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tls1 01-12-2013 03:08 PM

Furnace filters turn black quickly
Hi All,

I changed my furnace filters about 2 weeks ago. The old ones were dark dark grey (almost black!). I checked the new ones today and they are also almost black. Any ideas?

I've been reading and there's lots of talk of candles, mold and dirt getting into the returns. We don't use candles, we have dirt like anyone else. This did not happen in the summer when we were using AC, nor when using the heat pump to heat. In the last 30-45 days we have been burning more oil.

The furance was built in 1989. Our contractor said that it might be exhaust entering the filter door from a bad seal. I checked and there are no black streaks on the inside of the door.

Could it be a leak in the exhaust system inside the furnace above the filters and blower in the rear? The filters are equally dark grey edge to edge, and there are 2 of them! One horizontal and the second on an angle. There are the better quality filters that are pleated. $8 each x 2 each time I change them.

I am worried that exhaust is getting in the house, however our CO detector in the living room says 0.

Any ideas/thoughts?
Happy New Year!

bobinphx 01-12-2013 07:42 PM

ok.. first off I am not a pro. I have worked on many many ac systems for friends and family, not to many furnaces and no oil type. with that said, here goes.

I would suggest that maybe the black is combustion particals from the oil burning heater exhaust. so if we take that idea and run with it, where could it be comming from?

I suspect that something after the burners is leaking. 1989 is getting a bit old, so I think that maybe having the heat exchanger checked by a pro would be a good first step. The heat exchanger contains the flames and the exhaust gases on the inside and allows your inside home air to circulate aound the sealed outside area and pick up heat (really hot heat!!). So if there is a crack or a leak, soot could be comming out, as well as carbon dioxide, then circuilating in your home.

Once the exhaust gases leave the exchanger (cooler now), they enter the exhaust ductwork or flue or chimney or what ever name you want to call it. The exhaust gases may contain soot and for sure have carbon dioxide, as well as some condensation vapor. so I would also check for leaks in exhaust ductwork. The exhaust ductwork may also have rotted from the inside due to the condensate. so it may be hard to find. again a pro could help find this with a co detector and other tools.

an other idea is that your furnace maybe using inside air for combustion. That air has to come from somewhere else, thus the system may be pulling air that is "dirty" through the cracks etc of your home. your hvac system would not pull in outside air (normally), so this might be the why you dont have dirty filters in the summer. I would do a caveman type test and crack open an outside door with the furnace off and see how much air is moving in or out. Then turn on the furnace and crack the same door and see if there is an increase in air flow (ie negative air pressure indoors). You can use tissue paper to help see the air movement, both on and off.

Other here have much more experiance... I dont !!!!!

hope this helps.

tls1 01-13-2013 07:19 AM


Thanks for the info... I think condensation and age might be the culprit! In the rear if the furnace in the return area there is the blower at the bottom, above that are the filters, above that are the exhast tubes carrying the smoke from the chamber to the exit of the furnace. There are 5 of these tubes connected to square piece. It's hard to describe what they look like. They are used to prewarm the air. Anyways the do have some visible rust. Maybe there is a hole somewhere allowing exhaust to escape. its a very powerful fan back there...would not take much!

Also, note our home CO2 detector is rock solid at zero located in the main area of the main floor! Not near any duct or return.

I will post a pic of the rust part.

Thanks all!

Missouri Bound 01-13-2013 10:09 AM

Does this furnace also supply air conditioning, and is this black filter issue prevelant when cooling?

handyman_20772 01-13-2013 10:24 AM

Is there a opening on your return within 10' of the furnace itself? It's looking for makeup air when the furnace is running and you may have some backdrafting going on.

tls1 01-13-2013 11:08 AM

We have heat pump and it never happened with AC or heating during mild conditions. Speaking on the phone with our contractor, he thought the door to back of the furnace was not closed properly and exhaust was being sucked in. But that is not the case.

There is no return in the basement level. However I know there are leaks in the returns, because when the blower is running, it can suck the basement door closed. Heating or cooling. I added a vent to the basement door to lessen the effect.


beenthere 01-13-2013 04:35 PM

Do you burn candles.

tls1 01-13-2013 04:45 PM

only on birthday :) sadly a lot of them ...

kidding... we rarely burn candles...or anything else... no fireplace, etc...


bobinphx 01-14-2013 08:21 AM

you might want to take a look at the specs for your co detector. I have read that they are not so great at low long term levels.

here is a news report... for what its worth.

hvac benny 01-14-2013 09:52 AM


Originally Posted by bobinphx
you might want to take a look at the specs for your co detector. I have read that they are not so great at low long term levels.

here is a news report... for what its worth.

That's to reduce nuisance calls. CO isn't the only thing that sets off CO alarms. The best way to avoid CO exposure is having your appliances properly maintained (beyond just changing the filter). CO alarms are a last line of defense.

jagans 01-14-2013 09:52 AM

Is your furnace an updraft, or downdraft type. The reason I ask, is that the filter (s) need to be on the incoming side of the blower. You said your filter is above your blower?

tls1 01-14-2013 05:26 PM

1 Attachment(s)
I'm not sure if it's updraft or downdraft...but... here a drawing attached similar to mine.

On another note, my contractor told me that while the exhaust system may have a leak, burning oil does not produce carbon monoxide (CO). It produces carbon dioxide (CO2). Too much will still kill you :huh:.

However, he also asked if there was a funny smell like buring rubber (or burning anything)... no there was not. He's coming Wednesday morning to check.


jagans 01-14-2013 05:32 PM

This illustration is not very good, as it does not show incoming return air, outgoing plenum etc. but good luck with your problem. It sounds like dirty jets and a breached heat exchanger.

tls1 01-14-2013 05:33 PM

the cold air return is on the right of the drawing.

the exhaust flue prewarms the cold air . Then it is filtered, then to the fan, then to the chamber and up the left side into the house.

beenthere 01-14-2013 06:06 PM

That pic is of a low boy, up flow.

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