Time to check your air filters! This filter was blue when new.

It’s been a long winter for many of us, and that means the furnace has run a lot! The more it runs, the dirtier your air filter gets.

There’s a ton of information on the web about air filters, MERV ratings, and so on, so I’m not going to repeat that. I’ll give you some links at the end of the article. I’ll discuss the adverse effects that a dirty or improperly installed air filter actually has on your system’s efficiency and potentially, your health.

The old saying that you should check your air filter once a month isn’t accurate. It depends on the type of air filter, how clean you keep your house and how long the system actually runs. The main thing is, if you haven’t checked your air filter in a while, do it now!

A dirty air filter compromises your heating/air conditioning system more than you think. The dirt restricts the air flow through the system. In some cases, this can increase the power draw from the blower. In other cases, it just diminishes the amount of air that comes out your vents. In either case, a dirty air filter equals lower efficiency.

Did you know that a dirty air filter can also lead to fires? Yes, that is correct. If you have a gas or oil furnace, a dirty air filter can reduce the air flow to such an extent that the system will overheat. It should automatically turn off in this situation, but…

Lots of money is wasted on unnecessary service calls due to dirty filters. Heating systems may “lock out.” Cooling systems may ice up, and again, lock out. Systems can be damaged, requiring costly repair. So why not spend a couple of minutes and replace your air filter?

Please don't leave your filter hanging out! (and installed backwards...)

There are two problems in the above photo. Fist off, the filter is obviously not installed properly, or is too large. Second, it’s in backwards! See the metal grid on the filter? That grid is supposed to hold the filter against the flow of air, which is from right to left in this system. Always look for the little arrow on the filter that shows the direction of the air flow. It always flows towards the blower (called an “air handler”) from the ductwork. Sometimes they won’t have an arrow, so remember the trick of looking for the metal support grid.

But please, make sure you get the right sized air filter! Leaving the filter sticking out like this can reduce the efficiency of your system by 30%-50%! It’s true. If you don’t have the right sized filter and if you don’t air seal the filter slot, then where do you think the system is pulling the air from? Instead of pulling it in through your ducts, it’s going to suck the air in from wherever your air handler is located. If it’s in your attic, then it’s sucking fiberglass, dust, and cold (or hot) air from your attic, making the system work much harder. It’s a huge problem and the simplest one to fix.

I’m going to repeat that.

Leaving the filter slot open will often lead to a 30%-50% decrease in your system efficiency. Do not do this!

One last thing – please don’t buy those really cheap fiberglass filters that you can see through. They’re basically worthless. Yes, they’ll stop dog hair, but they don’t do much of anything for air quality. If you really want to capture dust and smaller particles, get at least a MERV 8 filter (for comparison, those fiberglass are around MERV 3). The higher the MERV, the better the filtration. But keep in mind that the better it filters, the more air resistance. Some “super” filters can filter so well that the air handler can struggle to move enough air through them.

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Air filter references

Allergy Clean Environments – Understanding MERV ratings

Heating and Air Conditioning FYI – Dirty filters can choke your system. A no-nonsense description of what a dirty filter does.

HVAC 4 U – First Line of Defense: Your Furnace’s Air Filter – Short description of various types of filters.

NordicPure – where I buy my replacement air filters. I buy them by the case so that I never have the excuse of not having the right sized  filter around! (This isn’t an ad. I get nothing from them if you buy their filters!)