Would you leave the air conditioner running?

I have a two-floor house, with two bedrooms on the second floor. 1st and 2nd floors have separate central AC systems. This summer, the two upstairs bedrooms are almost always unoccupied. I left the AC off on the second floor, but noticed today that the upstairs temperature reached 92 degrees. That can't be good for those rooms, can it? Should I be leaving the upstairs AC on? If so, set to what temperature?

we set our second floor to 81 during the day

I'm sure your empty rooms are quite comfortable. Unless there is a pet who cannot exit the room, or you have fine art on the walls, shut off the air conditioning if the rooms are unoccupied.

What max said. It's just costing you money.

One exception to that advice is if you have electronic equipment or certain furniture that requires a more moderate temperature in one or more of the second floor rooms. We had a piece of furniture collapse under circumstances such as you describe because the glue holding the furniture together dried out from the heat.

we do have a dog who likes our bedroom

I would leave it on, but at a higher temperature. I consider it the same as heat -- every professional will tell you to keep the heat on at a "reasonable" temperature during the day even when you're not home, rather than keeping it off/low and then cranking it up when you get home. Even though unoccupied, the heat that has risen upstairs will affect the temperature of the air downstairs.

I never thought about that. We just have window units and we leave them all off during the day usually. Super hot days like today we'll leave the first floor unit on for the dog

Also, how adequate is your attic ventilation? That can drop a few degrees off of your second floor temperature.

Yikes. I had been contemplating the effects on pictures and electronics. Never thought about the furniture!

Well, for now, I have it on, but set to 83 degrees. A compromise for the time being...

joan_crystal said:
One exception to that advice is if you have electronic equipment or certain furniture that requires a more moderate temperature in one or more of the second floor rooms. We had a piece of furniture collapse under circumstances such as you describe because the glue holding the furniture together dried out from the heat.

mumstheword said:
I would leave it on, but at a higher temperature. I consider it the same as heat -- every professional will tell you to keep the heat on at a "reasonable" temperature during the day even when you're not home, rather than keeping it off/low and then cranking it up when you get home. Even though unoccupied, the heat that has risen upstairs will affect the temperature of the air downstairs.

Not true. Every professional will not tell you to keep the heat or a/c on. It's a waste of energy if there are no other reasons to keep it on, such as a pet or keeping pipes from freezing. Only heat and cool when you're home.

In Italy we don't have air conditioning and we leave the windows closed and shades drawn during the day to keep out the heat. Fan's are used when people are in the room. And in the evening when it's cooler, the windows and doors are opened. It's quite comfortable for me and it's over 100-deg here right now.

I set mine to 82 when I'm not home and 77 when I am. If I let the house get too much hotter than 82 it takes too long to cool it back down.

I turned on the AC downstairs for the dog, and when we came home she was still upstairs on the sweltering bedroom.

Is your air conditioner running? You better go catch it.

Science fun fact;

-I guess we should all build homes designed and maintained like the African termites, - they keep their towering homes at a constant temperature despite huge daily swings in outside temps.

I learned this sometime ago and sometimes think of it as I'm adjusting all the floor fans, doors, windows, attic fan and window AC units

http://www.planetseed.com/relatedarticle/cool-termites

jeffl said:

mumstheword said:
I would leave it on, but at a higher temperature. I consider it the same as heat -- every professional will tell you to keep the heat on at a "reasonable" temperature during the day even when you're not home, rather than keeping it off/low and then cranking it up when you get home. Even though unoccupied, the heat that has risen upstairs will affect the temperature of the air downstairs.

Not true. Every professional will not tell you to keep the heat or a/c on. It's a waste of energy if there are no other reasons to keep it on, such as a pet or keeping pipes from freezing. Only heat and cool when you're home.

I guess you should tell that to Plungy and Phil Masucci, among others.

mumstheword said:

jeffl said:

mumstheword said:
I would leave it on, but at a higher temperature. I consider it the same as heat -- every professional will tell you to keep the heat on at a "reasonable" temperature during the day even when you're not home, rather than keeping it off/low and then cranking it up when you get home. Even though unoccupied, the heat that has risen upstairs will affect the temperature of the air downstairs.

Not true. Every professional will not tell you to keep the heat or a/c on. It's a waste of energy if there are no other reasons to keep it on, such as a pet or keeping pipes from freezing. Only heat and cool when you're home.

I guess you should tell that to Plungy and Phil Masucci, among others.

If they said that then they're wrong.

If I have a 16 room house and I keep 8 rooms at 74 and the other on a hot day at 100, I'll be saving energy and be more environmental friendly. Yes, it may take a bit more energy to keep cool the rooms that border the uncooled rooms because the few walls bounding the hot rooms are hotter. But its a damn lot less energy than cooling those additional rooms.

GGartrell said:
I set mine to 82 when I'm not home and 77 when I am. If I let the house get too much hotter than 82 it takes too long to cool it back down.

Mine doesn't take too long but the air coming out of the registers is 25 - 30 degrees lower than the return air. The only issue is one room which does not have sufficient registers for its size. It also helps to have a good air pressure flow.

It may be good to get a WiFi thermostat. You can track the temperature with your cell phone app and then turn on the air before you get home, the time needed based on the wanted temperature drop.

If we didn't have cats we would set it at 95 when not home.

