Some n00b questions about central AC. (Should it run all the damn time?)

I just moved into an apartment with central air conditioning for the first time in my life. I feel a bit out of my element here, and my Dad croaked a few years back so I don’t have a strong parent to guide me.

  1. Should it run basically all the time? I know it’s like 95 outside, and we’re in the middle of a heat wave, but it just seems like it always runs 24/7. Every once in a while - maybe once or twice a day that I’m aware of - it will go silent, but it cuts back on about 10 minutes later. Other than that it seems to be “blowing” all the time. Is this normal when it’s 85 outside?

  2. I set it at 72, which seems reasonably “nice and cool” to me. Is this normal? It’s nowhere near as ice cold as my office, but keeping it closer to 75 starts to feel a bit “lukewarm” to me.

  3. Opening and closing vents in different rooms. I closed vents back in the kitchen and the room that the thermostat is in because I’m rarely back there, with the idea that that would divert more AC to the other rooms where I spend more time. Does this make sense? The other thing that concerns me is that this makes the thermostat think that it’s constantly 80 degrees, so maybe that’s the explanation for #1 above.

But why should I keep the rooms that I’m not in cool at the expense of the rooms that I actually am in? What do you do about this? Does it make more sense to keep the entire apartment one contiguous “bubble of cool” by opening all vents, or does it make sense to let certain areas be hot so that the areas you’re spending your time in are more cool?

I think your suspicion on #3 is correct. If the thermostat’s temperature sensor is in an unvented room, it will think it’s hotter than it really is. Try opening the vent there (keep the door closed if adjacent rooms should stay warm) and see if the unit calms down a little.

72 is a perfectly good temperature to set it at. You may want to invest in a thermometer to check if your other rooms are getting cool enough. If not, get your landlord to have a technician clean and tune up the unit.

if you just moved in and turned it on, then yeah, it’s going to run a while. The interior of your home is hotter than shit, and the A/C has to try to cool that down.

that’s pretty low. try 78 or so. A/C also dehumidifies the air so it’ll feel cooler than the temperature alone would suggest.

the thermostat controls when the A/C turns on. if you’re preventing cool air from reaching the place the thermostat is mounted, then it’ll think its warmer than it actually is.

My emboldening.

You just moved in, in the middle of a heat wave, so I reckon just let the machine do its bit until the outside environment gets less unusual. You’re not really going to learn much in conditions that might not repeat for years.

Air conditioning is the greatest invention that ever existed. Let the damn thing run all the damn time.

There are two parts to central air: the fan and the compressor. The compressor turns on to cool the air. The outside is 23 degrees hotter than the inside. It probably is hotter than the inside even at night during a heat wave. Everything in the universe is working to warm your apartment right now. Of course the air conditioner will be running all the time. The laws of thermodynamics demand it. (And heat rises, so if you’re not on the first floor it all gets worse.)

What you’re hearing might, however, be your fan. Not having a way to blow the cool air into your room defeats the purpose of having an air conditioner. The only question is how much fan activity you require once your interior temperature stabilizes. Your thermostat should have settings that allow the fan to run all the time or just when the compressor is on to cool the air. Having the fan on all the time costs a few pennies more and so thrifty people advocate against it. The right way to look at it is that the fan keeps the air circulating and so cools you - and cools all areas as well. Since you’re in an apartment I assume all your rooms are on the same floor. Creating hot rooms without closing them off is self-defeating. Creating a hot room where the thermostat is located happens only on sitcoms, I would have thought. Having one set temperature creates more efficiency and allows you to move around more comfortably.

If you’re worried about costs, bump the thermostat up a degree or several. That’s the only way to effectively save money in an apartment. (For a house it does make sense to close vents - but only those on the first floor. Cool air sinks. Piping all the cool air to the second floor makes both floors more comfortable.)

It’s pretty warm, but it’s also possible the system is low on refrigerant. If it’s been running for a long time, check one of the open vents and verify that you’ve actually got a decent amount of airflow. If it seems like the air is just barely trickling out, then the evaporator coil may have iced up, which is a symptom of low refrigerant levels: humidity condenses out on the evaporator coils, and because the refrigerant is developing extremely low evaporator temperature, (due to the lower-than-normal pressure), it turns into ice and blocks the airflow. If you find this to be the case, you’ll need a service call; rather than replace parts ($$$$$), the tech will probably inject a sealant to plug leaks, and then recharge the system with more refrigerant. We did this back in 2007, and our A/C is still working great.

As for temperature setting: for energy conservation, most sources recommend choosing a temperature of 78F or higher. A light breeze from a fan can help if you’re still uncomfortable, but be careful: those things will kill ya. :smiley:

It was 113-114 here last weekend. My a/c definitely does not blow all the time. It is on about 50% of the time. I have 1300 square feet and a 3-ton a/c.

A fan in the room you are in will really help make you feel cooler. I keep the a/c at 80, but use ceiling fans and floor fans to supplement. They really do make it feel 6-7 degrees cooler.

And keep all the room vents open, and all the interior doors open. In order to cool down, the whole system needs to work together, and your returns need to pull the warm air out of the house.

Take a look at your thermostat. There is probably a setting for the fan, in addition to the setting for temperature. On all of the thermostats I know, the fan can be set to “AUTO” or “ON”. If it is set to “ON”, the fan runs all the time, mixing the air in your house. If it is set to “AUTO”, the fan only runs when the compressor is running.

If the fan is set to “ON”, I would suggest switching it to “AUTO” for a bit, to see how much the compressor is actually running. It the compressor is running all the time, then I’d contact the building management, and let them know the AC needs looking at, especially if you are paying the electicity bill.

