled into the fan from the right side, so this part of the ductwork is under negative air pressure.

So if there are any holes or gaps in the ductwork here then air from the room will be drawn into the ducts since they’re at lower pressure.  And one of the most common holes in the ductwork is the filter slot itself, and sometimes the gap at this slot is quite big.  So if this slot isn’t sealed well then air from the room gets sucked into the HVAC system.

Sometimes this is of little consequence, but many times you definitely don’t want to pull air from the furnace room into the HVAC system ducts.  If the furnace is in an attic (which presents other problems) then you don’t want to pull that unconditioned attic air through the system.  If the furnace is in a small closet (which again might present other problems) then this can create pressure problems that can lead to back drafting of the flue gases.  And if the furnace is just in a stinky mechanical room then you don’t want to pull in that stink and send it throughout the house.  The International Residential Code section 1602.2 says that you shouldn’t pull return air from a mechanical room, closet, or garage among other places.

The way to prevent this is to seal up the filter slot each time you replace the filter.  If there’s not a ready-made way to do this then you have several options.  You can buy flexible magnetic sheets that can be peeled off to get to the filter and then easily replaced.  Or you can simply use tape.  Use the cheap gray type of utility tape, and just leave the roll at the furnace so it’s always there when you replace the filter.

Inspection Overview
Why Get a Home Inspection?

Recent Posts

Whole House Fan

A whole house fan can be a great way to keep cool in the summer while saving money and energy.  Here are some important things to know about them. A whole house fan is found

AC insulation

Here’s a quick schematic diagram of a central air conditioner system.  There’s the outside unit, known as the condensing unit.  There’s everything inside the house, consisting mostly of the evaporator coil, a metering device, and

Water temperature

According to an article published in 2013 in the academic publication Journal of Burn Care and Research “Burns from tap water result in an estimated 1500 hospital admissions and approximately 100 deaths per year.”  J

Don’t let your pipes freeze

I’ve heard the question many times: How low should I set my thermostat if I leave town in the winter, to save money but prevent my pipes from freezing? 65 °F?  60 °F?  50?  I

Request Inspection

More Posts You May Find Interesting

Whole House Fan

June 17th, 2024|Comments Off on Whole House Fan

A whole house fan can be a great way to keep cool in the summer while saving money and energy.  Here are some important things to know about them. A whole house fan is found

AC insulation

June 10th, 2024|Comments Off on AC insulation

Here’s a quick schematic diagram of a central air conditioner system.  There’s the outside unit, known as the condensing unit.  There’s everything inside the house, consisting mostly of the evaporator coil, a metering device, and

Water temperature

May 31st, 2024|Comments Off on Water temperature

According to an article published in 2013 in the academic publication Journal of Burn Care and Research “Burns from tap water result in an estimated 1500 hospital admissions and approximately 100 deaths per year.”  J

Don’t let your pipes freeze

February 7th, 2024|Comments Off on Don’t let your pipes freeze

I’ve heard the question many times: How low should I set my thermostat if I leave town in the winter, to save money but prevent my pipes from freezing? 65 °F?  60 °F?  50?  I

Wood shrinkage

January 16th, 2024|Comments Off on Wood shrinkage

Let’s talk about wood shrinkage. Your hardwood floors are the place where you’re mostly likely to see the problems associated with wood shrinkage, but old panel doors can also experience problems.  In floors, as the

Window condensation

January 10th, 2024|Comments Off on Window condensation

Condensation on windows is a problem that I see a lot in the winter, and I see the damage from window condensation year round.  Here’s some information about the issue. Condensation will occur on a