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Question DetailsAsked on 7/29/2016

We have mold in our HVAC's plenum. Is a rebuild our only option or can the plenum be cut, treated, + resealed?

The evaporative coils and everything else looked clean...unit dates to 2012, and most of the ducts are hard, not flexible. I found a pdf on Internet that discussed cutting the plenum and spraying with mold-resistant coating, then resealing the unit as opposed to doing a complete rebuild...Any feedback as to how this (common?) issue is addressed will be greatly valued. If a rebuild is our only option, will an insurance claim be required? What is likelihood this issue will recur?

We live in Memphis, TN if there is a mold remediator with HVAC experience in the area...or a contractor with a lot of mold remediation experience. Thanks for your advice and assistance!

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3 Answers


If it has a terrible case sometimes replacing the ductwork is the only solution - but I have never seen a rigid metal duct that could not be adequately cleaned. Flexible corrugated ducting yes, but that product is a whole nother ball of wax you don't need to hear my opinion on, with respect to normal low-pressure/low velocity systems like almost all houses have.

Flexible ducting does not survive the cleaning process well - it tears, kinks, blows out, comes disconnected, etc. But rigid ducting can be cleaned by several air jet, air blast/rotating brush, and spray disinfection followed by air jetting or brushing systems. The ones I like best use a reverse jet air jetting tool, similar to pressure washing or sewer jetting, combined with VERY high airflow - typically around 5,000-10,000 cfm (cubic feet of air per minute), so the jetting tool (which is pushed through the duct to the end of the branch (which is taped shut) then backed out blasts loose the dust and buildup in the duct back towards the entry point, which is also where (or at the furnace plenum sometimes for a single setup point) the vacuum truck is pulling the air towards it so it cleans out the bulk of the material, then a final cleaning with same vacuum but a rotating brush is used throughout the length, then typically a mold killing spray lining is misted into the duct from end to end. The importance of the very high volume vacuum is it creates a high enough velocity in the ducts (they literally howl in many cases) that it pulls much of the matted material off the duct wall, and the air jet and brush finish that job for really tightly bonded locations or places (like at junctions) where the airflow is sheltered at a particular place.

The ducting is "cut" - but rarely is it actually cut through - usually the entry point is a place it can be taken apart, or a small hole is cut in the ducting for the hoses and later sealed with a sealed access panel, but rarely is the ducting "cut apart" destructively.

Air Duct Cleaning is the Search the List category - but you will have to weed out the $150-250 guys with a shop vac or carpet cleaning vac (which is nowhere big enough for ducts) from those with the expensive but truly professional equipment, which will normally (for about 2000SF 2-story home) cost about $400-600.

My opinion - the mold killing spray by itself is a waste - first it does not penetrate the dust mat so could not kill it all anyway, plus even if it kills it all right then, if the dust mat (which it needs as a substrate and food) is not removed it will just grow again. But done after the cleaning it can be a useful final step to surface kill the clean surface to force any mold regrowth to start from scratch with natural airborne mold.

Another factor - who told you that you have mold ? Someone with a handy-dandy test kit, which will read positive for almost any household surface that is not cleaned daily because there is substantial mold in the air all around us, at least when ground is not snow-covered. So the issue is not IS there mold because there almosat always will be detectable mold on surfaces, but is it a substantial buildup - which means it has an organic surface that it likes the "taste" of to grow on, and continual or near-continual high humidty (generally 50-60% or higher) to grow on.

If you don't find and cure the cause of the high humidity, the mold will just come back. Likewise, if the humidity stays below the critical 50% or so range, it will not have enough moisture to grow noticeably. Common causes - air conditioner not removing enough moisture from the humid incoming air (so not getting cold enough, or in rare cases is getting too cold so it ices up and does not remove the humidity - it just sublimates (changes back to vapor without liquid phase) the ice back into the duct air as soon as the A/C shuts down; or the A/C is short-cycling so it is not running air over the cold coil long enough (typically 10-15 minutes or more) to remove a significant amount of moisture from the air; or the condensate drain pan under the coil is not draining cleanly and is maybe ponding so the condensate is getting back into the air as it flows over it.

One other thing - are you sure you have significant mold - or did you look in your duct and see a fuzzy grayish coating and assume it was mildew or mold, when it might just be the normal fuzzy lint and dust buildup - which can buildup inches thick in systems with improper or no filtration. Generally, this sort of buildup is not a concern unless it is thick enough to signficantly obstruct the ducts - say commonly 1/2 inch thick or more. It looks bad, but it means the ducts are catching some of the dust moving through them, and except if the ducts are bumped hard or there is a strong earthquake or extreme low pressure system like hurricane or tornado pass over to pull it away formthe walls of the duct and loosen it up, it does not pass out into the house. Does mean your filter system is not working right or you are using cheap filters, though.

