Condensate flow and mold in mini-split ductless AC indoor unit

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SUMMARY

The discussion focuses on the mechanics of condensate flow and mold prevention in mini-split ductless AC indoor units. Users explore how condensation forms on the evaporator fins and the importance of airflow velocity in preventing water droplets from being blown into the room. Recommendations include using coil cleaner, maintaining clean filters, and employing vinegar or algae tablets to manage mold growth in the drain pan. The conversation emphasizes the significance of proper maintenance to avoid foul odors and microbial issues.

PREREQUISITES
  • Understanding of mini-split ductless AC systems
  • Knowledge of evaporator coil function and condensate management
  • Familiarity with cleaning agents like citric acid and vinegar
  • Awareness of air filter maintenance and its impact on air quality
NEXT STEPS
  • Research the use of citric acid-based coil cleaners for AC maintenance
  • Learn about the effects of airflow velocity on condensate management
  • Investigate commercially available algae tablets for condensate pans
  • Explore best practices for maintaining air filters in mini-split systems
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HVAC technicians, mini-split system owners, and anyone responsible for maintaining indoor air quality in ductless AC units.

Swamp Thing
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My AC has begun to emit a rather foul smell, which has led me to geek out on the inner workings of its indoor section.

For example, how does the condensed moisture make its way to the drain pan? I am assuming that the condensation happens on the fins of the evaporator. But in that case, what prevents the condensed water droplets from being drawn through the fins and through the blower, and then being thrown into the room?

This question in turn arises because I was wondering if a bit of diluted disinfectant or antifungal liquid, if poured onto the top of the evaporator, will end up mostly in the drain pan? If yes, then that will be a way for me to disinfect the drain pan without dismantling the unit.
 
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The water is already in the incoming air. When the air temperature drops when in contact with the evaporator, the air loses the ability to transport that much water, and thus the extra water is expelled from the air, falling on the evaporator. This is the same phenomenon as when the hot air from inside the house hits a cold window: The water leaves the cooler air and rests on the window.

Looking at this table, imagine you have air at 50°C with 50 % relative humidity; that's 41.5 g/m³ of water coming in. After crossing the evaporator, it's now at 25°C. The maximum possible relative humidity is 100 % or 23.0 g/m³ of water. The difference must come out of the air and thus accumulates on the evaporator. When there is too much on the evaporator, it drips away.

So what came out of the air because of a temperature change will never go back in, i.e. being picked up again, unless much dryer or hotter air comes in afterward.
 
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jack action said:
When there is too much on the evaporator, it drips away.
That is exactly my point. My question is not about re-evaporation of this dripping water (which is what your reply addresses, if I understand correctly).

My question is about mechanical transport of the dripping water, impelled by the blower, into the room. What prevents this?
 
Surface tension?

##\ ##
 
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hutchphd said:
Surface tension?

##\ ##

So if I slowly and carefully dribble some disinfectant-laced water onto the top of the evaporator, it will find its way into the drain pan and not damage other things, and not drip out of the air outlet?
 
Were I worrying about such things, I would
  1. Get a good spray bottle, put in a solution of water with a few squirts dish detergent (not the dishwasher stuff) and set it on fine mist
  2. Remove the "air filter sponge" on the inlet
  3. turn on the airflow and allow the sprayed mist to be inhaled by the air conditioner for a while suficient to use up some spray and dribble out the efflux pipe
I have never done this nor do I claim particular expertise
 
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Swamp Thing said:
My question is about mechanical transport of the dripping water, impelled by the blower, into the room. What prevents this?
The airflow velocity is too low (by design) to carry the droplets off the evaporator so instead they fall into the drain pan.
 
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Swamp Thing said:
So if I slowly and carefully dribble some disinfectant-laced water onto the top of the evaporator, it will find its way into the drain pan and not damage other things, and not drip out of the air outlet?
Please don't. The properties of water including surface tension, mass and vapor pressure along with the velocity keep it on the coil. The disinfectant might just evaporate into the air.

Just buy coil cleaner and spray it on the coil with the unit off. They are usually citric acid based.

And make sure you are using decent filters; an ac evaporator coil should normally be clean aluminum covered with distilled water. Not an attractive place for mold.

Unless you/the inhabitants are a smoker.
 
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Swamp Thing said:
... This question in turn arises because I was wondering if a bit of diluted disinfectant or antifungal liquid, if poured onto the top of the evaporator, will end up mostly in the drain pan? If yes, then that will be a way for me to disinfect the drain pan without dismantling the unit.
There are some commercially available tablets that keep certain type of algae to grow in condensate pans that are not fully draining out due to incorrect leveling of the air handler unit.

Lacking those tablets that slowly dissolve themselves, some people use small amounts of vinegar once a month or so.

Accumulation of dust in the coil (due to deficient filter) can produce all kind of problems, since disruption of air flow and heat transfer until microbial growth.

Is your air handling unit part of a mini-split system, as the title of the thread seems to suggest?
 
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Lnewqban said:
Is your air handling unit part of a mini-split system, as the title of the thread seems to suggest?
Yes, it is.
 
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