If a thermostat cools, and no one's around to feel it, does it still... Wait, what?

June 2, 2006 11:12 AM

Is it cheaper to keep your thermostat at a constant temperature or a to program it?

I've heard both arguments for years.

The constant temp argument says that your hvac works harder to restablize the temp of the house, therefore using more energy. The variable temp argument seems to rely more on common sense -- why cool or heat a house while no one's home?

So tell me MeFites... which is it?

If it matters, I live in NC (summer temps are firmly in the 90s by day, 60s by night).

posted by 10ch to home & garden (11 comments total)

I want to know too! Great timely question 10ch. We live in Florida, in our first home/no AC experience, being from Seattle originally. Total Noobs at AC usage/maintenance. We've only just started to use the AC, I've been turning it on when I get home from work, the house is in the mid-90s, it gets to about 82/83 when we go to bed. I turn it off and open windows and use the ceiling fan overnight. It *seems* to me more logical to have the AC on for only 5/6 hours a day and use less energy. *shrugs*
posted by Jazz Hands at 11:17 AM on June 2, 2006

Assuming the equipment's effectivity is not affected by the temperature it's operating under, heat or cool only when you need it.

The transfer of energy between inside and outside is proportional to the difference between the two. So to minimise temperature transfer, at the times when you don't care about the inside temperature you shouldn't be moving it away from the outside temperature.
posted by fvw at 11:22 AM on June 2, 2006

It's always cheaper to have the AC off when you're not in the house. That's why programmable thermostats exist. The government says you'll save $100/year, and the government doesn't lie.

If you wish to avoid the "hot house" problem, set the AC to come on an hour before you get home from work.
posted by jellicle at 11:24 AM on June 2, 2006

Googled "thermostat myths." From the first result: Another myth regards the efficiency of setting your thermostat down when you don’t need heating or cooling, such as at night or when no one is home. This myth states that a furnace works harder than normal to heat your home back to a comfortable temperature after the thermostat has been set back, resulting in little or no savings. This is not true, as has been proven by years of research and field observations. The longer your house stays at a reduced temperature when heating–or at an increased temperature when cooling–the more energy and money you’ll save.
posted by |n$eCur3 at 11:49 AM on June 2, 2006

How Jazz Hands is surviving Florida w/o the AC on at night, I will never know. I'm in Georgia, and the programmable thermostat seems to be the way to go. I've never really left it on all day, though, so I'm not 100% sure.

I say that, but I do wonder -- once I get it to say, 74 in here, it will stay at that temperature with little assistance - the unit probably comes on once every 2 hours or so. I wonder if that is more expensive or less expensive than trying to cool it from 85 in one fell swoop. Conventional wisdom says no, but I wonder. Our biggest problem is the clamminess that seeps into everything if the A/C doesn't cycle on every now and then.
posted by Medieval Maven at 11:49 AM on June 2, 2006

I think we can all agree that the "it works harder when turned on again" myth is clearly bullshit. Even if it had to run for 30 minutes continuously, that would still represent a great energy savings over having to run intermittantly for those 8 prior hours during the night.

So yes, if you have a programmable thermostat you will save money not heating or cooling the place when you are not there or when you are sleeping.

I think the source of the "leave it constant" myth is something else. It seems to me that what the myth is really trying to say is "avoid coming home to an uncomfortably unheated/cooled home and overcompensating by turning the thermostat to the other extreme." But with modern programmable thermostats, you can tell it when to automatically start and stop, and so in theory, you would never have to come home to an uncomfortable home.
posted by Rhomboid at 12:16 PM on June 2, 2006

Caveats!

From this site:

Programmable thermostats are generally not recommended for heat pumps. In its cooling mode, a heat pump operates like an air conditioner, so turning up the thermostat (either manually or with a programmable thermostat) will save energy and money. But when a heat pump is in its heating mode, setting back its thermostat can cause the unit to operate inefficiently, thereby canceling out any savings achieved by lowering the temperature setting. Maintaining a moderate setting is the most cost-effective practice. Recently, however, some companies have begun selling specially designed programmable thermostats for heat pumps, which make setting back the thermostat cost effective. These thermostats typically use special algorithms to minimize the use of backup electric resistance heat systems.

and

For steam heating and radiant floor heating systems, the problem is their slow response time: both types of systems may have a response time of several hours. This leads some people to suggest that setback is inappropriate for these systems. However, some manufacturers now offer thermostats that track the performance of your heating system to determine when to turn it on in order to achieve comfortable temperatures at your programmed time.


I've got steam heat and if you attempt too drastic a temp adjustment the blowoff valves start spewing steam.
posted by electroboy at 12:24 PM on June 2, 2006

In addition to setting the AC (or, in Maine winters, the heat) to come on 1/2 hour before you get home, set it to go off 1/2 hour before you leave in the morning. I love having a programmable thermostat.
posted by theora55 at 2:27 PM on June 2, 2006

Medieval Maven, although we catch a break by never using the furnace(okay, maybe one or two nights a year), we are trying to keep our energy bill from bankrupting us this summer. It is really only the last week that has become more and more uncomfortable, which is why I am so glad this question was asked.

If it gets into the 70s at night, with our windows open in our bedroom on the west and east walls, we get pretty good cross-ventilation...and a ceiling fan over the bed helps a bit too. I am getting more acclimated, if it is under 85 in the bedroom, I can get just cool enough to sleep. I guess I am miserly enough to put up with some discomfort if it saves money.
posted by Jazz Hands at 2:49 PM on June 2, 2006

Florida here too. I hate the AC and must say that at night we have been setting it to 90 in the new apartment, so that we don't get too hot. In the daytime it's been set in the highish 70s and if we stop moving boxes around I get cold. For what it's worth, we live on the 8th (top floor) of the building.
posted by bilabial at 3:09 PM on June 2, 2006

I'll take the other side of the argument. Contrary to what has been said here, I don't think the "set it to a constant temperature" is bullshit. A very good freind of mine, who installed my central air, was a HVAC professional. (I say was because he was killed at work a few years ago--he fell off a 3rd story roof in a rain storm.)

He told me, and it's what I've done ever since, to only use the programmable thermostat for heat, not AC. A forced air furnace can very quickly bring a cold house up to a comfortable temperature. Central air does not work the same way. I went from window units that put out an arctic blast that could cool a room in no time so this was counterintuitive to me also.

Central air (all AC actually) works by removing heat from the house, not "cooling it." AC units are sized appropriately to the volume of space that need to be cooled. (I probably should say should be sized.) A unit that is too large will cool very quickly but will cycle too much to do a proper job of removing the humidity from the house. Too small and it will run continuously and cool very slowly. Central air is designed to maintain a constant temperature. When that temperature is reached, the unit will cycle as necessary to maintain the temperature. If you use the program to set the temperature to a very high number while you are at work, you will find (like Jazz Hands above) that it will take hours to cool the house to a comfortable temperature. It will also run continuously for hours and it won't be very comfortable until hours go by. To me, that defeats the purpose of central air. My rule of thumb for cooling season is set it and forget it. (For me, the magic number is 72--I like it pretty cool. My wife on the other hand thinks that's too damn cold. I tell her to wear a sweater.)

Another point my friend made when I asked about the inside temperature rising on very hot days is that the AC is fighting to remove the heat that is getting into the house but it only has a capacity to remove so much. A properly operating system has the capacity to cool about 20 degrees F below the ambient outside temperature. On the hottest summer days that get close to 100 around here, my house gets up to 78 or 79 degrees with the air running full blast. Of course, 79 feels awful good when it's 100 outside.
posted by AstroGuy at 6:56 PM on June 2, 2006