Generally household dust can be filtered effectively with a basic media
filter. I might suggest changing to one of the better pleated filters and
see if that helps. I use one of the 3M filters.
Furnace filters are not really designed to protect your home from dust,
but rather to protect the furnace from dust, although they may help with the
home as well.
Since you say your dust problem occurs when using the furnace, I would
suggest it is not your home that is the source of the dust, but rather the
heating system and/or that fresh air intake.
A problem with your duct work could be pulling dust in or the ducts
themselves may have considerable dust. Normally I would not recommend duct
cleaning as most of the time it is nothing more than a way to separate you
from your money. In your case however I suggest it may be called for.
That fresh air duct could also be bringing in more dust than you are
thinking. Can you try shutting it off for a few weeks and see what happens?
As for the ducts, I suggest calling in a heating pro. Have him give
your furnace and ducts a going over. Likely it has been a long time since a
pro has checked them out. Be sure to explain your problem first. Hopefully
he does not clean ducts himself so he can be trusted to give you a unbiased
opinion on the ducts.
It is possible that a cracked heat exchanger possibly coupled with a
combustion problem is your problem. This is the problem you really want to
check on. If that is the problem, you will not fix the dust problem by
cleaning ducts or adding filters to the return air, but you are in real
danger due to the problem.
If that furnace is 20 years old then I would be really worried about it.
Also a replacement may save you enough energy to pay for itself.
Joseph E. Meehan
26 + 6 = 1 It's Irish Math
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