On 1/1/2012 3:06 PM, bob wrote:
On Jan 1, 11:06 am, "bob" wrote:
Due to allergy I installed a filter cabinet and a merv 12 4-inch
filter in
my furnace. I thought I'd be getting very clean air.

To verify, I cover two floor registers with furnace filters. After a few
weeks these filters turned dark grey and can be seen from either side
of the
filter. I flipped it over and there is no dust accumulation.

Is this normal? Where do the dark fine particles come from?

Possibly dirt that is already in the heating ducts from
before you put the MERV filter in the system?

I forgot to mention I had the ducts cleaned (vacuumed) by the same
company who installed the filter cabinet, which is 3 months ago. They
commented the ducts were very clean already.

The ducts have also been sealed 2 or 3 years ago, and although it is not
100% sealed, the contractor performed a blower door test and said it was
acceptably sealed. The reason I say it is not 100% sealed is because
during the summer, I can smell a faint crawlspace odor coming out of the
register when I'm in the bathroom with the ventilation fan on.

The furnace is about 5 years old.

The 4 inch filter is filtrete brand and cost $30 or so at hardware store.

I will set up a carbon monoxide alarm next to the register just in case.

Are there "serious" air filter for floor registers with merv ratings
like the furnace filters? I see foam-like filter that fits inside the
floor register, but they have no ratings.
Help us a little. What does your furnace use for fuel?

Filters usually don't fit tightly in the holder. Did you somehow seal up
around the edges of the filter?

Do you have little critters chewing into the heating vents, or the cold
air return ducts? Perhaps you are smelling them.

Get some "man sized" cleneex and wrap one or two of these under a floor
register. After a week, remove and examine for dust or whatever. Should
be clean if your filter is doing it's job. They are for collecting
evidence, not for regular air filtering.

Paul