I’ve always heard that during a heat wave, if the A/C is struggling to keep cool, you should close off any unused rooms and close the A/C ducts to those rooms as well. However, my roommate (who only comes by once per month and essentially just uses his bedroom for storage, hence the argument) claims that this is NOT a good idea, since closing the vents will “mess up the system”. I’ve never heard of that warning before, but a quick Google search shows there’s a 50/50 chance he might be right.
My WAG is that it would depend where the thermostat is. If the thermostat is in the room where the vent is closed you will never reach the temperature to shut off the A/C and it will run continuously. It may not be an issue since some A/C’s will shut off automatically if it runs for too long to prevent ice build up on the evaporator.
Another thing might be (again just a WAG) that if there were some partial restriction between the closed vent and the open vent it could put a strain on the fan. Of course that would only be a problem if the open vent were down stream of the restriction and the closed vent upstream.
Other than those two things I can’t imagine why that would be a problem. When I lived with my parents I always closed the vents to my room when I got too cold. I am sure someone with more knowledge of these systems can give you a better answer.
As long as you don’t close off so many that you start reducing airflow, you’ll be fine. If you reduce airflow through the evaporator, it’ll ice up and reduce cooling to the rest of the house (and the AC will work harder and cost more). But one or two, I wouldn’t worry about it.
Another thing, whenever people close off vents and shut doors to unused rooms in summer, I always suggest they check the room from time to time. If the room is cooler (or hotter in winter) then the rest of the house (say, for example, because the vent doesn’t seal well), then you might as well leave the door open and let the cool air into the rest of the house.
I was making idle chatter with the AC guy when he was fixing something in my house. He mentioned that closing off the other rooms can make the system work worse. Something about what airflow the system is sized for and by closing off the rooms you are making less air flow through the system. I’m not sure specifically why that would be a problem. I didn’t get the details.
It depends. If all your registers have to be open for proper air flow across the evaporator the closing some off it could cause some of the above discussed problems.
Note as well that a room which has closed-off ducts and a sealed door can sometimes develop a serious humidity problem, leading to mold and mildew.
Every house is a unique snowflake when it comes to balancing airflow. HVAC folks I’ve spoken to claim that almost every house they have ever seen, including brand new ones not even lived in yet, could benefit from balancing and tuning the airflow through the system (provided you have the proper main duct and ancillary duct dampers). It’s very possible you can re-balance the airflow such that a couple of closed-off rooms don’t cause any trouble at all.
Never thought about that. My house (being in the midwest) isn’t typically all that humid to begin with, but when I do close off a room (one is always closed another is closed about half the time), the return stays open. I would hope the return pulls enough conditioned air under the door to keep the room dry.
If the room is bedroom-sized, and there are no ''returns" in the room, and the t-stat isn’t in it or on the wall adjacent to it, you can usually close 1-2 of these rooms off w/out any issues, provided the rest of the registers and returns are working/open.
That probably covers 80% of the scenarios people are looking to address.
If you aren’t one of those 80%, seek professional advice.
In the frozen North here… the A/C is part of the forced-air furnace system, using the same ducts, vents and returns.
I find the A/C struggles to cool the house despite cold air and good flow. I’m sure it’s because the flow is “short-circuited” between the vents and returns - you can feel cold air below your knees, but it takes hours to fill the rooms.
So, the vents and returns are positioned to optimize flow when the heating system is on, but fail for A/C: they are both at floor level. I put cardboard in front of the returns and it helps (the system brings in fresh air from outside as well as from the returns).
From an undergraduate course I took, air flow should be adjusted according to the heat loss of each room. Calculate the percentage of heat loss from each room due to exterior surfaces and windows, and adjust dampers so that the room receives the same percent of the total air flow.