Air duct vents sweat when AC is cooling

[frostedflake]

Hello everyone!

I have this house with installed fan & coil units and the vents from where the AC enters to the rooms starts to sweat (condensate) and drips water to the floor.

i know some causes may be:


high ambient humidity.
that the air flow of the fans may be poor.
that the ducts run a bit longer than usual.
no propper duct sealing.

i would like to know if there could be any other major reason that may be causing the vents condensating.


thanks in advance for reading this post, enjoy the weekend!.

Frostedflake

[B G Scott]

Only one reason, the temperature of the outlet is below the Dew Point temperature of the room.

[MilosBog]

What? Does this thing occurs only on the particular place or ? Please describe when..
BG scout has told what is the physics. What are the room temperature?Check in the project what are the temperatures of inlet and outlet air. If you have a ducted fancoils maybe you can increase the air flow so the temperature will be higher than the dew point..Please describe when and what

[Magoo]

the supply air temp is below the dew point, possibly because air flow has reduced due to blocked filters. Dirty coil face.Uninsulated supply ducting. Cavity air space may require ventilation.

[S_Line]

Thats a pain when the Grilles Condensate, ive seen this happen before. It is down to the Dew point.

The only way you will be able to stop it, is too increase(yes) the temp of the cooled air going into the room.
Increasing the fan speed will help.

[frostedflake]

Thanks so much for your insight on the subject, i will try to elaborate providing more information.

Dry bulb temps

Ambient temperature outside: 75 - 78 F
Evaporator air temperature: 52 F
Air vent air temperature: 57 F

As some of you suggest that one way to go around this issue is to increase the temperature of the cooled air, does this mean the room will take longer to reach the desired comfort AC temp?

again i appreciate the time taken to read this post and to contribute.

frostedflake

[dougheret0]

In a similar case I worked on, the building was secure so the doors were kept locked. Employees only had a key to the front door. Employees were sneaking out for a smoke and blocking the side and rear doors open so they could get back in. The AC system had negative pressure - more exhaust than ventilation air - so in central Florida, outdoor air with a dew point higher than 74F was being sucked in the open doors.

[Gary]

Evaporator air temperature: 52 F


I assume this is the air temp leaving the evap. What is the air temp entering the evap?

[Gary]

As some of you suggest that one way to go around this issue is to increase the temperature of the cooled air, does this mean the room will take longer to reach the desired comfort AC temp?


Just the opposite. Assuming the air temp is raised by increasing the air flow, the volume more than compensates for the temp change and the room cools faster.

[mad fridgie]

The direct cause for the condensation as stated above, The air is to cold.
Apart for looking at direct airflow problems, you should also look at you refrigeration capacity/system, If this is occurring in more temperate weather (not hot), it could be that your SCT (discharge pressure) is somewhat lower, compared to hot weather, for which the system is designed. The system will balance giving greater capacity and lower air temperatures.

[Roadrunning123]

Check your filters and evaporator coil condition and !!! Is all the windows closed ?
Also check room temp and surface temp of outlet.
If your reaching dew point then your return is too small or not enough air removal in the given area, also check that your superheat is between 7 and 9 ktd deg C

[Sledge]

I agree with what is said above:

52f is too cold leaving the evap.

reasons could be many;

poor air flow, poor system design etc.

[Gary]

Condensate is forming because the supply temp is lower than the room dewpoint. Is the supply temp too low or is the dewpoint too high?

[Gary]

In a similar case I worked on, the building was secure so the doors were kept locked. Employees only had a key to the front door. Employees were sneaking out for a smoke and blocking the side and rear doors open so they could get back in. The AC system had negative pressure - more exhaust than ventilation air - so in central Florida, outdoor air with a dew point higher than 74F was being sucked in the open doors.

It seems like dumping cold air into the room would reduce the indoor dewpoint. I'm wondering if this system isn't also running in negative indoor pressure... possibly indicating leaks in the supply air ducts?

[frostedflake]

Thanks so much to everyones input on this issue, as i read from your replies i'll elaborate on the information.

the 52F temp is the air temperature leaving the evaporator coil, being 68F the temperature of the air entering the coil after an hour of operation or so.

as for the negative or positive pressure of the house I inspected the balconies/windows/doors.
where there are glass doors there is very little air entering the house.
being at an 8th storey building there seems to be very little air entering the windows.
The entrance door on the other hand, right at the bottom creates a very strong air draft into the house.
as for the system air supply ducts I have inspected them and no leak was found so far.
So i think the house has overall negative air pressure.

How do i calculate the indoor dewpoint?
I know the outdoor temperature during measures was 75-78F with 70% humidity.

i found this link http://www.inspectapedia.com/Energy/Dew_Point_Tables.htm and i'm trying to figure it out.

Thanks for taking the time to read this thread and kudos on the new site design/look!!

Frostedflake.

[Gary]

Generally, the difference between air entering the coil and air leaving the coil (dT) should be about 20F. Your dT is 68-52=16F. You have too much airflow. You need to switch the fan to a lower speed.

Why are you keeping the room at 68F?

[frostedflake]

Thanks for your reply. I'll try correcting the dT's and post the results.
One of the vents that sweat the most has already very poor airflow, but the main problem with that particular one is that the evaporator maintenance was neglected for 3 years so you can imagine the amount of dirt accumulated thus restricting propper air circulation.

The customer decided that the room feels great only with the thermostat set at 61F. Not much I can do about that other than pointing the extended runtimes of the compressor end at the electric bill.

[Gary]

Running the temperature below 70F can freeze the coil. It's an air conditioner, not a refrigerator.

[Gary]

Try the dT change (reduce fan speed) and the customer may not feel the need to keep it that cold, because the humidity will drop.

[Magoo]

Hi Gary the Guru,
can you elaborate please.

[Gary]

Reducing the airflow lowers the leaving air temp. Lower leaving air temp means lower coil surface temp, which means more moisture removed from the air as it flows through. Also this extends the run time, so more of the room air is dehumidified during the run cycle. Higher airflow gives us more cooling (and higher efficiency). Lower airflow give us more dehumidification (and lower efficiency).

[frostedflake]

It is really amazing you share your year's of knowledge with us.
I appreciate the help and will post the results.

Thank you very much,

Frostedflake

[meenakshidp]

Many homeowners these days are experiencing sweating ducts. Most of these sweating ducts carry cool supply air, although sometimes return air ducts can sweat, especially if the house is kept very cold and the return ducts run through cool vented crawlspaces.

The outside surfaces of the ducts are below the dew point of the air in their immediate surroundings. This is the reason why duct sweat.

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