Should we close off A/C vents in unused rooms? (condo, home)

[shanrx]

We just bought our first home and have quite a few rooms we won't be using for awhile, pretty much our whole upstairs. We have 2 A/C units that run seperately, 1 upstairs and 1 downstairs. We aren't sure if we should close the vents in the upstairs rooms since we won't be using them. Right now we have them closed, but we feel like the A/C is running all the time. We have a 2 story family room that opens to the hallway upstairs so we aren't sure how the cooling works with that. Also, does anyone know what we should be setting the upstairs thermostat to? I know we could ask an HVAC person, but I just wanted to get some of your experiences and suggestions. Thanks!

[raptorhigh]

I'll start by saying I'm not an A/C expert, but would like to pass along what I've learned. If you have 2 zones, I'd imagine you have 2 thermostats. Unless they did something like add temperature sensors in other rooms, the thermostats should be in rooms/areas where you have the vents open. As for shutting down vents in unused rooms, I'd just make sure you have pressure operated damper attached to the main HVAC blower. Again, not an expert, but it's my understanding that in an over-pressure scenario (like one created by closing vents), that barometric bypass damper will open to relieve the excess pressure. On my HVAC setup, this just looks like a joint in the duct with a metal weight attached to a rod. It is very close to the blower, and will actuate when the system is blowing air into a zone that has multiple closed vents.

That said, I've heard from a few HVAC guys that closing vents is a bad idea. The AC system is sized for a certain square footage, and reducing that can lead to efficiency issues. I really doubt closing one or two vents is going to cause you trouble, but just passing on what I've heard.

[ChelleC]

I am not an expert either and it sounds like raptor has way more experience than I do, but would like to say from my own experiences...

Shutting vents will not help with your costs if that area is still open to the rest of the house. All it will do it make your unit run longer and harder trying to maintain a constant temperature. The only way to keep your unit from cooling those areas is: 1) Shut the doors leading to those areas (assuming there are no sensors in the areas) so that the cool air doesn't keep going in and your main unit doesn't keep trying to compensate for it being warmer in those areas OR 2) shut off your 2nd unit AND the areas.

If the areas you are shutting the vents in are open to the remainder of the home, you would be better off opening the vents back up and just turning that thermostat to, say, 75 or so. That way it will keep the area from getting too warm, but it won't be running as much.

If your 2nd unit is required to keep your main living quarters cool, then you may want to talk to an HVAC company about re-zoning your system or about ways to control your cooling costs. Just remember, heat rises and if you cut off the air to your upstairs it's going to be very uncomfortable up there.

[Tober138]

Originally Posted by shanrx

We just bought our first home and have quite a few rooms we won't be using for awhile, pretty much our whole upstairs. We have 2 A/C units that run seperately, 1 upstairs and 1 downstairs. We aren't sure if we should close the vents in the upstairs rooms since we won't be using them. Right now we have them closed, but we feel like the A/C is running all the time. We have a 2 story family room that opens to the hallway upstairs so we aren't sure how the cooling works with that. Also, does anyone know what we should be setting the upstairs thermostat to? I know we could ask an HVAC person, but I just wanted to get some of your experiences and suggestions. Thanks!

If your A/C on the second floor is a separate unit from the first floor, instead of closing vents, why not just turn the upstairs unit off - or set it for a higher temperature that will be tolerable if you have to go up there for a minute, but will not use that much energy.

[Whytewulf]

I close the vent in one room only, above the garage. If you don't go upstairs, I recommend as Tober said, putting that one a few degrees high then your lower. Hot air rises, so you should be ok.

[car421]

Don't close the vents.
Closing Off Vents and Rooms to Save Energy

[Whytewulf]

Originally Posted by car421

Don't close the vents.
Closing Off Vents and Rooms to Save Energy

Good little article. I will have to share that with the folks as it comfirmed what I suspected.

"The energy penalty associated with the register closing technique was found to be minimized if registers farthest from the air handler are closed first because this tends to only affect the pressures and air leakage for the closed off branch. Closing registers nearer the air handler tends to increase the pressures and air leakage for the whole system. "

Which is why I only close the one room off at the end of the system

Though funny story when I first moved in, I close nearly allt he vents upstairs except my master bedroom. Then one night as I am lying in bed and the AC came on, a torrent of air came at my face from the one vent above my bed. I thought my roof had been blown off. haha, I went and opened more vents.

[BigJon3475]

Say you have a 2 ton system. It should be moving 800 CFMs of air. Lets say there is 20 supply vents and you close 4. You'll now be moving 160 CFM less than 800 which is enough to put straight aqir systems in the danger zone for refrigerant flood back. Most heat pumps have what's called a suction line accumulator which is like a storage tank in certain modes and it also acts as a last resort protector for your compressor. It can still get floodback if it's way overcharged or airflow is deminished past a certain point. Now the loss of cfms by shutting vents off depends on a lot of different factors including duct size, type, length, air temp, humidity... etc etc etc. Basically your system is designed to condition the envelope not certain parts pof the envelope unless it's setup as a zone system with baffles and adjust fan speeds. Also something else to think about is interior walls are not insulated to you allow a room to get blazing hot turns it into a giant radiator. So you are crippling your system while allowing heat closer to the rooms you want comfortable and at the same time killing what little protection you do have like insulation on exterior walls.

[car421]

Originally Posted by BigJon3475

Say you have a 2 ton system. It should be moving 800 CFMs of air. Lets say there is 20 supply vents and you close 4. You'll now be moving 160 CFM less than 800 which is enough to put straight aqir systems in the danger zone for refrigerant flood back. Most heat pumps have what's called a suction line accumulator which is like a storage tank in certain modes and it also acts as a last resort protector for your compressor. It can still get floodback if it's way overcharged or airflow is deminished past a certain point. Now the loss of cfms by shutting vents off depends on a lot of different factors including duct size, type, length, air temp, humidity... etc etc etc. Basically your system is designed to condition the envelope not certain parts pof the envelope unless it's setup as a zone system with baffles and adjust fan speeds. Also something else to think about is interior walls are not insulated to you allow a room to get blazing hot turns it into a giant radiator. So you are crippling your system while allowing heat closer to the rooms you want comfortable and at the same time killing what little protection you do have like insulation on exterior walls.

What he said.

[ohioaninsc]

I only have 1 unit for my condo, but I just keep my thermostat at 78 degrees...it still is cooler than outside, and it doesn't run that much...of course, somehow even at 78, my room temps seem to hover around 76.