Since that thread deals mostly with heating, I was wondering if the conclusion is the same for cooling. In a nutshell, is it cheaper to keep your thermostat at a constant temperature all day and all night, or would it be better to set the thermostat higher for times when the house would be unoccupied and then turn it down when you get home?
The consensus in the heating thread seemed to be that variable temperature is better than a constant temperature. Since heating and cooling are two different animals, I thought it was worth ressurecting this question.
As an addendum to the general question, I present you with a specific situation – mine. I live in a 600 sq. ft. apartment cooled by an 18,000 BTU window unit. Would it be better for me to leave my AC on low all day to maintain the temp., or should I turn the AC totally off while I’m not there and then crank it up when I get home?
In your case, (Window A/c0 the higher you set the temperature and the more time you turn it off, the more you will save. That is absolute and it would also be truie of heating by air.
My home is a slight exception. It would be true also except for the following: I have hot water heating. Suppose I turn the thermostat down and leave. Then I come home and turn it up. The furnace starts heating the water in the system pretty fast but the rooms are still not at the set temperature. When the room reaches the set temperature the water is way hot and has accummulated a lot of heat which not overheats the room. This is wasted energy but you can avoid this if you turn off the furnace before the room reaches the set temperature. It takes a little manual playing with the thermostat but you get the hang of it fairly quick.
In the case of air heating/cooling this is not an issue.
Whether it’s the heater or the AC, the most efficient thing to do is to only turn it on when you need it. The “takes more energy to make it warm/cool than to keep it warm/cool” argument is bogus, as explained in the previous threads.
An even more efficient way is to open the windows when the outside air is cool.
My superintendent says that the ‘turn it on when you need it’ camp misses out on the energy it takes to cool your furniture/rugs/clothes/closets down to the desired temperature – so long as you aren’t gone for more than a day, it made sense to just leave the thermostat set and let er’ rip. Thoughts?
That’s the one and only argument for the “leave it on” camp - that it takes energy to cool the contents and components of your house, including air, walls, furniture, clothes, fish tank, whatever. And this is false, for reasons endlessly explained in the previous thread.
Here’s an analogy. Let’s say you have a bucket with a small hole near the bottom. The more water you put in the bucket, the faster it leaks because of the incrased pressure. If you don’t need the water for an hour, which takes more water - continuously topping up the bucket for an hour, or letting the water level drop and filling it back up after one hour? The answer is that it’s always better to leave it alone and re-fill it later. It doesn’t matter if there are sponges or cups or other junk in the bucket.
The situation is the same for air conditioning a house. The rate of heat flow into the house is proportional to the temperature difference. If you leave the house for an hour with the AC off then come back and turn it on, it will need to expel the heat that has entered the house in the past hour. But if the AC was on during that hour, even more heat would have entered the house during that time because the house is cooler. So the work done by the AC which is left on for an hour is larger than the work needed to restore the temperature after being off for an hour.
I wonder just how much energy is wasted by people across the US who leae their AC set to 70 F for no reason at all on a 3-day trip, under the mistaken assumption that it will “save energy”? This really ought to be something that gets debunked more publicly.
I used to turn mine off every day before I go to work, and then turned it back on at 6:30 when I got home. The house cools in about 20 minutes. Of course, since I had a PC overheat, I now must leave the AC on at a high temperature so the house doesn’t get too hot and fry the UnaBoard server. Sigh.
For the record, since one can argue theory all day:
TXU kindly includes on our electricity statements a thirteen-month usage history, including a record of average daily temperatures during the first and last usage months on the statement. During this year’s May billing cycle the average temperature was exactly the same as during last year’s May billing cycle (we got more rain this year, that’s the only climate difference though). Our major change in our energy habits between the two is that we have started making sure that the lights are all off and the thermostat is turned up to 80 degrees F (during A/C season-and yes, mid-April is A/C season in Texas ) whenever we leave for more than a few minutes.
Our electricity consumption was down about 30%, according to TXU’s records. Turn things off when you’re not using them, central air included.
I have turned up my thermostat to 80 in the morning when I left for work. My a/c stops running for maybe 3 hours. When I get home and turn it back down, it runs for a constant 8 hours. I’m in Texas where it’s been over 100 for 40 days in a row, though. I’m staying with a constant temp for now.
You will be consuming more energy. The recooling the stuff in the house argument is along the lines of letting the cool out or keeping in the dark.
Leaving the AC on all day will require more run time on your compressor. But confort can be given up by turning up the stat when you leave. A properly designed system should cool a house only a few degrees an hour.
) whenever we leave for more than a few minutes.