Thermostat Setback, what is most efficient setting?

[boots918]

I have a programmable thermostat and I want to set my stat higher during the day when no one is home.

I assume there is a optium increase to set it to. I dont want the system to labor long and hard to recover from a 10 degree increase but i also dont want the unit to run all day.

I live in the SouthEast where it is HOT (95) and humid.

Please give me some ideas on this.


thanks

[thorton]

My thoughts on this is....I would set the upstairs temp cooler and set the lower level warmer. Why? Because, I'm assuming the bedrooms are on the upper level? Thus one would want the upper floor cooler for sleeping, right? Plus cool air falls naturally, anyways. Thus, the slightly overcooled upper floor air will fall and help cool the lower level anyways.

Thorton
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[engineerguy]

A poorly insulated house with an oversized air conditioner could have a large setback. A well insulated house with a properly sized air condtioner will need a small setback.

You have to decide how much discomfort you are willing to tolerate while it brings the temperature down. Your largest savings will come from setting the thermostat to the highest temperature at which you are comfortable.

[boots918]

ok , but lets say we like the house to be 75 during the evening and night.

what is the highest temp that would be effcient for the system to recover from during the day while we are at work?

Is it best to just leave it @ 75 all the time or turn up stat to 80 or 85 during the 12 hours we are away?

Keep in mind it is hot and humid here in the southeast.

[thomefan2579]

i live in ohio and leave the a/c at 70 all summer long even when im gone at work. if its nice i open the windows and off it goes. a/c's take longer to pull down so that 10 degree set back may take like 7-10 hours to pull down depending on design. by then your fast asleep

[watercop]

When a heat pump in heating mode is set back say at night whilst everyone is sleeping, and then set up an hour or so before everyone arises, it is possible, even common for the system to use MORE energy despite having been set back. This occurs with crude thermostats which interpret the set up temp as a stage 2 or more heating call and activate much more expensive to operate heat strips.

More sophisticated thermostats now can work around this, but meanwhile, setbacks got a black eye as a money waster rather than saver.

Cooling mode is different - any amount of daytime setback will save some energy and money, and it is not harmful, in fact can be beneficial for an AC unit to run for hours at a time.

However, there are 2 disadvantages to cooling setbacks: 1) Properly sized cooling equipment will only keep up on hot days - if it gets behind owing to a setback, it may well stay behind, never satisfying setpoint until well after dark. Occupants may find that uncomfortable. 2) During the setback period, say from 9 AM for 3-6 hours, the AC will run little, if at all. During that time infiltration will fill the structure with very humid outdoor air, pushing relative humidity well past the recommended limit of 60% (You did say this house is in the hot humid Southeast). Repeated extended intervals of high humidity contribute to Indoor Air Quality problems such as mold.

[coordinatesales]

I generally tell people 2-5 degrees setback is fine for most applications. Many people are concerned with recovery time for large setbacks. They usually only consider sensible heat (what your thermometer tells you). Latent heat (which is due to humidity) takes MUCH longer to recover on most systems. Too large of a setback results in higher humidity levels in many areas of the country and even though the system gets the temp down to 75, you probably won't be comfortable due to humidity levels. In the winter that's not so much of an issue.

Personally, my unit has a 2 degree setback in the summer and a 4 degree setback in the winter.

[watercop]

Despite all I said earlier, a mild setback, along the lines of what Co-ord says, will probably work okay. Don't count on it to save much more than 5-10% of HVAC costs

[cissado]

As a homeowner, I let the humidity determine my temps at home. If the humidity is out of the range I want, then the stat is not set to be that high. I rarely want my humidity to be over 50%, so whatever temp that may mean, that's what I put the T-stat to, whether I'm home or not. Every house and system is different. I agree with the above statements about IAQ and mold. Just because you're not home doesn't mean mold won't grow. Keep the humidity in a good range.

Yes, mold needs a stretch of time to grow and flourish, but why take a chance. Just keep the humidity in the "no mold" range at all times. You may want to keep it in the higher end of the range when not home, but I certainly don't shut the system down altogether.