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Why Do Air Conditioning Vents Sweat?

Air Conditioner Supply Vent With Mold Because Of Sweating

Back in the summer of 2005, I got a call to figure out a moisture problem in a new house.  The homeowners took me into the master suite, where the first thing to catch my eye was the towel folded neatly on the floor.  It hadn’t just fallen out of the laundry basket; it was there to catch the water dripping from the air conditioner supply vent in the ceiling directly above.  Another one was on the other side of the bed, and those two vents were dripping steadily.  It didn’t take me long to find the problem.

Air conditioner vents get cold.  Water vapor loves cold surfaces.  Sometimes it leads to mold, as you can see in the photo above.  Other times it leads to the paint coming off and rust forming on the vent (photo at bottom).  If you’re lucky, it’s just an occasional nuisance that doesn’t lead to long-term damage to the supply register, indoor air quality issues, or the need for towels on the floor.  And if you’re really lucky, you wonder why I’m even writing about this because you’ve never seen it.

One cause, multiple reasons

There are a lot of reasons why this happens, but there’s only one cause.  Let’s start with the cause.  Condensation occurs only when humid air is in contact with a cold surface.  If your air conditioner supply vents are sweating, you have a cold surface (the vent) and near it you have air with water vapor in it.  The threshold for condensation to occur is that the temperature of the cold surface has to be below the dew point of the water vapor in the nearby air.

That’s simple, right?  Humid air with a dew point above the temperature of the supply vent causes condensation.  Now, as for the reasons, we can start with the two big ones.  Either the air is too humid (dew point too high) or the supply vent is too cold.  That means the fixes are simply reducing the humidity of the air or raising the temperature of the vent, depending on which of the two reasons is causing the problem.

Dew point and the temperature of conditioned air

But how do you tell which is the culprit?  Is the dew point too high or the supply vent too cold?  The special number here is 55° F.  Well, it’s special if you keep your house at or close to the indoor design conditions recommended by the Air Conditioning Contractors of America (ACCA).  Those conditions are 75° F for the indoor temperature and 50% relative humidity.  Those conditions correspond to a dew point of 55° F, our special number.

If you don’t keep your house at 75° F and 50% relative humidity, the table above shows the relative humidity levels that correspond to a 55° F dew point when you go below or above a temperature of 75° F.  As you can see, the colder you keep the house, the higher the relative humidity can be to keep your 55° F dew point.  Conversely, if you keep the house warmer, you have to keep the relative humidity lower or the dew point will rise above 55° F.

But what if you’re not on or close to one of those lines leading to a 55° F dew point?  The higher your relative humidity is, the higher the dew point is at a given temperature.  That means you’re more likely to get condensation because the supply vent doesn’t have to be as cold.  At a temperature of 75° F and relative humidity of 60%, for example, the dew point will be 60° F.  A supply vent at a temperature of 60° F or below will sweat in that case.

If your indoor temperature and relative humidity are at or close to any of those rows in the table above, your dew point is about 55° F.  That means as long as the vent itself is warmer than 55° F, you shouldn’t see any sweating.  How cold is conditioned air?  When it comes off the cooling coil, it’s usually about 55 to 60 degrees Fahrenheit.  By the time it reaches the supply vent, it might be a bit warmer, depending on where the ducts are located and how well they’re insulated.  Even if the air gets to the vent at 55° F, though, the vent itself is in contact with the warmer room and probably will be warmer than the air passing through it.

The starting point of figuring out whether your problem is humidity or supply vent temperature is to measure them.  Getting the indoor air temperature and relative humidity isn’t hard, and once you have those you can easily find the dew point.  The Dew Point Calculator website makes it easy with sliders for the three variables.  Once you’ve determined whether humidity or vent temperature is the culprit, you’re in a position to eliminate the problem.

Why the indoor humidity might be too high

Here in the southeastern United States, the summer dew point is often above 70° F.  The more of that high dew point outdoor air that gets into your house, the higher your indoor dew point will be.  In some places, like condos on the beach, that outdoor humidity gets into the house because people open the doors and windows while the air conditioner is running.  Easy fix:  Close the doors and windows.

More likely, the house stays too humid because of unintentional infiltration.  Most houses are too leaky.  The fix is air sealing.  Get a blower door test, seal up as many leakage sites as possible, and the indoor humidity will go down in summer.  A house above a vented crawl space is connected to a big source of humidity, especially when it has holes as large as the bathtub drain hole shown below.  Encapsulate and condition the crawl space or make the floor above the crawl space airtight.

