Just finished a big remodel on the house. As a result, I have to crawl
about three times as far in the crawl space to change the furnace
filter. Our furnace guy told me that I could install the filter behind
the grate of the cold air return (the opening's big enough for a 14 x
20 filter; the one under the house is 16 x 20). This sounds great to
me, and a simple fix. Is it as simple as installing a frame to hold
the filter behind the grate?
Yes and no - How's *that* for a solid answer? <g>
You can indeed install a filter behind the grate of the cold air
return to catch the dust and debris from inside the house.
The problem will be that you not will catch any dust or debris that
enters the cold air return between the filter and the furnace. In a
perfect world (and maybe in your case) that section of ductwork will
be air tight so that nothing can leak in through any seams as the air
rushes towards the furnace. Absent that perfect world, you will be
getting some dirt pulled into the furnace. Not a good thing.
One might suggest that you use a filter behind the cold air return but
leave the one on the furnace also, but you need to make sure you don't
restrict the air flow to such an extent that you cause other problems.
There are certain filters that claim to not restrict air flow, even
when loaded up, so you might want to consider that type if you are
going to "double up". The next few times you change the "convenient"
one, crawl over to the one at the furnace and see how it looks. After
a few trips you'll get an idea of how much debris enters via that
section of ductwork and determine how often to change the one in the
crawl space. But once again, I caution you to make sure you are not
restricting air flow to the furnace by having 2 filters in the return.
That may require the help/opinion of a professional, at least to get
it right the first time.
Also, having the filter at the return will increase the negative
pressure in the return duct along the entire length. So more outside
air will enter through any leaks. I'd at least make sure to carefully
inspect it and seal any leaks which are found.
older home and when I started to put the filter rack (that's what they
call it) in the hall ceiling the blocking that made a box for the return
duct to hook to wasn't square. I had to hack out part of the blocking
to make room for the filter rack. It was about a quarter of an inch
on one end, but it makes a mess when you are doing that sort of thing.
Now though it doesn't take but a couple of minutes to change my filters.
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