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.