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  1. #1

    Warming home for a few hours, then cooling -- Make sense?

    Say I want my home at 74 degrees when I'm home (or when family is home).
    Does it make sense to use a programmable thermostat and keep the home @ 74 except for a relatively small number of hours? I'm worried the cost of "cooling down" the home will eat up the savings.

    For example, perhaps no one is home between 7:30 AM and 3 PM. Would moving the temp up to 80 during that time save me money? Or, would the additional cost of cooling it down from 80 to 74 around 3 PM eat up the difference?

    Thanks

  2. #2
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    Try using only a 1 degree per 2 hours gone set back. In your posted times, that would be 4 degrees, or setting it to 78. More the that, and it will take until you are in bed for the house to cool down on the hotter days.
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  3. #3
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    Saves energy and removes more moisture because of the extended run to drop the temperature. The question is how long will it take your a/c to get down the desired temperature? I have been doing that for several years and it really improves the dehumidification done by the a/c.
    Many closely sized a/cs will not be able to drop the temperature during peak heat. That is the limiting factor.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
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  4. #4
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    Also keep in mind because you are removing more moisture you will feel comfortable at a higher temperature.

  5. #5
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    Not trying to hijack the thread but:
    Heat is entering the house whether AC is running or not. Heat will be stored in house if AC is not running? AC will still have to remove the same amount of BTU's regardless of when it does this, right? Difference is outdoor temp when the AC is running. Lower outdoor temp = lower head pressure = lower current usage. Make sense? Or will it be negligible?
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  6. #6
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    Every house and cooling system can be different. But I hit the hold button on my thermostat four years ago. It stays set at one temp all the time. I use a lot less kWh.

    jim
    Common sense isn't very common anymore.

  7. #7
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    Quote Originally Posted by Keith73 View Post
    Not trying to hijack the thread but:
    Heat is entering the house whether AC is running or not. Heat will be stored in house if AC is not running? AC will still have to remove the same amount of BTU's regardless of when it does this, right? Difference is outdoor temp when the AC is running. Lower outdoor temp = lower head pressure = lower current usage. Make sense? Or will it be negligible?
    After the house gets to x temp. No more heat enters it. So your not paying to continually remove that heat all day.
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  8. #8
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    The less difference between inside/outside temp the less heat gain there will be. On 100 degree days my house only gets to about 85 inside, even if the A/C is off for a weekend. Dunno why it doesn't gain the extra 15 degrees, maybe concrete slab floor?

    We have time of use billing for electricity with peak periods from 2-7pm weekdays. Turning the A/C up during those hours makes a HUGE difference in the bill !!

  9. #9
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    These are all good posts. The heat gain on any home is dependent on the temp difference between inside and outside. The greater the difference, the greater the heat transfer.
    Keep in mind if your a/c is undersized, you may not be able to recover in a timely fashion. All so, the materials in in the home absorb more moisture with cooler temps and higher %RH. Typically as the temps rise inside the home, the %RH declines. Hence materials dryout.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  10. #10
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    Quote Originally Posted by beenthere View Post
    After the house gets to x temp. No more heat enters it. So your not paying to continually remove that heat all day.
    I would guess then, that it depends on how long it takes to get to x temp and how long it takes to recover. If you don't reach x temp during that period then it may have been just as well to leave it running.
    Every problem gives us the opportunity to choose from several solutions: stupidity, ignorance, indifference, or brilliance. Which one will you choose?

  11. #11
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    Quote Originally Posted by Keith73 View Post
    I would guess then, that it depends on how long it takes to get to x temp and how long it takes to recover. If you don't reach x temp during that period then it may have been just as well to leave it running.
    No nessesarily, because with long run times you'll have lower humidity, so you'll still end up saving energy and if hte hosue is a coupel degrees warmer, you'll still be comfortable.


    Because of reverse stack effect and more air leaks my upstairs is alwasy 6-10% high relative humidity than downstairs. As such, my downstairs is kept 2-4 degrees warmer but is still comfortable.

    I actually run my system with humidity control as the priority, and temperature setpoint as a secondary priority. It will therefore overcool by as much as 3F to reach the humidity setpoint. In that way, the systme maintain a similar level of comfort no matter what the run time of the unit is.

    The other way to look at it is, the system adapts itself and the setpoint is higher when it's hotter outside, savings energy and lower when it's cooler. But we're comfortable regardless.

    It sems to be working. I recieved our first electric bill and it was 50$ less than I was expecting. We went form a 1200 square foot single story ranch with 10 windows and minimal afternoon shade.... to a 3200 square foot 2 story home with 42 windows, similar tightness and insulation with a little more shade, but less efficient AC unit... but the bill was only 40% higher than in our last house... whcih was a relief. Although it does have the roof deck insulated with 5-6" of open cell spray foam and I installed ECM motors on both furnaces. I think that might have saved my almsot 20% off my bill.

    The biggest difference? The AC units are not significantly oversized as in my last house and I keep it 3-5F warmer as a result. I also have 2 zones, so I only need to cool the upstairs at night.

  12. #12
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    The greater the thermal mass of your house is, the less turning it up during the day will save you.

    Programmable stats work great on oversized furnaces - for properly sized cooling equipment, not so much.

    I wouldn't increase it by more than 2F or so.
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  13. #13
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    Quote Originally Posted by Keith73 View Post
    I would guess then, that it depends on how long it takes to get to x temp and how long it takes to recover. If you don't reach x temp during that period then it may have been just as well to leave it running.

    If you allow it to rise 4 degrees. But it only rises 2 degrees. You still save money, because you didn't pay to maintain those 2 degrees lower temp all day while you were gone.
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