High efficiency (HEPA) filters with adequate airflow (see specification in furnace owner's manual (A/C air handler manual if it has a separate air handler) will also remove a lot of the incoming mold spores, reducing the tendency to grow mold in the ducts.

A good Heating and A/C (HVAC) tech can check the coil operation and operating temperature and A/C line pressures (high and low side) to determine if it is operating correctly, and time the cycle and check with a hygrometer (air moisture meter) to see if it is actually removing significant moisture from the air. Unless you have constant high humidity incoming air, making sure the A/C is operating correctly would be an important first step and common cure for mold growth (once existing buildup is removed).

Common causes of high household humidity other than A/C not removing it at the coil -

1) high basement or crawl space moisture coming into the conditioned space

2) high attic humidity getting in

3) leaky doors and windows letting in too much outside air, wood subfloors over unconditioned basement or crawlspace without vapor barrier, or generally "leaky" house

4) running the A/C with doors or windows open

5) significant number of indoor plants or uncovered terrariums or fish tanks

6) taking showers or hot baths without running the bathroom fan during the back or shower with the door closed, then leaving the fan going (or setting timer) for typ[ically 1/2-1 hour after wards to dehudify the bathroom

7) cooking without kitchen fan running (and for at least 5-10 minutes afterwards) - especially boiling and frying and woking

8) in some new homes (less than a few months old) wetness in the framing from air-dried lumber or framing that got wet before dying-in passing into the air

9) running unvented gas heater or fireplaces, releasing the water vapor (which is a SUBSTANTIAL portion of the exhaust gases, into the living space - to the tune of several gallons a day in may cases

10) excessive makeup air inflow to the furnace/air handler, so you are bringing in constant flow of humid outside air so it cannot keep up and reduce the indoor humidity enough

11) wrong-sized ducting at A/C evaporator coil, so significant airflow can bypass the coil and hence does not have the moisture removed at the coil, but that humid air hits the cooled air just downstream of the coil and condenses out the moisture on the cool ducts, promoting mold growth. This is a common and under-diagnoed problem, because many A/C installers just slap the coil into the duct without paying attention to proper duct sizing for it or installing seals or baffles around it to block of bypassing. I have seen installations where only about half the cross-sectional area was blocked by the coil - with the result the vast majority of the air took the "easy route" - the lower air friction route - around the coil.

Basically speaking, if you don't have excessive humidity in the airflow and the A/C is removing a significant at the coil, and don't have a major outside humid air inflow source, you should not have a duct mold problem.

You can find some other comments in previous similar questions in the Home > Air Duct Cleaning link in Browse Projects, at lower left.

Here are links to a couple of articles on the subject too -


One other consideration - if you do get your ducts cleaned, and they do not have sealed joints (sealed with seam sealer or mastic), that is the best good time to have them sealed with internally sprayed/misted joint sealer. One type is a spray-on coating that covers the surface including joint openings, the mist type is a leak-sealer that flows with the air (in ducts with exit registers sealed) and migrates to the places air is escaping, and basically gums up the exit point until it seals it.

Answered 2 years ago by LCD



Thank you for your reply! We discovered mold growth on a couple of walls upstairs recently. One wall was directly under HVAV unit #1, which had a clogged condensation line which makes sense. The evap coil was found to be rusted (which probably contributed to the clog). But there is another wall with mold that is a mystery and still very wet according to wall humidity tester...only thing over head is a vent (but recently that vent was ruled out and it was recommended we go into wall to check the wall stack's insulation). We, therefore, had an inspection done on our HVAC (2 units) for 3,600 sq. ft. home (upstairs about 1,000 sq.ft.). The other unit #2 services most of lower floor except 1 supply to moldy wall (directly under other, formerly clogged unit). It was within this unit that the HVAC tech discovered a buildup of debris (and mold) within the plenum, on the black interior lining, though that evap coil looked clean. I don't recall seeing fuzzy white if it matters, but it was built up around the edges and smelled musty, so I took his word for it. Initially, he thought a rebuild might be necessary, but later he recommended having a mold inspector test the plenum and maybe spray/treat it...then let filter do the rest. He wasn't too worried about the mold extending into the ducts...but I like the sound of the cleaning techniques you described. We had the ducts cleaned by ServiceMaster 2 years ago, but I don't know how thorough or what equipment/techniques they used.

Some history on house prior to mold issues: My husband turned off AC upstairs, so temp was 84, when I went up last...and former home owners had closed about 50% of the ceiling vents, which we also discovered recently (maybe they knew about mold?). We have been in the house about 2 years without (known) mold issues until now...though we did have to replace the roof last year (now ridgeline vents), which had leaked prior ( covered up by former home owners) + they apparently let upstairs bath mold has probably been an issue in the past.

