# Yet another AC question **URL:** \ **Category:** Factual Questions\ **Created:** [September 12, 2026, 4:06pm UTC](https://boards.straightdope.com/t/yet-another-ac-question/1033016 "2026-09-12T16:06:09Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![burpo\_the\_wonder\_mutt](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/burpo_the_wonder_mutt/32/365_2.png) [@burpo\_the\_wonder\_mutt](https://boards.straightdope.com/u/burpo_the_wonder_mutt)\ **Post date:** [September 12, 2026, 4:06pm UTC](https://boards.straightdope.com/t/yet-another-ac-question/1033016/1 "2026-09-12T16:06:09Z")
The house we’re renting has a “Florida room”: 90% windows and I can’t vouch for insulation in the roof/ceiling. And, get this — there are two AC vents and a large return _in the ceiling_. We have the door to the backyard open several times a day for K9s to relieve themselves and I keep hearing my Dad saying _We’re not air conditioning the outdoors_. It feels like we are. I closed the vents, but plenty cool air still escapes. The return really unnerves me; isn’t it sucking in 100 degree air as opposed to floor-level cooler air? Yesterday, I cut some cardboard pieces to size and taped them in place over all three grills. The AC vents leak no more and the return is sealed (man, was it sucking air; it practically yanked the cardboard out of my hands and held it there while I taped it in place). And, of course, the **Voice of Doom** is now harping at me that I’ve done something untoward to the AC system; even Googling couldn’t tell me if what I’ve done is not good (plenty of comments about adding another room would overwork the system, but nothing about closing off a “room”). What say you, Dopers? Best case scenario, could we expect a drop in our electric bill? Thanks. ---
**Author:** ![filmore](https://avatars.discourse-cdn.com/v4/letter/f/7993a0/32.png) [@filmore](https://boards.straightdope.com/u/filmore)\ **Post date:** [September 12, 2026, 6:23pm UTC](https://boards.straightdope.com/t/yet-another-ac-question/1033016/2 "2026-09-12T18:23:31Z")
AC systems are designed for certain airflow amounts. If you limit the air through the system, problems can happen. One is that the coils can ice over. Without sufficient air flowing over the coils to warm up the freon, the coils will ice over from the humidity. Eventually the coils will be covered in a block of ice and air won’t flow through the system. It can even cause ice to form along the tubing to the coils. Another problem is stressing the blower motor. By making it harder to pull air into the system and out the vents, the motor has to work harder. Blocking the intake is like if you cover the opening of a vacuum cleaner hose and the motor gets louder. Closing the vents isn’t so bad. Covering the intake will likely lead to problems due to the amount of air that will be reduced. What you might want to do instead is look for ways to reduce heat loss in that room. Do things like tint the windows, add insulation, plant trees that shade the room, etc. Another option is to get an HVAC company to move those vents somewhere else. That will keep the airflow the same without it going into that room. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [September 12, 2026, 8:10pm UTC](https://boards.straightdope.com/t/yet-another-ac-question/1033016/3 "2026-09-12T20:10:57Z")
Eh. How many other returns are there? Did you close off 10% or 90% of the total return area? I’d not worry about 10%, but 90% is courting failure. Can you deduce from the HVAC installation whether it was resized upwards when the Florida room was added? ---
**Author:** ![Hampshire](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hampshire/32/4887_2.png) [@Hampshire](https://boards.straightdope.com/u/Hampshire)\ **Post date:** [September 12, 2026, 8:36pm UTC](https://boards.straightdope.com/t/yet-another-ac-question/1033016/4 "2026-09-12T20:36:44Z")
If the Florida room is sealed off from the rest of the house and you sealed off both supply and return you should be good. However, you may want to find where the temperature sensor(s) for the thermostat are. If there’s one in the Florida room the AC will never shut off. ---
**Author:** ![jjakucyk](https://avatars.discourse-cdn.com/v4/letter/j/3d9bf3/32.png) [@jjakucyk](https://boards.straightdope.com/u/jjakucyk)\ **Post date:** [September 13, 2026, 12:17pm UTC](https://boards.straightdope.com/t/yet-another-ac-question/1033016/5 "2026-09-13T12:17:56Z")
> [@LSLGuy](#): > > Did you close off 10% or 90% of the total return area? I’d not worry about 10%, but 90% is courting failure. Right, I’d worry if you’re blocking off more than say 30% of either the supply or return capacity of the system. Some furnaces/blowers are better at handling variable resistance than others. Whether it’s from closing off vents, a restricted air filter, or anything else. Those blowers will ramp up or down automatically depending on the load. Others won’t, but some do better pushing against restricted ducting and will simply deliver more air to the rest of the house. There’s no way to say for sure in your case because there’s a lot of variables. That said, is the return vent approximately 2x the size of the supplies? That would suggest it’s sized just for that room, which would be good. If the return is significantly larger, then it’s pulling air from other parts of the house too, and blocking it off would be a problem. Are there relatively balanced supplies/returns in most rooms of the house? Approximately what percentage of the system does this room represent? ---
