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Condensation Problems in multiple rooms

Condensation Problems in multiple rooms

Condensation Problems in multiple rooms

(OP)
Hi all, I currently have a project where supply air grilles in several rooms are experiencing condensation. The design is such that outdoor air goes through a 100% fresh air AHU(PAHU) before going through localized FCUs in each room. From my understanding, condensation can be reduced by either reducing the dew point of the air at room condition or raising the surface’s temperature. Would the following methods help to reduce the condensation?

-Increase FCU off-coil temperature (thereby raising the temperature of the supply air grille)
-Reduce PAHU off-coil temperature (to cool and dehumidify outdoor humid air and reduce absolute humidity of the air before it passes through the FCU)
-Reduce the temperature set-point for the room condition (at same RH, the dew point is reduced)

I’m still relatively inexperienced in this, hope more experienced engineers will be able to provide insight as to whether my line of thinking is right.

Thanks all!

RE: Condensation Problems in multiple rooms

Tell us more about the project. Is this a hotel? Is condensation happening in unoccupied rooms? What is the condition of outside air supplied to FCUs? Is ventilation air cooled to dew point and then reheated to room temperature? Are the rooms in the internal zone or perimeter zone?

RE: Condensation Problems in multiple rooms

In order to condense, heat must be removed from water vapor. Since heat in the air is already removed by the cooling coils, the place the air can further lose heat is air that is outside the air handling system. That is the room air. So the room air is too humid and water is condensing much the same way as onto a cold drink glass.

Increasing the temp of the air delivered to the air grill will reduce the amount of heat removed from room air at the grill.

The other proposal don't seem like they would help - if there is a source of water vapor in the room then reducing the off-coil temp will increase the heat absorbed at the grill and the amount of condensation. Driving the room temp down will simply drive the relative humidity in the room higher which, in my experience, is rarely comfortable.

Has the bathroom ventilation been inspected for function? Are there leaks into the building? Is there infiltration of air into the rooms, particularly in excess of the ability of the system to displace outside?

RE: Condensation Problems in multiple rooms

You need to sketch this system and add some data like temperatures and RH.

From you description, outside humid air goes though an initial cooler ( your PAHU) then is blown into individual rooms where there is a FCU.

what is this FCU doing? further cooling? heating?

If cooling why are you surprised you're getting condensation?

what do you mean by the grills are experiencing condensation. This can only happen if the grills are colder than the incoming or or that the FCUs are blowing water through to the grills. Are the condensate drains blocked??

We can only see what you tell us and so far it doesn't make sense.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Condensation Problems in multiple rooms

Dew point ins pace is too high. I bet you also have comfort problems due to high RH.

Climate? In the rainforest, or the Mongolian desert? What room settings and operation? Is it a steam kitchen, or office?

Maybe recirculate air and dehumidify before you let fresh air in.

RE: Condensation Problems in multiple rooms

Aside from all the input you’re getting and your correct comments amount the contact of high dewpoint to cold surfaces and correcting that which is important to make sure is correct keep in mind sometimes it’s not the design itself.

Your post seems to indicate it’s only certain rooms so it’s possible it’s related to the spaces - do they have leaky envelope or an open source of water evaporating in the space.

Also sometimes diffusers get poorly installed and not completely sealed to the ceiling around them and higher humidity plenum air leaks at those cracks into the cold diffusers surface in what is otherwise a room with a well controlled dewpoint.

RE: Condensation Problems in multiple rooms

The condensation on grills does confirm the dew point of room air is higher than the surface temperature of airflow grills. When high moisture, air contact a dry surface, then the air is unable to maintain that moisture level, condensation will start. This is like the reverse of vaporization. I have seen the same issues, mainly in the entrance area. If It is happening in the rooms, please check the following observations
1.The FCU is correctly sized, right? Suppose, the oversized FCU may shall satisfy the room sensible load, and the remaining moisture will be condensed to the nearest cold surface, like a grill.
2.When you decrease the FAHU off coil temperature, it will reduce the working time of FCU and subsequent moisture removal. So, increase off coil set point of FAHU. by which, a constant thermal load will get the FCU and reduce Rh levels in the room

RE: Condensation Problems in multiple rooms

Simply tell the occupants to stop boiling kettles and washing items up in those rooms.

RE: Condensation Problems in multiple rooms

I have run across this. Now in normal HVAC design the space is pressurized with conditioned outdoor air at about 0.5 CFM/SF to prevent infiltration of outdoor air. The condensation is caused by infiltration of outdoor air which is caused by 2 things. First there is a crack or opening that lets in outdoor air in the ceiling cavity. The second is that the air handler outdoor air damper is stuck in the closed position so there is no outdoor air to pressurize the building.
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