Sure, we can make a good guess it is in the furnace or somewhere near it. It is even more likely it is there if that is where we have changed it in the past.
What I am really asking is do you know if it STAYS where it is when the furnace is operating?
With all the current talk about indoor air quality, clean furnace filters are important year-round if the forced air system has features that allow for the circulation of air throughout the home—-whether it is in heating or cooling mode. A huge percentage of heating systems I inspect have poorly fitting filters, or have filters that are easily pulled out of place during normal operation of the furnace. This is especially the case for furnaces that have cheap cardboard type replaceable filters.
This picture is of a furnace filter that is clogged with dust. The position of the filter in the picture, is the position the filter assumes when the furnace is operating. The blower tries to suck the filter towards it. Obviously this would allow for much of the air to by-pass the filter. In other words it allows lots of the unfiltered air to circulate into the home.
One might ask: “Does the filter stay in place—–when there is no one inside the furnace to watch it?” Many times the answer is no. Most filters that are not “engineered” to stay in place (like Electronic Air Cleaners), do a very poor job of filtering the air and in many cases might as well not be there. Unless the filter slides into a channel that traps all four sides, some amount of air is likely to get by—-un-filtered.
Properly maintained electrostatic air cleaners or expensive HEPA type filters will almost always do a much better job of cleaning the air than cheap cardboard type filters.
Of course it is always going to be relative to how often they get changed or cleaned.
Charles Buell, Seattle Home Inspector
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