AC setback - rule of thumb?

bri0822June 29, 2012

Does anyone know of the most efficient setback temp for the AC in summer? I live in eastern PA and have kept my AC at 78 during the day and 74 in the evening, but I notice it runs a lot in the evening stay to stay at 74 after being set back.

This week, I held the temp at 74 all day and the AC seems to run a lot less and still hold 74. I'm wondering if just holding the steady temp is more efficient.

Overall, my electricity costs are low as the house is sealed well and I'm running a 2 year old Bryant Evolution. Does anyone have any thoughts? Thanks.

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tigerdunes

I don't think there is any hard and fast rule of thumb. Every home cooling load is different with different weather conditions plus you have to consider any extra cooling capacity you might have.

For instance, if your system is closely matched to the load, then probably on a 4 degree setback the system will run continuously for a long period of time. False economy in my opinion. Now if you have extra capacity, the setback might not be as big a deal and your system is able to catch up in a reasonable time-say a few hours.

On the first situation described, I think that is hard on a system , poor comfort for those inside not to mention false economy.

IMO

    Bookmark   June 29, 2012 at 1:38PM
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ionized_gw

The rule of thumb is that the greater the setback and the longer the setback the less it will cost you. Simple physics tells you that.

Continuous running is great for most machinery including cars and air conditioners. If you are gone all day, set the t-stat at its max temp for the day and figure out what time you need to have it come on to be comfortable when you return. It might take some playing around to figure out a time with varying weather.

    Bookmark   June 29, 2012 at 1:48PM
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david_cary

I think a good rule of thumb might be a setback that takes 3 hours to recover from. That may satisfy those worried about a system "working too hard" but still be a decent size setback.

The reality is setbacks are less helpful for a/c season. So minimally helpful that I wouldn't go too far.

    Bookmark   June 30, 2012 at 2:12PM
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tigerdunes

A good point by David Cary.

As far as recovery times, setbacks for AC cooling and HP heating are entirely different and longer recovery than fossil fuel heating from oil, propane, and natural gas. There are obvious reasons here primarily sizing.HPs if sized correctly are very good at maintaining thermostat setting but can be very poor in recovery both in AC and heat mode.

IMO

    Bookmark   June 30, 2012 at 6:11PM
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ionized_gw

Where does the "3 hours" figure come from? Why should anyone worry about a system "working too hard"?

What is the logic or data that supports this,"The reality is setbacks are less helpful for a/c season. So minimally helpful that I wouldn't go too far."?

If correctly sized, a BTU is a BTU and fossil fuel heat pump recovery will be exactly the same as a heat pump. I would not say "poor recovery" for AC or heat. "Slow recovery" is more appropriate. Large set-backs are the most economical and comfort will be maintained if done intelligently or if adaptive thermostats are used. Care must be taken if expensive modes of back-up heat are employed. Unfortunately, that is beyond the minds of most homeowners.

In the case of the OP, having the system run a lot in the evening to maintain a temp is fine. It might reflect the need to cool the thermal mass of the house. As long as comfort is maintained, that is not a worry, nor is it a good reason to use a lower-temp, more expensive, set-back every day.

    Bookmark   July 2, 2012 at 3:00PM
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