One way that outdoor air gets inside intentionally is through ventilation.  When you turn on a bath fan or a range hood, you exhaust indoor air and cause outdoor air to come in.  Clothes dryers do this, too.  On the whole-house ventilation front, heat recovery ventilators (HRVs) bring in outdoor humidity; energy recovery ventilators (ERVs) also bring in humidity but less than HRVs.  (An ERV is not a dehumidifier.)  Ventilating dehumidifiers reduce the humidity.

If the house doesn’t have a lot of infiltration and isn’t over-ventilated, look for indoor sources of humidity.  Some possibilities are:

  • Showering without running the bath fan
  • Doing a lot of cooking
  • Having a lot of aquariums (You know who you are, afishionados!)
  • Steam showers, hot tubs, indoor pools, water features

Two more humidity trends are working against you, too.  New houses keep getting more insulation and air sealing, which reduces the cooling load so air conditioners don’t run as much.  Also, some of the more advanced air conditioning systems have less ability to remove moisture.  In some cases, the only way you can get the humidity low enough to prevent the vents from sweating is to use a dehumidifier.

That humidity’s got to be coming from somewhere.  Your two fixes here are to reduce the source of the humidity or use a dehumidifier.

Why the supply vent might be too cold

If the indoor humidity is OK, the AC vents still can sweat if they get too cold.  The two main causes of excessively cold vents are low refrigerant level and low air flow through the system.  You’ll need an HVAC company to check the refrigerant level, but you may be able to do something about low air flow.  Here are some of the main causes of low air flow:

  • Dirty filter
  • Excessively restrictive filter
  • Return or supply vents blocked by furniture, rugs, or other items
  • Dirty cooling coil
  • Excessively restrictive duct system

You’ll probably need to hire a pro for the last two, but if you suspect low air flow, the first thing to do is check the filter.  Then check to see if all the vents are unblocked and the louvers open.

I’ve also heard of cases where the air flow was too low in multi-stage or variable capacity air conditioners operating at low speed.  Ideally, the refrigerant and air flow change together in just the right amounts, but complex systems don’t always do what you want them to do.  Sometimes the air flow is fine at one speed but too low at another speed.  In addition, multi-stage or variable capacity air conditioners are designed to give you long runtimes.  If cold air is always blowing out of the vents in a house that’s on the edge of having sweating vents, that might be just enough to get those vents wet.

Start with the basics

The cause of sweating vents is simple.  Air that’s humid enough finds a surface cold enough.  When vents start beading up with water—or worse, dripping—either the humidity is too high or the vent is too cold.  It’s not hard to figure out which of those two is the culprit.  Just find the dew point of your indoor air.  If it’s close to 55° F, the problem lies with the vents being too cold.  If it’s above 60° F, the problem is probably the indoor humidity.  From there, you can sort through the potential sources of humidity or low air flow to see if you can fix the problem yourself.  Otherwise, call in an HVAC pro to find the problem and stop those vents from sweating.

Oh, you’re probably wondering about that house with the dripping supply vents in the master suite.  Turns out it had a problem with humidity, not supply vent temperature.  They had spray foam insulation installed beneath the roof deck to encapsulate the attic and make it part of the conditioned space.  But the spray foam contractor missed part of the eave, so it wasn’t sealed well. 

To make matters worse, the master bedroom had a tray ceiling with a gap all the way around, connecting the bedroom to what should have been a sealed attic.   Negative pressure in the attic from duct leakage and in the master bathroom when the exhaust fan ran pulled in humid Georgia air, and it didn’t mind the gap.  It got into the bedroom through that tray ceiling gap, finding those nice, cold air conditioning vents.  The spray foam contractor sealed the eaves, and the problem went away.


Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard. He is also the author of the Energy Vanguard Blog.  You can follow him on Twitter at @EnergyVanguard.


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This Post Has 17 Comments

  1. Thank you for this simple
    Thank you for this simple explanation of this sticky problem on a hot humid summer day. Gotta go check my vents.

    1. You’re welcome, Dean.  It’s a

      You’re welcome, Dean.  It’s a common problem.

  2. Keep an eye on the moisture
    Keep an eye on the moisture drain for the A/C unit, if it clogs the moisture backs up and is picked up by the passing air.

