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Condensation on supply grilles

Condensation on supply grilles

Condensation on supply grilles

I have a problem I have never encountered before.

A chilled water system with a series of fan coils. Each fan coil is installed in a bulkhead with a short section of supply duct terminating in a sidewall grille. We are getting condnesation on some of the grilles. Has anyone ever experienced this?

I am figuring it is either caused by warm air in the room passing over the grille and causing the moisture to condense. Another possibility might be that when the unit cycles, the warmer air it blows through the duct is cuasing condensation on the grille which will still be cold.

Any other possibilities of what is causing this and what can be done to prevent it?

RE: Condensation on supply grilles

Any time that the dewpoint in the room is greater than the supply air temperature (and therefore the supply grille temperature) you can have condensation on the grilles.  Are the grilles that are sweating close to an unconditioned room, or a door to outside?  Are they close to any sources of water vapor, like a kitchen?

One thing that I have found to be helpful in this situation is the use of plastic grilles.  They will tolerate a higher dewpoint without sweating.


RE: Condensation on supply grilles

No, there is nothing I can see that would cause an unusual amount of moisture in the air. That's why I find it a bit perplexing.

RE: Condensation on supply grilles

I have observed this in large lobbies and high solar load zones.When you use a constant volume system in an area where the room sensible load varies substantially,the room relative humidity level goes up very high when sensible load drops(for eg solar load variation dependent on time of the day).You either need to use reheat or raise off coil temperature to avoid this problem.

RE: Condensation on supply grilles

How is outside air being supplied ?  Is treated air being supplied to the fan coils or is the outside air directly being ducted to the fan coil ?  

Also check for the possibility of any opening above the false ceiling through which there's an ingress of outside air.


RE: Condensation on supply grilles

Just some thoughts to consider.
Were the fancoil coils selected to avoid moisture carry-over?  
Is there the possibility that the static pressure from the fan is drawing moisture in from the drain pan?

RE: Condensation on supply grilles

SAK 9, I think you may be right. There is no option to install reheat, but the water supply temp could be increased a bit to increase the temperature off the coils.

We have reduced the fan speed (3 speed control) and the condensation is not nearlly as bad, but still present.

The outside air mixes with the return air before reaching the coils. There are no obviosu signs of outside air coming in.

It is definitely not moisutre carry over. The water is definitely condensing on to the grilles.


RE: Condensation on supply grilles

Is the condensation happening throughout the day ? or is it happening at some specific time everyday ?


RE: Condensation on supply grilles

Sometimes also, fan coils by themselves do not have enough latent capacity to handle the latent load because their coil is not deep (not enough rows available) enough. Supplemental primary air at lower temperature is sometimes required to handle latent loads. Thus verify that the primary air quantity and temperature is adequate to meet the ventilation air requirement and also the latent load of the space if can't be handled by the fan coils.

RE: Condensation on supply grilles

Another option would be to dump some of the supply air into the ceiling plenum or return air duct.This would take out the element of overdesign in the supply air volume to the space.It is possible to have it automated by installing a motorised damper which reponds to high relative humidity levels say 70% and above in the occupied space.

Though this may sound crude and inefficient,it does work!

RE: Condensation on supply grilles

You have to introduce the primary air to the space to make it handle the space latent load. The latent load absorbed by the primary air is:

Btu/hr latent load sbsorbed by primary air = CFM primary air x .69143 x 7000 x (lb moisture per lb dry air at room indoor design - lb moisture per lb dry air at entering primary air condition)

Make sure this is equal to or greater than the room latent load (from people, latent equip gain and infiltration if any)

RE: Condensation on supply grilles


Will there be a condition that the chilled fluid valve closes totally during a day? As you already have a fresh air intake, the moisture in fresh air may be condensing on a cold grille. I have seen in many ocassions, condensing of moisture in fresh air upon controlled space walls and ceiling when the chilled water supply is totally cut off.

You can minimize this by crack opening the chilled water bypass.

When you reduce the speed, the fresh air flowrate also gets reduced and that is why there may be less condensation.

Just close the OA damper totally and observe.


RE: Condensation on supply grilles

Are you in a very humid climate?

Is there a lot of infiltration in your building. If you are recycling most of your air (i.e. 90%) than the room air will eventually dry out due to you removing the moisture and the space humidity will fall (hopefully to say 50%). If however it is a shop where the doors open constatntly and let in humis air, you have problems.

As already suggested, you would need to cool then reheat to minimise this problem.

I also like the idea of plastic grilles.

Also, try a full recirc unit which is dedicated to dehumidification only to try and lower the space humidity.(we use wall mounted dedicated units for this if we get a problem)

Try a company called Calorex who specialise in this type of unit.

Friar Tuck of Sherwood

RE: Condensation on supply grilles

To quark,

Yes, the valves will close off from time to time, some more than others as some are oversized because they changed the partiiton layout so some units have a much smaller area to cool than previously. I have not beem able to do it yet, but I plan to increase the water supply temp. to keep the valves open for longer periods. THis may have a benefit of saving some energy too.


It is an office building, so there will be some infiltrationdue to doors opening, but it should not be as much as a shop.

I will keep you guys updated as I go on.

RE: Condensation on supply grilles

Check the latent capacity of the fan coil. If you increase the supply air temperature you would have less latent capacity. As I said in my previous post you have to supply primary air with humidity below room design. Also with this system start up is critical. The primary air must be started say one hour first before the fan coils to dehumidify the space.

RE: Condensation on supply grilles

This is a stupid question, but are you getting moisture carryover from the cooling coil?

RE: Condensation on supply grilles

No, it is defintiely not moisture carryover.

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