Thread: Mold on furnace plenum register

Any idea what would cause this? 22 year old mid efficiency forced air gas furnace. Register was in the plenum, removed for pictures and to check the interior of the plenum. No other anomalies were found. Air conditioner A coil was clean.IMG_2671.jpg

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Moisture and food (in this case the dust/dirt on the vent) provided an environment for the mold that was already in the air a place to grow.
I'm guessing either the air in the home was a bit too humid or the grill was subject to enough of a temperature difference to form condensation. This is not that unusual in air conditioned properties here or in refrigerated spaces (walk in coolers, etc.)

I'm familiar with what causes mold to grow. But in over 5,000 inspections I've never seen it in on a plenum register before. Also keep in mind that I'm in Minnesota where we run furnaces 8 months out of the year.

Originally Posted by Ken Rowe

Any idea what would cause this?

Then

I'm familiar with what causes mold to grow.

Now, I'm not sure what you are asking.

As to the specific mechanism that caused the high moisture, one could only speculate, but you were there. Why would this particular register see more moisture or dirt than others in your area?
I generally take it as a sign of poor house keeping and/or filter maintenance with the possibility of personal habits that increase moisture such as not using exhaust fans for showers, more people than normal living in a space, cooking moisture, etc.
I don't see it as any big deal unless it is systemic so I would not be overly concerned if it is as limited as you report.

Should You Have the Air Ducts in Your Home Cleaned? | Indoor Air | US Environmental Protection Agency

Originally Posted by Jim Luttrall

Then



Now, I'm not sure what you are asking.

As to the specific mechanism that caused the high moisture, one could only speculate, but you were there. Why would this particular register see more moisture or dirt than others in your area?
I generally take it as a sign of poor house keeping and/or filter maintenance with the possibility of personal habits that increase moisture such as not using exhaust fans for showers, more people than normal living in a space, cooking moisture, etc.
I don't see it as any big deal unless it is systemic so I would not be overly concerned if it is as limited as you report.

I guess I was hoping for someone to chime in saying this is typically caused by a short cycling furnace or a/c. Or some other problem associated with the furnace or a/c. Since the furnace is located in a finished basement, maybe someone would comment that it's due to excessive moisture in the basement, maybe even due to past flooding of the basement.

Just a SWAG guess, but the paint on the register happens to be some kind that can support mold or something was spilled, splashed, applied to the register that supports mold. In your humidity, that was all that was needed. I frequently see mold growing on vinyl siding on the north sides of houses, so it doesn't take much for mold to find a way.

Next SWAG guess is that a previous owner had lots of plants or aquariums, etc that created excessive humidity which led to mold surviving on an unlikely surface. But, if that was the case, there is likely mold in lots of places, both visible and hidden.

My SWAG would be that the HO continued to use the higher MERV rated pleated air filters, which worked so well and without complication during those 8 Heating months, during those four months the AC was utilized. Excessive drag for the moisture ladden yet cooler air. Dew point + too-low air volume, at a drag point; and at a location which was overlooked in housekeeping and servicing by both the HO or occupant & the service technician.

Plenum registers in basement for fuel-fired furnces fitted with aftermarket split AC coils (already compromising & creating drag) should NOT be present in the area containing the fuel fired furnace which is NOT sealed combusion unit AND direct vented, esp. if a standing pilot OR one of those since (recalled or was it just class-action lawsuit attempts and/or settlement) ... notorious "pulse" furnaces: and most especially not in the presence (same room) as a Cat I WH.

2nd SWAG is that the clean AC coil you mentined, whomever cleaned same with compressed air blew all the collected gunk towards the plenum register you photogrphed, and did so when the furnace season was over or not yet begun.

BTW looks more like BPM (even black dirt) or algae, not 'mold' perhaps mildew, to me. If the unit isn't sealed and DV, or nearby Cat I WH, could be recycled soot.

I see that on occasion, here in the sub-tropical climate of Mississippi, caused by a missing or not properly installed condensate trap allowing the fan to disperse moisture in the plenum. Not sure if it would apply to your climate, could it be left over from the summer cooling months?
Trane - HVAC Trapping Design Flaws.pdf

Originally Posted by Joe Hendrix

I see that on occasion, here in the sub-tropical climate of Mississippi, caused by a missing or not properly installed condensate trap allowing the fan to disperse moisture in the plenum. Not sure if it would apply to your climate, could it be left over from the summer cooling months?
Trane - HVAC Trapping Design Flaws.pdf

Thanks Joe. That's a great write up. Any idea who wrote it or where it came from? I'd bet the this is what caused the growth.