    1. Interesting point, Harold.  I

      Interesting point, Harold.  I’m trying to understand how that would be different from normal operation.  Once the AC has been operating for a while, there’s going to be water on the coil and in the drain pan even when the drain isn’t clogged.  If the drain does clog, there will be more water in the pan but why would it evaporate more than during normal operation?

      1. The standing water is
        The standing water is evaporated if the blower should be operated in the “on” position since the coil is not being cooled in the refrigeration process. If the drain is blocked, causing an overflow, then there is excess water in the equipment, on the floor, or around the coil, however the system is installed.

  3. If there are leaks in
    If there are leaks in ductwork in attic, doesn’t the house go negative and pull in moist hot air around improperly sealed ceiling vents? I have been literally “rained” on by dripping vents.

    1. Thomas, yes, that can happen

      Thomas, yes, that can happen with ducts in an unconditioned attic, but it depends on whether the leaks are predominantly on the supply side or the return side.  With more supply leakage than return leakage outside the enclosure, the house will be under negative pressure.  Here’s an article I wrote about that:

      The Sucking and the Blowing — A Lesson in Duct Leakage

  4. Great post Allison! I’ve
    Great post Allison! I’ve highlighted several points and passed it along to many of the HVAC contractors that I work with. Had a discussion with one of them the the other day who was insisting that the supply air should be around 45 degrees Fahrenheit. Jeffrey will find this hard to believe. He was so impressed with the work that he saw here recently!:)

  5. I would add lots of house
    I would add lots of house plants to that list of humidity sources. I’ve encountered a case of this being a factor, the living room looked like a jungle. Also, looked at a home that more or less had a conservatory with it’s own ERV and it should have been an HRV because the space was still fighting humidity. Here in Michigan hydroponics and grown rooms are starting to be factor as well. People aren’t considering the humidity and moisture effects on the structure.

    1. Good point, Andy.  Lots of

      Good point, Andy.  Lots of house plants can be as bad as lots of aquariums.

  6. Allison-
    You Wrote… I’ve also heard of cases where the air flow was too low in multi-stage or variable capacity air conditioners operating at low speed. Ideally, the refrigerant and air flow change together in just the right amounts, but complex systems don’t always do what you want them to do. Sometimes the air flow is fine at one speed but too low at another speed.
    How would I confirm or eliminate this as being the issue? Flow hood I suppose?

    1. The easiest way to find out

      The easiest way to find out if this is causing a problem is to measure the temperature of the supply air at the vents or the surface temperature of the vents themselves.  If it’s 50° F or below, it’s probably a problem.

  7. ………Or, incorrectly
    ………Or, incorrectly installed “pull through” type evaporator coils – commonly found on electric air handlers, installed without a condensate drain trap. As these systems run they pull condensate water through the untapped drain line and instead of draining, throw water all over the face of the coil, re- humidifying the very air that the air conditioning system is intended to dehumidify.

  8. You had me until this part:
    You had me until this part:
    “Why the supply vent might be too cold
    …The two main causes of excessively cold vents are low refrigerant level and low air flow through the system.”
    Are you telling me that supply air can be TOO cold, and that it could be caused by, among other things, LOW refrigerant levels or LOW air flow?
    For the love of Air Supply! How can any of this be true?
    Yours truly,
    Traumatized in Tuscaloosa

    1. Hi. Low refrigerant results in low temperatures in the evaporator. Low temps are a result of low pressures due to low refrigerant. If the coil is much colder than 40 degrees, it can result in colder than normal supply air (until the coil freezes). If the air flow is too low, it spends more time in contact with the coil, again, resulting in colder than normal supply air

  9. Reducing the indoor airflow rate will result in colder supply air but it will also result in more latent capacity, thus lower humidity levels in the house. So lowering the airflow rate might help reduce this sweating.

    Low refrigerant charge does mean lower evaporator pressure and thus lower temperatures in part of the coil, but the refrigerant flow rate is greatly reduced due to choked flow in the expansion device. The result is that much of the coil contains superheated refrigerant which results in the overall evaporator capacity being lower and thus the supply air temperature will be higher. So low refrigerant charge does not contribute to the grill sweating unless I am missing something here.

    The biggest issue that I have seen contributing to supply register sweating is humid air from a vented attic leaking around the register. The solution is to seal and insulate where the supply duct passes through the ceiling or wall surface.

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