Cheaper to turn A/C to 85 degrees during day?

I work during the day, my husband stays home but is outdoors at least half
the time. He usually turns the A/C thermostat up to like 85 degrees since he
won’t be inside. I get home around 5pm, the house is WAY warm and I turn the
thermostat down to 75 degrees. I’ve noticed that the A/C then runs non-stop
for at least an hour, sometimes more, in order to cool the house…
we have a small (approx. 1000 sq. feet) house, the thermostat is in the small
hall that is in the center of the house. So, the question is…would it be more
economical to just keep the thermostat on 75 degrees all day or is it better to
turn it way up at 8am and then turn it down at 5pm and have it run non-stop for 1-2 hours?
PS-- I live in N. Little Rock, Ar…it’s been about 97 degrees here…

Turning up the thermostat saves power.

  1. Each time the airconditioning starts, some power is wasted establishing the operating pressures, cooling connecting lines, etc. Also there is a brief power surge required to start the compressor. Running for the hour straight is thus more efficient than cycling on and off several times.

  2. How much heat leaks into the house depends on the difference between indoor and outdoor temperatures…this heat leakage must be offset by the AC. So keeping it cooler requires pumping heat out of the house at a higher rate, which requires more power.

  3. The operating effiency of the AC depends on the temperature difference, as well as the evaporator (indoor “radiator” part) temperature. Raising the T-stat allows the the evaporator to run warmer, and reduces the temperature difference, both of which improve effiency.

Your husband is right in this case. If we assume that you keep the thermostat at 75 while you are there and set it to 85 when you leave then it takes several hours for the house to heat up to the temperature required to start the A/C. If you had left it at 75 the A/C would have been running off and on all of that time which more than makes up for the hour it takes to cool the house down when you get home. In addition the heat that comes into the house depends upon the temperature difference between inside and outside. So if the house is at 85 inside instead of 75 less heat comes in so the A/C has to work less often to keep the house at the higher temperature.

If coming home to a warm house and having to wait for it to cool down is bothersome, and it would bother me, you might think about getting a programmable thermostat so that you can have the regulated temperature reduced from 85 to 75, say, 45 minutes before you arrive.

P.S. You are absolutely right about everything else, so just smile and give your husband this one tiny victory.

Kevbo & DavidSimmons,
Thanks a bunch! I’ve learned something new today. The war on ignorance
racks up a small victory!
And my husband will completely LOVE IT that he’s right. He may go into
shock. Maybe I should spare him the trauma & not share this with him—
nah, he can call his son and they can share their “Yea, she actually said I
was right! No, really, seriously” stories. Geez, I love that man.
So, the thermostat will stay at 85 during my workdays.
Thanks again.

As long as the OP has been answered I have a related question. The heat gained by the house is proportional to the difference in temperature between the inside and the outside, or more accurately the log mean of the temperature difference.

The efficiency of the heat transfer is proportional to the temperature difference between the hot side, the fluid in the condenser tubes and the cold side, the outside air.

Would it make sense to lower the set temperature when the outside temperature drops below a certain temperature? You’d move more BTU’s outside per watt consumed, although you would gain heat faster at the same time.

I’m thinking of dropping the temperature 3 or 4 degrees in the pre-dawn hours, then putting it up to normal when the outside heats up.

When will people learn to use search function?

What are you talking about?

If your husband does a lot of running in and out it partially negates the cost savings.

Better he should just stay inside and watch TV or take a nap.

This topic does come up several times during each summer season, but not nearly as often at the Whole Nine Yards so I’d give it a pass. :slight_smile: A search will turn up several other discussion of the A/C during the day problem.

Well I’m pretty sure he knew he was right, what might be traumatic is hearing it from you.

As a comprimise you can get a programable thermostat so you can program it to start cooling before you get home.

A final consideration: Turning the A/C way back causes temperatures and humidity to increase, creating a breeding ground for key allergens and potentially increasing indoor air pollution.

If you have allergies/sinus problems, you might want to run a ceiling fan from time to time, or strikes a compromise in the temperature during the day.

As long as the same question doesn’t come up ALL the time, who cares? Many of the threads I find most interesting, I wasn’t looking for in the first place. Someone else asked them, and I enjoyed/learned from reading them. Again.

This is good news to me. I just installed a programmable thermostat and I have it set at 85 from 8AM to 6PM. I’m waiting to see the savings on my bill. :slight_smile:

Sorta in the same vein: the condensing unit of my AC system gets baked by direct afternoon sun (and this being the Sacramento Valley, were also talking air temps often in the 100-110°F range). I’m planning on screening the unit, mostly for aesthetic reasons, but can I expect more efficient cooling from the unit in shade?

Yes, so long as the screening does not restrict airflow by any appreciable amount.