Sweating AC Grille

[Pete]

Have a HVAC supply grille above the washer/dryer cabinet, in the salon, that has run condensate down the paneling leaving multiple white water stains. This is my second time dealing with this refinish project and I would like to make it my last. The grille is bronze colored aluminum used back in the 70's. I seem to recall that some type of heat strip on the back of the grille was a recommended solution. Anyone successfully deal with this type of condensate problem?

 

[Briankinley2004]

Usually condensation is due to improper airflow. I guess it could be heat from the dryer if it has a leak in the vents. I would address that before I worked on the finishes.

 

[wndsr]

I would also suggest that you make sure the drain tube for the condensate pan is flowing properly. The blower may be blowing excess condensation on the grille

 

[Colwyn D]

My ac outlet vent does the same. I figured it was because the aluminium aacted as a cold sink. I've just re made them with a teak surround but using the aluminium louvres. Unfortunately its that cold in New Smyrna, Fl. at the moment that last thing I need is AC so can't tell you if it works.

 

[madhatter1]

AC controls guy. Air leaving the AC coil is chilled and condensating (100% relative humidity) which is what it is supposed to do to strip the air of moisture. Exiting the diffuser and mixing with room temp the relative humidity instantly drops due to temperature rise. This is all dependent on airflow. Could be dirty filter, dirty coil, air leak before diffuser, or just poor design of ductwork.

Just like correcting a Diesel that runs too hot it is all about the basics and rarely ever complicated.

Verify all is correct step by step. Like the Diesel running high temps it's usually a combination of all having an airflow restriction that add up to create the issue.

 

[Pete]

Appreciate the suggestions and info. Let me go one layer deeper on the situation. First, the air flow is significant and there is no condensation on the directional vanes. The condensation is on the face of the frame. There is no air flow across the face except for perhaps a slight amount of room air from the venturi effect of the discharged conditioned air. I believe the problem is simply an aluminum frame with excellent thermal conductivity, being cooled from behind by the conditioned air flow, and the frame face exposed to room air with high humidity. In essence the frame face is acting as an evaporator by dropping the air in contact with it below its dew point, resulting in condensation.

That is my understanding of how the frame becomes wet with condensation, leading to dripping onto finished wood. I am not an expert on this subject, so feel free to set me straight if my brain has it wrong.

Giving that understanding, my thought is the frame temperature must be increased above the dew point of the room ambient air, somehow. Three alternatives that come to mind: 1) replace the air register with a teak framed unit, which will have greatly reduced thermal conductivity, maybe enough to stop the sweating, 2) put some sort of heat strip on the back of the register face to raise the temperature of the frame above the dew point the room ambient air, 3) introduce additional air flow in the conditioned air after the evaporator to lower the discharge temperature at the register. None of these sound great to me for various reasons, thus my request for more brain power by the members be applied to the subject.

Thanks for your help.

 

[Briankinley2004]

You could have air leaking at the back of the register box. If it is conducting through the aluminum you should be able to make a thermal break by using some adhesive backed weatherstripping on the grille. It is available in numerous widths and thicknesses. It would be applied to the back of the flanges and wouldn't be visible.

 

[Pete]

Brian, like your idea. Worth a try. Thanks

 

[Briankinley2004]

Let us know if it works. They solved the sweating aluminum window problem years ago in a similar manner. Rubber strip between the window frame sections to stop sweating. Hope it works and you haven't lost much if it doesnt

 

[Reefgeorge]

What is the humidity of the air in the boat with the AC running and having this problem? Does it matter what the conditions are outside when this happens? What are the other outlet vents doing?

George

 

[Pete]

Brian, implemented your suggestion. 2" wide, 1/8" thick adhesive backed foam rubber sold for this very purpose. Applied two layers to everything other than the actual face flange and directional vanes. Have to wait for Summer in eastern NC to get results.

George, the discharge from the offending vent is located in the salon of the 48 LRC. It is directly across, about 10 feet away, from the entrance door from the cockpit. It is directly above the washer/dryer although these were not operational during the latest event, right after Florence went through New Bern, NC, when we were living on the boat using generator power. Directly to port is the galley. As for humidity level, that time of year and most of the warm weather season, the humidity outside goes close if not to 100% overnight. During daylight, mid afternoon, it may drop in the 50% range. Big part of the problem, is frequent opening and closing of the door to the cockpit. The unit is a 24K BTU with two evaporators of equal size one in the position under discussion and the other low under a corner built in table, directly to the starboard of the cockpit door. And the two evaporators deliver large volumes of really cold air. I have not measured the actual temperature drop across the evaporator coils. Which leads me to one other observation about the installation. When the unit was sized way back when, it had to be able to handle 95 degree outside air, while cooking on the stove with the exhaust fan running, the dryer also operating and a few openings of the cockpit door. Now you take that same system, no cooking, no dryer, evening temperatures and humidity. The system is clearly oversized for this environment. Sounds like the ideal environment for a two speed compressor. Maybe the modern chiller approach helps with this problem. As for the other discharge vents, the ones below in the sleeping/shower quarters do not exhibit the problem. The pilothouse overhead vent will occasionally start some minor sweating but nothing like the profuse sweating of the problem vent.

 

[Reefgeorge]

I suspect that a combination of an oversized unit coupled with high internal humidity is the cause. What is the discharge temp at that vent vs the temp in the cabin and have you ever measured the internal humidity when this is happening? A feature of an oversized unit is poor internal humidity removal because you get to set point too fast and the discharge temp can be very cold.

George