We live in the midsouth, so high humidity is definitely an issue. The HVAC tech did not test humidity of evap coil or anything else. Should he have done this? Also, should we have the system "balanced"?...We have had some heavy condensation on an overhead stairwell vent (vaulted stairwell ceiling). Makes sense hot air rises, hits that vent, and presto condensation...but should it be dripping that much water on the carpet? I asked about balancing the vents, but a condensation-resistant spray was recommended instead.

Thanks much for your help and advice. We are having all sorts of issues.

Answered 2 years ago by CHome


One thing I did not answer before - mold is almost never covered by insurance - both because most homeowner policies usually specifically exempt it, and also because it would be considered a chronic or long-term water/moisture problem rather than an acute one like from a pipe leak, so would generally be excluded under that clause as well. Check you policy, but almost certainly not covered, and if there is anything indicating it wouldnot be covered I would NOT contact your insurer about it - both because any call about possible coverage sometimes is counted as a claim and raises your rates (yeah that is slimy, but they do that) - and also because them knowing about a moldy house situation could cause them to cancel your insurance.

On your answer-back - the moisture-resistant spray (as opposed to a mildewcide spray) sounds like a misapplication in your case - might protect duct metal somewhat against rusting from high humidity, but as far as the moisture goes - not going to do anything about mold.

The tech would have checked for moisture issues (coil temp and moisture removal and such) only if a problem were suspected there - and as long as there is water draining from the coil drain tube (should be a slight trickle or fast drips starting a few minutes after A/C kicks on and sometimes continuing a minute or three after it shuts off), it is not freezing up, and is effectively cooling the house down as required then it is almost certainly working as it should.

I think your problem is explained by what you were saying about the upstairs unit being turned off - you had a fully-conditioned living space throughout the house [possibly excluding basement if you have one] - now you have a conditioned downstairs with unconditioned but still connected upstairs, and the moist (from outside humidity, cooking, bathing, body evaporation and breathing moisture) hot air tends to rise to the top floor, which I would suspect is showing high humidity up there, at least early in the morning when coolest. Then you run one A/C release point up there (or is it two including the stairwell vent ?) - of course you would expect serious condensation as the hot humid air fills the first part of the duct when the unit is off, then rapidly cools down by probably about 20-30 degrees when the A/C kicks on - mold would certainly be expected in the outlet area of the duct and on the register (which will be cold when and immediatelyi after unit is running but have humid room air passing by it), as well as possibly on walls if the A/C runs long enough to cool any walls down significantly.

Solution - balancing between registers might be needed but usually with a thermometer you can do that yourself, gradually adjusting (not blocking off more than about 25% of total flow areas for a given A/C unit) the registers so each room is roughly the same temperature (measured at the same elevation in the room - usually at head height in center of room) when the A/C unit turns itself off. Usually you adjust using adjustable (if you have them) return duct register rather than inflow, so you are not reducing the total cooled air inflow, just which rooms it is going to - but if one room is running really cool because the air is coming in and then flowing out under the door to other areas, you can control that with the inflow register or by leaving that door partly open usually.

Basically, I think the crux of the problem (after unblocking the blocked drain line in one unit) is to bring the other unit back into service - could be set at a slightly higher temp if you want but you will have to watch that does not cause one unit to work too hard trying to cool the entire house by itself. Of course, keeping different temps upstairs and downstairs is easier if you have a stairwell door. Running both units versus just one should not cost significantly different - might even be cheaper because you would not have one unit running excessively long cycles, which is hard on them. And also maintains the unit that was turned off - it is not real good for an A/C unit to sit long periods without running, No choice in the winter but asking for corrosion and gumming up of the compressor oil by leaving it sit unused in the summer too.

If there is a power problem with both units occasionally running at same time, or one starting hard because other is already running, it is possible to install unit control relays to prevent either thermostat from calling for cooling (and hence unit from starting) while the other is calling for cooling. Sometimes called single-run relays in the trade - not commonly needed but sometimes in older houses with smaller amperage services or breaker/fuse panels.

One other possible solution method, if your A/C(s) are short-cycling - only running a few minutes at a time before shutting off, is adjusting the setback temperature (available on many or most digital thermostats) so the temperature difference between on and off settings is greater, making the unit run longer to get the indoor temperature back down to where it shuts off. This longer run recycles the air in the house through the evaporator coil more in each cycle, so it removes more moisture from the air, dropping overall humidity.

Back on the balancing - unless both units are feeding into the same duct system, I would try the DIY route and possibly adjusting doors in rooms before paying the $300-500 a balancing test can cost.

And of course, look at the sources of moisture I gave in the previous answer and see if you can reduce some of those - especially easy is kitchen and bathroom fan use to exhaust moisture from those sources. (And from any other open source of water which I did not mention like sauna, hot tub, pool, etc.)

Answered 2 years ago by LCD

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