Barometric Damper/Humidity Problems

[bartman0531]

Howdy,

I have a client who's condo on the gulf coast built in '82 that has been experiencing humidity problems. They have reported condensation forming on the ceiling, which serves as the slab of the unit above. Aside from the usual suspects, leaky windows and an AHU in need of service, there is a barometric damper to the outside of the unit. My only experience with barometric dampers have been in chimneys or gas stoves to regulate combustion air. And i guess my question is why would this even be here? There are no gas appliances in the unit and it has electric heat. Is it to supply outside air to the AHU? That sure seams like a bad idea given the coastal climate. I have attached a copy of the mechanical plan. Note 8 is the barometric damper.

Any expertise would be welcome.

 

[Shophound]

If I read the prints correctly, there's an arrow showing air exiting the building where the damper is located. Is this correct in that the barometric damper vanes swing outward?

Does the condensation form when the condo is unoccupied and the a/c set point raised to save energy? Or does it occur regardless of occupancy? If the former, the owner will need to set his thermostat lower (assuming the a/c is working properly and able to control the condo's indoor dew point) than where he has when the unit is unoccupied to avoid condensation. What may be happening is the condo above his is occupied continuously, or the owner maintains his indoor temperature lower, which makes the ceiling/slab colder by turn. The unit below will suffer if the owner does not keep indoor humidity controlled. If he does not want to run a/c to do this, he should consider either a whole-house or free standing dehumidifier. Whole house has the advantage of evenly controlling humidity and disposing condensate without buckets, and the a/c can be left off or set at higher temperatures when the unit is vacant.

 

[DRWeig]

Answers to Shophound's list of questions is necessary to give a more thoughtful response regarding the condensation problem.

On the "why a barometric damper" question, I'm going to guess that it's for pressure equalization of the condo as the gulf coast hurricanes pass through, keep the glass windows from bursting out? That's just a guess, mind you.

Good on ya,

Goober Dave

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[willard3]

Dehumidifiers breed legionella in an occupied space.......not that good an idea.

 

[bartman0531]

Shophound,

The tenant has told me the condensation is most notable after returning to the unit but they noticed this while the unit is occupied too, but to a lesser extent. The unit above has a long term tenant. I did a little experiment and shut off the AC in the unit for 24 hours and took temp/RH readings. The ambient temp and RH% crept up to around 77/69RH% and the ceiling ceiling surface temp was as low as 70 in some areas. They must have had the AC cranked in the unit above.

The scenario you described is more or less what I had concluded but at this point I need to come up with a recommendation for repair. It's going to go something like -- Service the AHU, clean coils, verify CFM's at registers; seal(or replace) the leaky slider doors that face the prevailing winds, possibly install window tint to reduce solar gain to help the AHU keep up.

The only thing I am struggling with is the damper, and yes the vanes swing outward. Like DWReig mentioned this could be to equalize pressure in a wind event. This was my first thought as well but I suspect it is very leaky and a big source of air infiltration and am considering recommending that it be closed off.

 

[KiwiMace]

Insulate the ceiling. The damper is a red herring. It is probably there just to moderate the internal pressure against the outdoor air flow, perhaps they thought they were making a super-tight building to prevent this very problem.

 

[Shophound]

If the ceiling surface temp holds steady at 70 degrees, your condo will see condensation on the ceiling if the given interior dry bulb temperature of 77 and a relative humidity level of 79% is achieved. You may have localized areas near the ceiling, due to variances in the ceiling material and level of air circulation in your condo beneath it, combined with envelope leakage, where the %RH is higher than that.

In each condo, does the cold air supplied via the HVAC system discharge from the ceiling, sidewalls, or floor?

 

[bartman0531]

Discharge is on the walls just below the ceiling. The some of blistering on the ceiling was just in front of the vents closest to the AHU.

 

[Shophound]

>>>>Discharge is on the walls just below the ceiling. The some of blistering on the ceiling was just in front of the vents closest to the AHU. <<<<

Are you experiencing condensation on the ceiling inside your condo at the vicinity of the supply registers? If so I would either redirect the discharge air so it does not blow onto the ceiling, or if the existing registers do not easily allow this to happen, change the registers so the air is thrown far into the room instead of scraping against the ceiling. Some supply registers are really cheap and do a poor job of mixing the discharge air with the room air. This can also occur if a given supply outlet is not fed enough air from the AHU (below optimal FPM and CFM). End result in either case is a cold plume of air that chills surrounding surfaces to dew point, leading to condensation and mold.