**Author:** ![burpo\_the\_wonder\_mutt](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/burpo_the_wonder_mutt/32/365_2.png) [@burpo\_the\_wonder\_mutt](https://boards.straightdope.com/u/burpo_the_wonder_mutt)\ **Post date:** [September 13, 2026, 1:09pm UTC](https://boards.straightdope.com/t/yet-another-ac-question/1033016/6 "2026-09-13T13:09:57Z")
OK, each of the three bedrooms has a supply vent and a return vent; all the same size. The rest of the house — living room, kitchen, dining room and family room — are all part of the “open floor plan”; numerous supply vents and one huge return in the middle (kind of a utility core). The bathrooms, of course, have a supply and no return along with the walk-in closet. The Florida room is separate from the rest of the house via three sets of sliding glass doors. The storm doors from FL rm to outside are subpar — not really airtight. There are two supply vents, they are both in the ceiling and in front of a SGD. The return is also in the ceiling, right up there with all the hot air. It’s slightly smaller than the two supply vents combined (as per **jjakucyk** ’s comment — thanks, TIL). The AC has shut off and re-started several times (as per **Hampshire** ’s comment — also, thanks, TIL). My biggest concern was sucking all that hot air back into the system; I’ve never seen a return in the ceiling before. Thank you all for your help. [**LSLGuy** , happy birthday!] ---
**Author:** ![Fear\_Itself](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fear_itself/32/19637_2.png) [@Fear\_Itself](https://boards.straightdope.com/u/Fear_Itself)\ **Post date:** [September 13, 2026, 1:50pm UTC](https://boards.straightdope.com/t/yet-another-ac-question/1033016/7 "2026-09-13T13:50:49Z")
> [@burpo\_the\_wonder\_mutt](#): > > The return really unnerves me; isn’t it sucking in 100 degree air as opposed to floor-level cooler air? Cooling the hot air makes the room more comfortable. ---
**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\ **Post date:** [September 14, 2026, 4:02pm UTC](https://boards.straightdope.com/t/yet-another-ac-question/1033016/8 "2026-09-14T16:02:35Z")
I’ve never lived in a house with a central AC system, but my understanding has always been that the return is supposed to be in the ceiling, or at least high up on the wall. You don’t want to blow cold air into the room and then suck that same cold air back out. It’s more efficient to let the cold air warm up, rise to the top of the room, and then recycle that through the AC system. ---
**Author:** ![Hampshire](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hampshire/32/4887_2.png) [@Hampshire](https://boards.straightdope.com/u/Hampshire)\ **Post date:** [September 14, 2026, 4:22pm UTC](https://boards.straightdope.com/t/yet-another-ac-question/1033016/9 "2026-09-14T16:22:48Z")
Ideally a cooling system would have supply vents up high and return vents down low while a heating system would conversely have the supply vents low and the return vents high. Since homes heating/ac ducting now serves double duty you get a mix vent locations. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [September 14, 2026, 7:41pm UTC](https://boards.straightdope.com/t/yet-another-ac-question/1033016/10 "2026-09-14T19:41:46Z")
That’s kinda true, as @Hampshire just explained. But the OP case has a return from an uninsulated all-windowed room in the sunny tropics. The problem isn’t whether the return is high or low. It’s that it exists at all. ---
**Author:** ![Dark\_Sponge](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dark_sponge/32/121_2.png) [@Dark\_Sponge](https://boards.straightdope.com/u/Dark_Sponge)\ **Post date:** [September 15, 2026, 12:10am UTC](https://boards.straightdope.com/t/yet-another-ac-question/1033016/11 "2026-09-15T00:10:50Z")
Having both the vents and the return in the ceiling is the optimum configuration for cooling the room (hopefully spaced apart). The cold air is sinking down and mixing with the hot on the way before settling at the bottom (where you are) and the hottest air at the top is being sucked out, leaving the room cooler. I don’t understand the point of blocking off the vents & return. To stop cooling the room altogether? I guess if you don’t want to cool the room at all that would work.