I have lots of my auditing customers who ask me about duct cleaning, and if they should do it.  It seems like the only way to get them really clean is with some sort of roto brush, but it also seems like that process is going to possibly do more damage than good.  Obviously this really depends on the duct system and the quality of the craftsmanship, but I was just looking for the Pro's opinions.  Thanks!

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"They are taken in by the return ducts and circulated around the home by the HVAC system. A regular duct cleaning can help reduce the number of these particulates building up and being pushed back into the home."

If they are circulating through the duct system and being pushed back into the home, what is the build up that needs cleaning made of?

Kristoff Irwin had an episode on his Positive Energy podcast about this. He interviewed a guy with the epa (i believe) who is also doing his own research actively on larger systems. 

Basically yes it does help but sometimes the cleaning can be more harmful. If you are cleaning with a solvent it will score the metal and fins in the coils. This creates more surface area in the ducts and allows them to get dirtier and the coil to be less efficient and allows more breeding ground for bad bacterias and debris. Thus making you want to clean them more often and just keep the cycle going. So good buisness for the duct cleaners. Not for your system.

They found a hot steam clean with a probiotic solution is best. The heat breaks down the crud and the probiotic solution coats all surfaces with a good bacterial that doesnt allow the bad stuff to adhere and fights it off. The guy had a large system in an industrial building that automatically sprays the probiotic solution for 15 min every week in the system. Hasnt had to clean it for 3 yrs now. He used to clean it twice a yr.

Yes I did say probiotic solution.

Why are they calling?

Get 4 or 5 why's.

Are supplies dirty?! Why!!

In most cases, changing from tiny return with restrictive 1" filter to Big A$$ Drop with 4" media filter will be MUCH better way to improve comfort, health, and durability.

Ductless air conditioning systems allow you to break away from larger units that weave ductwork throughout your home. With a ductless ac installation, small groups are installed in each area of your home providing heated or cooled air directly where it is needed. Here are 5 main benefits of ductless air conditioner installation. 

There is some good info in that article but there are also errors. I give the author credit - I think he is reporting accurately on his experience. However, in most humid climates the air leaving the cooling coil is saturated and may keep the interior of the ducts wet. It is VERY common to find microbial grown in the air conditioning supply plenum and nearby supply ducts in humid climates.

Speaking of personal experiences - mine match several of those previously posted (Ed Minch and Chris Heenen). I would NOT recommend duct cleaning except on an "as needed" basis. I personally change that "as needed" to the more direct "in the most unusual and severe cases". Flex duct and duct board are virtually uncleanable. The damage caused by the cleaning process is greater than any potential benefit. If there is a need to "clean flex ducts" I would recommend the ducts be replaced instead of cleaned. The usual response is that unless the ducts are accessible (attic or basement) the cost to access the duct is too high. My experience is that if you "clean the ducts" you will be creating access to replace the ducts soon. That cost is hidden at first but quickly becomes apparent. I recommend skipping the step that causing problems with little benefit and going straight to the solution that gains the benefit without causing hidden problems.

There are several 'duct cleaning' issues that I would recommend - clean the coil, clean the blower, clean the return plenum. Those are generally accessible and can provide most of the benefit of even the best duct cleaning. And I recognize that there are exceptions.

Of course - your mileage (experience) may vary. And that does not mean either of our experience are wrong - just different.

The air is only saturated because it is very cold.  The amount of "actual moisture" in the air is very low.  Any condensation that happens should be outside the ducts, not inside the ducts.  That would be an inadequate insulation and air sealing issue.  If that water is finding its way inside the ducts that would be a problem for sure.

I agree the air is saturated because the temperature is at the dew point. The "amount of moisture" is irrelevant - the air is saturated. At a 55 degree dew point there is plenty of moisture to wet the inside of the ducts. Any condensation outside the ducts is due to the dew point of the air outside the ducts.

We can discuss theory or "what should be happening" forever. The bottom line is I have NEVER opened up an existing supply plenum and not found it coated with microbial growth in my humid climate. Never. There is an old saying that if the data and theory don't match - keep the data and get a new theory. So the theories about blower heat or heat thru the duct insulation are consistently inadequate to raise the surface temperature and prevent microbial growth.

I suspect that in drier climates the theories are more applicable.But not in my humid climate.

To reach dew point at 55 degrees F, the humidity in the duct after the condenser would have to be above 99%---I think that would be unusual.  It is more likely warm moist air is finding its way into the cold duct and condensing.

I'm not even sure how to address that comment.

First - If we are talking about the "condenser" (sic) then we are talking the outdoor unit of an air conditioner or a heat pump in heating mode. Neither of which will condense water from the air.

Second - in a humid climate, if the air conditioner does not get the supply air down to 55 degree dew point, the indoor humidity cannot be controlled. Remember the outdoor air dew point is around 70 degrees. Humidity can never be reduced by exhaust or ventilating with outdoor air. FYI - look on a psychrometric chart and you will find that 74 dry bulb and 50% RH is a 55 degree dew point. So the typical design condition for cooling is 55 F dry bulb and 55 F wet bulb.

After typing this it dawned on me you might be talking about a dehumidifier that uses the cooled air to cool the condenser. In that case the 55 degree air gets reheated by the condenser.

Yes of  course, I meant the indoor coil.  Obviously the coil at 40 degrees is going to be well below the dew point sufficient to condense moisture out of the air where it can be drained away, so on that surface or very near it the humidity is going to be close to 100%, but as the air moves away from the coil the humidity is going to drop considerably and will very quickly be above the dew point.  Again this air will not be the culprit in condensing moisture at the ductwork past the coil---but air intrusion from the hot humid air outside the ductwork easily can be.

I hate beating a dead horse but -

Quoting "...very near it the humidity is going to be close to 100%, but as the air moves away from the coil the humidity is going to drop considerably and will very quickly be above the dew point."

I would like to see / hear your analysis of how this will happen. If the cooling is still on, the supply air temperature should continue to be at 55F and 100% RH. And that air is constantly being replaced with fresh supply air at 55F and 100%RH. What is the source of heat that is raising the dry bulb temperature? Or the source of moisture removal that is reducing the RH? The inside thermal mass of the ducts? What is the thermal mass of duct board or flex duct? The heat flux thru the duct insulation? Lets see, the interior of the duct is at 55F because there is a constant stream of 55 F air over it. The outside of the duct insulation may cool off as the duct cools down but the interior does not really warm up until the cooling shuts off. What does change is the distance down stream the interior of the duct is at 55F will increase farther and farther down the duct until the cooling shuts off. That means the point at which the RH drops below saturation keeps moving farther and farther down stream. In a humid climate, the interior of supply ducts is typically saturated any time the cooing is on.While the cooling is on, the pressure inside the ducts normally means any air leakage is from the inside of the ducts to the outside.

Again, I am lost with your logic. Can you show me how the situation you described happens?

In a climate with a lower outdoor dew point (like Seattle), you might see a 40 degree coil surface temperature. You might even get a leaving dew point temperature in the low 50's. In a humid climate, it is rare to get a coil temperature much below the return air dew point. Once a point on the cooling coil reaches the dew point of the air passing over it, the surface temperature of the coil is virtually locked down. Attempts to lower the coil temperature on the refrigerant side just condense more water out of the air without the coil getting significantly colder. If you want to lower the coil surface temperature, you have to reduce the water entering the coil by reducing the air flow or provide some other means to dehumidify the incoming air.


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