Furnace Filters

Furnace Filter Solid State Home Inspection

If you have a forced air furnace in your home, you have a furnace filter. The furnace filter has two roles in your home, first, it prolongs the life and effectiveness of your furnace by keeping dust out of the heat exchanger. Second, it filters the air in your home as it circulates to improve your homes air quality. Filters come in different sizes and qualities which means home owners need to do a little research before going to the store to buy some for the heating season.


Where is My Furnace Filter Located?

Most commonly, your furnace filter is located right next to the main furnace unit in a little slot. Look for a vertical strip of metal about 1" wide and 12-16" tall which should have a removable metal cover. You may need a screwdriver or other prying instrument to get the metal cover off as they can be tight. Once the cover is removed, the filter should be visible and slide out easily for replacement. Presuming the filter in place is the right size, you can read the size of the old filter and can replace it with another filter of the same size. Note: Do not try and remove the filter while the furnace is running, it will be very difficult to remove and may fall collapse in removal damaging the furnace blower fan.


Most disposable filters are made of cardboard frames with paper or mesh in the screens. There are reusable (washable) filters which are a little more robust and electronic filters available. If your filter has a electric cable, it is electronic and only needs a wipe down. These filters won't need replacement.


What Type of Filter Should I Buy?

Air filters have an industry standard rating system called MERV (minimum efficiency reporting value) with higher numbers being better than lower numbers . Filter prices generally follow MERV ratings and can range from $2 for an unrated filter at a dollar store to over $30 for a HEPA filter with 13+ MERV ratings.


Filters with MERV ratings around 8 are recommended for most households as it will pick up most dust, mold, and pet dander. If you have home occupants with asthma, allergies, or other breathing problems, consider buying a filter with up to a 12 MERV rating. Ratings over 12 are often considered hospital grade.


Higher MERV ratings mean tighter air flow through the filter and this can cause the furnace to work harder than intended by the installer. If you are planning on putting in a higher MERV rated filter, you may want to consult with your furnace service technician to make sure you won't damage the furnace air blower system.


How Often should I Change My Furnace Filter?

The standard industry recommendations are every 4-6 weeks through the heating season. Different filters haver different recommendations and some furnaces/thermostats are even able to monitor the hours of usage on a filter and alert you to change the filter like some cars ask for oil changes. It also depends on the lifestyle of your home and the amount of dust and dirt. If you have a large family and pets, filters will need changing much more often than if you live alone.


With a practiced ear, you can often hear when your furnace is working harder to pull air through a clogged filter. The pitch of the furnace goes up and the volume level increases.


We recommend you buy enough filters in one shopping trip at the beginning of the season to last the year so as not to run out and forget to change the filter regularly If you change the filter every 4 weeks, that would be one per month. Often retailers bundle packs of 3 together for a better price than buying singly. Two packs of 3 would get most home owners through the season.


Is it Possible to Install My Filter Wrong?

In short, yes. As Home Inspectors, we see the following problems all too often in home inspections:

  • Installed Backwards - Most filters are designed for air to flow one way through and there are arrows indicating the direction of air flow. Air typically passes through the filter into the main body of the furnace, so the arrows should usually point at the furnace cabinet.
  • Wrong Size - Filters need to be the correct size for the filter slot. We have seen undersized filters in place (meaning most air goes unfiltered) or oversized ones forced in (meaning air is restricted passing through).
  • Missing Filter Slot Cover - Filters are only effective if all the air passes through them. When filter covers are missing, air can be pulled into the furnace around the filter.
  • Missing Filter - All to commonly, we don't see filters in the filter slot at all which is an obvious maintenance problem.
  • Clogged/Dirty Filter -  It can become very obvious when filters have not been changed for a long time. This is an easy maintenance item and a good indicated of homeowners attention to their home.


Final Thoughts

When it comes to maintaining our furnaces, there is very little for homeowners to do. Annual maintenance is always recommended but once the heating season has started, all the furnace needs is a little maintenance in the form of a new filter every few weeks. Maintaining our filters will help extend the life of our furnace and reduce the risks of a breakdown in the middle of winter potentially saving you thousands of dollars in repairs or replacement. If you need help with understanding your furnace system in a home purchase, ask your home inspector so you will know how to keep your home safe and solid.


By James Bell - Owner/Operator of Solid State Inspections Inc.