Electronic equipment, when not on, can usually handle hot environments. When on, some parts of my computer go to 130 degrees. I'm sure when not running a room temperature of 95 would be OK.

mumstheword said:
I would leave it on, but at a higher temperature. I consider it the same as heat -- every professional will tell you to keep the heat on at a "reasonable" temperature during the day even when you're not home, rather than keeping it off/low and then cranking it up when you get home. Even though unoccupied, the heat that has risen upstairs will affect the temperature of the air downstairs.

That is a different scenario than the OP's, who's rooms in question are largely unoccupied. You're speaking to an operating cycle of hours, they are speaking to days/weeks/months.

Right. My concern is whether I would be doing any damage to the rooms or to their contents by allowing the temperature to rise into the 90s. (And the humidity along with it.)

Sundays said:

mumstheword said:
I would leave it on, but at a higher temperature. I consider it the same as heat -- every professional will tell you to keep the heat on at a "reasonable" temperature during the day even when you're not home, rather than keeping it off/low and then cranking it up when you get home. Even though unoccupied, the heat that has risen upstairs will affect the temperature of the air downstairs.

That is a different scenario than the OP's, who's rooms in question are largely unoccupied. You're speaking to an operating cycle of hours, they are speaking to days/weeks/months.

BG9 and I agree. Here is one of the myths about energy debunked:

7. It takes more energy to cool a building when the AC has been off all night/weekend than it does to keep it running at a higher temperature. The facts: It always saves energy to shut units down completely. Keeping a cooling unit running over night or the weekend wastes unneeded energy – even if you have it turned up to 85°. As we mentioned in Myth #3, changing the temperature on your thermostat doesn’t make it work faster or slower, it simply turns things on and off at different rates. By using a programmable thermostat, you can be sure that your home, work or community building is at a comfortable temperature by the time it’s being used; and that it’s conserving energy and not cooling when it isn’t needed during nights, holidays and weekends. If you’re programmable thermostat doesn’t have an option to turn “off” during certain times, consider adding a step to your opening and closing procedures that includes turning the thermostat on and off when appropriate. During the winter months it is important to keep your building at warmed enough to avoid freezing pipes, however in warmer climates and seasons it is safe to turn your cooling units off completely.

BG9 said:

mumstheword said:

jeffl said:

mumstheword said:
I would leave it on, but at a higher temperature. I consider it the same as heat -- every professional will tell you to keep the heat on at a "reasonable" temperature during the day even when you're not home, rather than keeping it off/low and then cranking it up when you get home. Even though unoccupied, the heat that has risen upstairs will affect the temperature of the air downstairs.

Not true. Every professional will not tell you to keep the heat or a/c on. It's a waste of energy if there are no other reasons to keep it on, such as a pet or keeping pipes from freezing. Only heat and cool when you're home.

I guess you should tell that to Plungy and Phil Masucci, among others.

If they said that then they're wrong.
If I have a 16 room house and I keep 8 rooms at 74 and the other on a hot day at 100, I'll be saving energy and be more environmental friendly. Yes, it may take a bit more energy to keep cool the rooms that border the uncooled rooms because the few walls bounding the hot rooms are hotter. But its a damn lot less energy than cooling those additional rooms.

Houses and furniture have stood for far longer than AC has been in existence. The rooms will be fine. I'd keep the shades drawn. If there are delicate valuables in there (perhaps original photos/artwork) consider moving them to another room.

Something I would recommend to keep running while you aren't home is a dehumidifier in the basement.

Sundays said:
Houses and furniture have stood for far longer than AC has been in existence. The rooms will be fine. I'd keep the shades drawn. If there are delicate valuables in there (perhaps original photos/artwork) consider moving them to another room.

Definitely keep the shades down, if possible. Its not just delicate valuables. We have wood floors and we noticed a color change on areas hit by the sun. There's also some fading of fabric covers.

if you should leave AC and just set it higher depends on how low you normally keep it. I have been in houses where they set the AC to 68 and walk around in sweaters! But if you normally keep it at 80..higher wouldn't do much!

re keeping AC/heat on when your not home....if I leave my AC off during the day...it will never cool down enough at night on the very hot days. So on the very hot days I do leave the AC on...with central AC you can leave the thermostat set to a warmer temp while you are gone for long periods.

BG9 said:

Sundays said:
Houses and furniture have stood for far longer than AC has been in existence. The rooms will be fine. I'd keep the shades drawn. If there are delicate valuables in there (perhaps original photos/artwork) consider moving them to another room.

Definitely keep the shades down, if possible. Its not just delicate valuables. We have wood floors and we noticed a color change on areas hit by the sun. There's also some fading of fabric covers.

True, but this has nothing to do with the temperature. It is the UV radiation in sunlight that fades and damages fabrics and floors. Many of the houses in this area were originally designed to have awnings over the windows for this reason.

Cat + apt = Window A/C running all day while I'm at work. But, only in two blocked off rooms. And the cat does stay in there all day long (even move her food in there when it's so hot). And, I still worry about leaving it running re: any electrical issues during the day while I am work.

Not that I'm suggesting that pet owners shouldn't leave the A/C running when they're not home, but: A friend of mine has a Siamese cat that likes to take naps up in the attic when the temperature is around 100 degrees up there... some cats are apparently "heat seekers." And when I was growing up, we had dogs (all Boston terriers) that would lie in the sun even on the hottest days of the year, panting but looking happy...

It takes far too long to cool my house if I leave the air off on a hot day. Everything gets hot, me not the least of them. If my house is 78 after walking from the train, I am happy.

This does not address unicorn33's question, but the other comments left on this thread.