As an AC cools the air it pulls condensate from inside. Ive noticed that this condensate is often discarded via drain a line away from the unit, why is this the case?

Wouldn't it be more efficient to take the cold water flowing from indoors and have it pass over the hot outdoor condenser coils, improving efficiency through evaporative cooling?

The water is clean from any minerals being condensed from the air so buildup wouldn't be a concern? Ive been unable to find out why the condensate is discarded when water is commonly used as a method of energy transfer.

i can see situations like large rooftop mount HVAC systems not having this however whats stopping small split systems or even window mount from implementing this?

  • $\begingroup$ Likely help dust buildup to eventually block the coils. $\endgroup$
    – Solar Mike
    Commented Jan 28 at 12:01
  • 1
    $\begingroup$ Water+cheap metal+outdoor air sounds like a recipe for rust. There are uses for condensate water as water rather than coolant though. $\endgroup$
    – Abel
    Commented Jan 28 at 12:32
  • $\begingroup$ I thought the same but theres the case of the evaporator coils indoors that are constantly getting drenched in water. $\endgroup$ Commented Jan 28 at 13:23
  • $\begingroup$ Completely normal thing to do - products.geappliances.com/appliance/… $\endgroup$
    – Phil Sweet
    Commented Jan 28 at 21:35

5 Answers 5


Designing a product for installation at site means that the designer has no idea or control over where the condenser coils are going to be.

Will they be located over a door step? Over a car park? Over some other electrical device? You just don't know. But you would be responsible if water dripping results in slipping, mess or damage.

Which means you can't just have an uncontrolled amount of water dispensed onto the coils, where it could then be dripping down onto who knows what.

With the existing drain line you can direct the installer to direct the drain water to a safe location. And this is a straightforward enough task that the law (mostly) accepts that it is the installer's responsibility. But having the condensing coils sometimes drip would be harder to avoid and much more likely to be viewed as a fault.

Plus the issues the others have brought up of having dust and water combine into a festering slime within the condensing coils over months and years of no cleaning.


There are indeed proposals to combine the air conditioner's heat pump action with that of evaporative cooling to increase the overall efficiency of the air conditioner. One way is to run the cold condensate over the hot coils and use both the "cold content" of that water and the evaporation of it off the coils to remove heat from them, as you propose.

To prevent the collection of airborne dirt on the wet condenser coils from ruining their heat exchange capacity requires that the condensate water be used to cool the incoming outside air before it reaches the condenser coils.

If you can devise some means of accomplishing this, then the resulting combined cycle air conditioner can be made worthwhile.


Window units blow the condensate over the condenser.

I suspect that issue is that you haven't seen the nastiness that condensate lines turn into. You don't want that all over your condenser pad and unit.


Portable a/c units and refrigerators sometimes use the condensate for cooling -- perhaps because they need a way to get rid of the condensate, but yes, it improves efficiency, particularly for portable a/c units, which are notoriously inefficient to start with.

Older refrigerators had large condensate trays, which sometimes overflowed, because they were just using natural evaporation.

You don't put the condensate "on coils", because the coils are designed for air cooling. When you use the condensate for cooling, you use an evaporation tray.


Air conditioners often double as dehumidifiers for several reasons.

  1. Control environment to mitigate microbial growth namely fungus/mold and bacteria that thrive in humid environments.
  2. Dry conditions deter corrosion of parts to ensure longer AC unit life.

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