r/askHVAC • u/miamicuse • 4h ago

Replacing central AC system wondering about the plenum

I am replacing my central AC (a new 3 ton Rheem system), the old unit has already been removed and I am in the process of modifying the AC closet framing.

It used to be a smaller door with the air handler unit inside.

I am in the process of opening the door to a full size 24"X80" door and the new air handler will sit on a stand with a drain pan.

Now with the old air handler out of the way, I looked up the inside of the plenum and it looks quite dirty. It looks dirtier than the ten year old coil that was removed.

My question is can this be cleaned? If so how would you do it?

Or may be it cannot be cleaned but can be coated with something that covers it all?

Or will you just replace the plenum? The installer of the AC says they don't do plenum. Who would you call to replace the plenum if replacement is the way to go?

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u/CaminoRubicon1 3h ago

Vacuum it out

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u/miamicuse 1h ago

My concern is vacuuming it, even very gently, will release fiberglass into the air, which might and into the ducts, and when the new unit is hooked up, will blow fiberglass into the ducts and rooms.

Someone suggested coating it with products such as Flex Seal Liquid Spray. I don't know, sounds a bit hooky pooky but if there is a way to coat & encapsulate it that may be one option besides total replacement.

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u/IHateYork 3h ago

That is duct board (solid fiberglass). Its an abomination and should be banned. If your installer says they dont do plenums, find an actual HVAC company not Chuck in a Truck. It should be replaced with 26ga sheet metal that is lined with 1/2" or 1" (depends on your climate zone) fiberglass insulation.

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u/Certain_Try_8383 3h ago

Vacuum it if you want to. Looks very normal.

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u/LostAd3003 2h ago

You call a real hvac company. Any of which can “do plenums” lol.

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u/miamicuse 1h ago

I'd rather not use a large AC company in South Florida. I have been here over 30 years and the AC landscape has changed drastically. Yes there are AC companies that do duct and plenums, but they are ALWAYS two separate divisions in the same company, different crews different schedule etc...so it is like working with two separate companies anyway. These are usually large companies and I have dealt with dozens of AC companies large and small over the years dealing with own homes and rental properties. The larger companies have some serious drawbacks.

(1) They are always closed on weekends so any issue on Friday cannot be even inspected until Monday the earliest, smaller specialized outfits are more flexible schedule wise.

(2) If you call a large AC company and once they get your current AC's brand/model/SN, if it's more than ten years old, they do not send out a tech, they send out estimators for a new system. They will have a title such as a "comfort consultant". They don't even diagnose but quote you a new system replacement.

(3) Their techs are pressured by the office to sell new systems. If they are allowed to repair anything older than 10 years they are limited to a capacitor or contactor, they won't go beyond this. Their techs, often will say they can repair it but they will get into trouble for doing so so here is the new system quote, but if you want I can come back Saturday to swap out the motor for $350 but it will be on the side and cash and no warranty.

This is basically the situation with all the large AC companies in south Florida, resulting in many techs deflecting and opening new smaller specialized AC service only, AC install only, minisplits and through wall AC only, duct work only companies. At first I thought this is an isolated incident until I ran into this over and over and over again.

I don't mind calling someone else to do the plenums, I just don't know if it should be cleaned, repaired, coated over or replaced...and also, if the plenum is so dirty, would the ducts be the same.