If you have downstairs neighbors, their heat will rise to your apartment too. When I lived in a top floor apartment, the a/c almost never stopped in summer, but my heat almost never came on in winter while downstairs neighbors were complaining about their heat bill.

Modern, energy efficient refrigerators run ~80% of the time or more. The physics of cooling (removing heat) is very different than heating (adding heat) and it takes more energy to remove heat than it does to add it (using conventional cooling methods).

Because of this - it’s more efficient for cooling devices to maintain a more constant temp than to allow greater fluctuations - which heating systems may do using less energy.

The result is cooling systems activating more often for shorter bursts.

If you were to heat or cool (using equivalently powered appliances - both fan forced) the same space at the same starting temp either up 10 degrees or down 10 degrees, heating that space +10 degrees will take much less time and energy than to cool that space by the same factor.

Another example - your comfort zone is 75 degrees, but you will tolerate temps up to 80 degrees. Your house doesn’t hit 80 degrees until 2 PM. Thinking you will save energy, you wait until 2 PM to turn on the AC which is set at 75.

Truth is, if you would have activated the AC set at 75 when your house reached 75 (say at 10 AM), you are using less energy for the same 24 hour period as you would if you turned on your AC 4 hours later. Not only does the AC have to work harder to cool the house that extra 5 degrees, but it’s doing so during the hottest part of the day.

For the same reasons - zone heating (adjusting vents, and only heating those parts of the house you use) is feasible and saves energy, but zone cooling does not (typically - though this can vary).

Personally - we keep our house at 74 during the summer (69 during the winter).

Condensing (heh heh) what others are saying, and basing it on our house -

  • the thermostat’s brain controls the whole system. Make it realize that it’s not so damned hot (several methods are advanced above)

  • in this heat, particularly, don’t try to get the house temp down to 72. It’s really a losing battle and unnecessary, really.

  • if you set it at, say 78 or 79, and go outside, and then come back inside, you’ll see how great that feels. It’s drier - which is really the key to body comfort

  • and then, use some floor fans to keep the air moving when you’re in a room. You’ll be amazed at how cool you’ll feel with some air motion and some drier air.

  • if you can, get one of those programmable thermostats. I have ours go up to about 80 at night, and with a slow ceiling fan moving the air, we sleep very comfortably. Set it at the lowest temps when you’re home and let it warm up a bit when you’re out. A bit of savings there, too.

And, if you’re a cheapskate like I am, you won’t hear the damned thing running on and on and on, and you’ll actually sleep better.

xo, C.

Here’s another way of thinking about it…

First a definition of sorts: Air conditioning doesn’t make things cool as much as it removes the heat, so that you feel cool. It moves heat from in your home to the outside. That heat is expressed in “Btus”; about the energy in one match.

Nature want to equalize things, so hot will always move to cold (less hot…); thats why a glass of ice water left in a room will eventually be the same temp as the room.

When it’s 95° outside there are millions of Btus----matches----that are trying to infiltrate your home. And they are…the hotter it is the more of them there are and the faster they enter your home.

And through something called a ‘load calculation’ they can be measured believe it or not. As an example, might say that *in your particular home *23,000 Btus are entering your home when it’s 84° outside. When it’s 94° outside 31,000 Btus are entering your home.

Your A/C system is designed to essentially “pick up” the Btus and put them back outside. Of course they get back in----and your A/C picks them back up and puts them outside again. And…your A/C system’s ability is rated in Btus; it’s ability to pick up Btus and move them outside.

So you’re A/C system has a “fixed” output----let’s say 36,000 Btus, and the “input”----the amount of Btus entering your home is variable, meaning the hotter is outside the more (and faster) they get in.

So practical application looks a little like this:

As an example you have a 36,000 A/C system and you set your stat at 74°.

At 76° outside 5,000 Btus are entering your home. Your A/C responds periodically and removes the heat. It’s “at rest” most of the time.

At 80° outside 15,000 Btus are entering your home. Your A/C is at rest much of the time.

At 85° outside 27,000 Btus are entering your home. Your A/C is running more, but it isn’t struggling to maintain the 74° you set it at.

At 90° outside 31,00 Btus are entering your home. The A/C is running a lot, but because it’s ability to remove heat is greater than the heat thats getting in, it can still maintain 74°. It still “rests” from time to time.

At 93° outside you’ve reached the balance point. 36,000 Btus are entering your home. Your A/C has the ability to remove 36,000 Btus. It will be able to maintain your setpoint, but it is now running non-stop.

At 97° outside 41,000 Btus are entering your home. Your A/C can remove 36,000 of them. Your house is getting a bit warmer because not all the Btus were removed. The thermostat is set at 74°. The A/C is running not stop. The actual temp in the house is 76°. You’re past the balance point and the system will be unable to maintain setpoint. The infiltration is greater than your A/C system’s ability to remove it.

Clear as mud? I thought so.

eta—the numbers are WAGs and have no RL correlation.

My AC installer friend tells me that when they size units they try to ensure that they run at or near capacity for the area they’re cooling – starting up the compressor sucks the biggest jolt of electricity, so the fewer on/off cycles, the lower your electric bill will be.

Also, closing too many vents in unoccupied areas can be a bad thing. If you don’t have adequate airflow through your system, the evaporator can freeze up and you’ll lose cooling to the open vents as well. And if your AC is in the attic (like mine is) and the system ices over (like mine did) and the drain pan is unable to handle the condensation and melt-off (like mine was), your ceiling may have to be repaired due to water saturation (like mine has).