mold growth on air supply register

**Jack Wingo** · 09-08-2012, 10:41 PM

Every supply register has mildew that has reappeared after thorough :cleaning 6 months ago. Any ideal what is the cause.

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**Rick Cantrell** · 09-09-2012, 04:17 AM

Originally Posted by Jack Wingo

Every supply register has mildew that has reappeared after thorough :cleaning 6 months ago. Any ideal what is the cause.

Mold/ mildew will only grow where there is moisture.
Moisture on the registers is likely from condensation.
Check for:
partially blocked/ clogged cooling coil
Dirty air filter
Crushed duct
Oversized AC unit for this house
Too much moisture in the air; cooking, showers
Also hole in return duct

**Raymond Wand** · 09-09-2012, 04:43 AM

Indoor Fungal Habits

**Markus Keller** · 09-09-2012, 10:43 AM

I'm guessing you guys don't install humidifiers in FL? Or maybe there is one and its turned up too high.

**Jerry Peck** · 09-09-2012, 05:36 PM

Originally Posted by Jack Wingo

Every supply register has mildew that has reappeared after thorough :cleaning 6 months ago. Any ideal what is the cause.

Register and supply plenum box not sealed to drywall ceiling, this allows hot, moist attic air to leak through and condense on metal supply register.

Originally Posted by Markus Keller

I'm guessing you guys don't install humidifiers in FL? Or maybe there is one and its turned up too high.

Markus,

Humidifiers ... down here ... the only humidifier we use down here is called 'outdoor air', and we try to keep it turned as low as possible.

**Robert Slight** · 09-10-2012, 07:19 AM

Mildew means moisture. These cases are a process of elimination. I'd start with the evaporator coil. Check the freon pressure (HVAC technician) but also check to see if the condenser is oversized. Short cycling condensers don't give the evaporator coil enough time to remove moisture from the conditioned air.

**Scott Patterson** · 09-10-2012, 05:36 PM

I think Jerry has it right. I have also seen this in cases of poor housekeeping where the dust builds up and starts to grow.....

**Joe Hendrix** · 09-13-2012, 05:21 AM

Don't forget to take a long good look at the evaporator condensate drain trap. Poorly designed or missing traps will let moisture enter the air steam causing the problem you have described.
http://0323c7c.netsolhost.com/docs/T...aws%5B1%5D.pdf

**Thomas McKay** · 09-14-2012, 08:11 AM

I agree with Robert; seen many cases where the homeowner has replaced the proper sized condenser with a larger one expecting better performance. What they get is a wet house and mildew buildup on the supply plenum. Actually had one homeowner boast that he had installed a 5 ton condenser at his 1500 Sg. Ft. home what I found was beads of condensation on the interior walls. Best to stick to the mfg. suggestions for sizing the condenser or heat pump.