Furnace filter doesn't filter?

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?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Possibly dirt that is already in the heating ducts from before you put the MERV filter in the system?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 1/1/2012 3:06 PM, bob wrote:

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

*Some* dust is normal. Places it comes from and where you might want to check. . . Did you clean or replace the ductwork when you installed the expensive filter. [I'm guessing that a 'merv 12 4-inch' filter is expensive-- I don't know anyhting about them] I'd replace it myself if you're up to the task. Buy a good respirator before you begin. You won't believe how much crap accumulates in a decade or two.
Did you at least check all the ductwork for points of infiltration? The air gets filtered at the furnace. if there is a pinhole in a duct it will suck dirt out of whatever cavity it happens to be in. that's why some folks put filters on the registers.
Jim
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

When did you last have your ducts cleaned out ?
You might want to have a closer look at those dust particles under magnification to have a better idea of their source.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

It is the dirt built up in your duct work from before you installed the filtration system...
You should have your duct work cleaned, pressure tested and properly sealed at each connection point and then insulated...
Less dirt in your air with the filter, none can get sucked in at the joints in the duct work and with the duct sealing and insulating you would be losing a lot less heat to the basement or attic of your home (or whatever other unfinished spaces it runs through)...
~~ Evan
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

The portion of the system AFTER the filter he just installed has positive pressue. Nothing is being sucked into it. On the other hand, the vacuum in your empty skull, I;m not so sure about
Evan, clueless as usual
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Who dragged Canadians into this? I'm not Canadian, you;re just confused as usual.

Show us how dirt gets in with positive pressure. Enough dirt that it's showing up at the registers.

Show us how dirt gets in with positive pressure. Enough dirt that it's showing up at the registers.

And you're the guy who told us not too long ago that nitrogen had to be recovered from an AC system and not released into the atmosphere. And how you NEVER, EVER want to cut and re-glue a PVC exhaust pipe because everyone is gonna die and that requires special skills. Meanwhile every idiot around is installing dryers, for example.
What a fool.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
MERV stands for Minimum Efficincy Reporting Value. It's a scale from 1 to 16. A MERV of 12 is currently popular with the heating and AC industry for filters. It should do a pretty good job of stopping particles above 1 micron in size. Smaller particles will be less well impeded. Your dirt may be smaller particles. Why the focus on dirt? How's the allergy doing? That's the important thing, not dirt. Are you allergic to things that are bigger than 1 micron?
Dave M.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.