Will closing all but one vent in an apartment with central air cause leaks in the duct work?

July 17, 2009 3:38 PM

Will closing all but one vent in an apartment with central air cause leaks in the duct work (I mean, there are flaps on the vents for a reason, right?)?

I rent a four room apartment with central air (South Eastern United States about 40 minutes South of the Appalachians). I really only care about keeping my computer room cold, which is the smallest of all four rooms (the others being the bedroom, living room, and kitchen) and also is where the thermostat is located.

I'm thinking about sealing all the vents in my apartment with the exception of the one vent in my computer room to save on cooling costs, because I can tolerate the other rooms being 85 deg. Fahrenheit, but not my poor computer (and seal of the computer room itself with some thick blankets suspended from curtain rods). It has occurred to me, however, that doing so could increase the pressure in the ducts and perhaps cause a leak, since the conditioned air from the central unit now only has one vent to escape from.

I have considered buying a single room air conditioner (and have also read the other ask mefi threads about single room air conditioning options). It seems to me that window units are much more efficient than portable units, but they pose somewhat of a security risk (my apartment being at ground level), and I don't know that my landlord would be so happy about my doing so.

Is there a way I can figure out what my the rating of my ducts is? Can anybody suggest other low cost solutions to bring the ambient temperature of my computer room down (Googling doesn't really present many solutions, although my Google-fu may also be a bit inadequate)?
Is my only recourse a window-mounted unit? Are there ways to address the security risk this presents?

posted by weakcore to Computers & Internet (9 answers total)

Response by poster: erm slight typo .... I meant to say "and seal OFF the computer itself with some thick blankets ..." though I'm sure you've figured that out by now.
posted by weakcore at 3:45 PM on July 17, 2009

Since you rent, you could talk to your landlord about the status of the ducts. The age of the apartment will likely have a lot to do with the quality of the ducts, and your landlord might know something.

An important consideration is the location of you thermostat. If it's in a different room, you might have to leave those vents open as well or risk your air conditioner running nonstop (bad for the environment and your pocketbook). If you close the vents in two of the four rooms, I'm fairly certain that would be fine as far as the ducts are concerned. That's really only double pressure and the ducts should be well equipped to handle that.
posted by scrutiny at 4:15 PM on July 17, 2009

Are you asking if this will damage the ductwork? No, it won't. I'd only be concerned if the thermostat isn't in the computer room because then you have a real problem. The AC would never turn off as the thermostat would never cool.
posted by chairface at 4:20 PM on July 17, 2009

Best answer: An important consideration is the location of return air ducts. Your furnace can't pump cold air into a room unless there is an equal amount leaving. Make sure that your cold air returns are unobstructed.

The worry with sealing ducts off is not causing a leak in a duct (furnace ducts are generally not completely airtight anyway), but damaging the blower fan in your furnace itself. You could shorten the blower motors life if you force it to try to blow air into a blocked off system.

As some of the above posters have stated, thermostat location is important. If you decide to do this, in order to see any energy savings you would need to move the thermostat to your computer room. The furnace doesn't know what vents are blocked off, it just blows until your thermostat tells it that it is cool enough.
posted by davey_darling at 4:24 PM on July 17, 2009 [1 favorite]

Not sure about damage to the vents, but I know that when I lived in an apartment building in New York, the Super came through all the apartments once a year to make sure they were all open because people (including me) kept them closed routinely with no ill effects obvious.

All I know is, when they came around to force them all open, I suddenly had a horrendous infestation of mice. :/
posted by Edubya at 4:45 PM on July 17, 2009

Best answer: Closing off some or most of the vents won't cause any harm to the ductwork. The blower in your central A/C doesn't develop much pressure.

What it will do is reduce the total airflow through the unit, which causes the evaporator and condenser to run colder. Combined, there will be several effects:

• The blower motor and compressor will use less power when they are running
• The refrigeration cycle will be less efficient
• The air coming out the vent(s) which are still open will be colder
• Once warmed up to room temperature, the air will be dryer (less relative humidity)
• You might get more noise (from higher air flow through the remaining open vents, and blocked off vents can make their own noise)
• If the evaporator coil gets cold enough, it might freeze up, and you'll have no airflow at all once it's completely iced over. If this happens enough, you can damage the coil from the ice forcing joints apart.

You'll save some energy from the blower and compressor motors being less loaded, and from more (and colder) air going into the room with the thermostat, so that the system doesn't run as long. Offsetting that, there will be a loss of efficiency, and a cool room surrounded by warm rooms will pick up some heat from them, so will require more cooling to maintain temperature.

I routinely close off half the vents in my house to balance the temperature in the summer. That doesn't seem to cause any problems, though it can get a bit breezy in some rooms from the higher airflow there. Going down to 1/4 of the vents open seems dicey to me. Also I notice if a large portion of the house has no airflow, the air gets kind of stale. I would probably try partially closing/blocking some of the vents instead of closing them off completely.
posted by FishBike at 4:47 PM on July 17, 2009 [1 favorite]

It's just on the edge of possible that by closing off 75% of your vents you could cause your evaporator coil to ice over. This isn't an irreversible problem, you would just need to turn the A/C off for a day to let the ice melt. And it would be pretty obvious as the both the cooling and air flow would stop or be severely reduced.

Also be aware of where your thermostat is located. If it isn't in the conditioned space then you risk icing if the temperature drops too low.

Finally there is a risk of condensation in the cold room when you open the door and exchange all the cold air with warm moisture laden air. This is more of a problem if you have high RH levels in the unconditioned space.
posted by Mitheral at 6:17 PM on July 17, 2009

Response by poster: Thanks for the responses (FYI, my thermostat is conveniently located in my computer room) ....

davey_darling, I had not really even considered the return air vent, but now that you mention it, it is located in the hallway leading to my computer room, which ends up fitting nicely with my plan, as I can simply close the door on the opposite end of the hall instead of having to hang a second blanket.

FishBike, thanks for mentioning the stale air thing. I had almost forgotten about that. My apartment is fortunate enough to have ceiling fans in all the rooms.

Mitheral, do you think that even with the condenser sitting in the sun all afternoon (it is in a position where it gets a good deal of sun, which I'm sure reduces its efficiency too) the evaporator coil to ice? I usually crank the A/C down when I'm not in the house and aren't downloading anything--putting my computer in hibernate when not in use-- and plan to continue doing so even after plugging up the unused vents (or just stick with downloading things at night when I sleep). I guess I will have to be more ninja like when entering and exiting the air-conditioned space ....
posted by weakcore at 7:05 PM on July 17, 2009

The evaporator coil is inside your furnace so sun exposure won't affect it. Icing is a remote possibility anyways; I mention more so you are aware of it.
posted by Mitheral at 11:30 PM on July 17, 2009