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  1. #1
    Join Date
    Jul 2011
    Location
    Arkansas
    Posts
    81

    To set back, or not to set back?

    I get this question all the time from customers and I give them the advice that I use for myself. I recommend setting back besides hp's with electric backup during heating.

    I turn my stat up to 85 in the morn when I leave for work and drop it down to 68 when I get home. My reasoning is, its not running all day costing money and as long as you dont mind dealing with the heat for a while after you get home, this is the least clostly mode of operation. Not to mention the system runs non stop for hrs at peak efficiency instead of cycling all day, drawing start current and running for 5-10 mins before the refrigerant balances out.

    Same for heat. The furnace is more efficient after the heat exchanger has had time to warm up. So setting it back and turning it up when u get home should be the best option as well... right?

    I feel confident in my reasoning but want to ask more experienced techs if Im right or wrong. For my own sake and for the sake of all the customers who ask my advice.

  2. #2
    Join Date
    Aug 2009
    Location
    Jurupa Valley, CA
    Posts
    1,781
    Really, it all depends on the home's tightness. If it is a tight, well insulated home, then it probably doesn't matter, as there is likely to not be a lot of 'catch-up' waste. However, if the home has a lot of loss/heat gain, then it may end up using more energy maintaining during the day than it would to get it back to comfortable level after work.

  3. #3
    Join Date
    May 2012
    Location
    San Diego, CA
    Posts
    352
    I do 85 during the day... 75 F (starting like 5 minutes before people come home)... 78 F while sleeping and morning.
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  4. #4
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,429
    Theory 1: Setback does save money, but not as much as correctly sizing equipment to begin with. Quick recovery means your unit is too big, so setback for now and replace with smaller equipment when the time comes.

    Theory 2: More utilities are going to time of use rates where power is more expensive during times of peak demand. In our area going warmer from 2-7 weekdays will has the biggest effect on your bill since that's when power is most expensive.

  5. #5
    Join Date
    Feb 2004
    Location
    Midwest
    Posts
    9,932
    As far as AC setbacks go, there's no general answer to that, because the number of variables is mind boggling. The ones mentioned in this thread are just the tip of the iceberg. You have to pick a precise, specific scenario.
    And the energy savings must be weighed against the disadvantages.

  6. #6
    Join Date
    Nov 2007
    Location
    New Jersey
    Posts
    110
    I can't speak for the cooling side, but on the heat side setting temps back more than a few degrees, maybe 4-5, does not decrease operating costs. As objects in the heated area cool off, they reach a point where they seem to act as chillers when the system is trying to come back to the higher temp. This can become an issue more so if the heating system has been properly sized. If there's a new, high efficiency system, setback may turn into a difference between comfort and cost.

    Additionly, if the area in question is not well insulated the system may never catch up on very cold or windy days. In some cases folks do well to just stay even.

  7. #7
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Quote Originally Posted by 54regcab View Post
    Theory 1: Setback does save money, but not as much as correctly sizing equipment to begin with. Quick recovery means your unit is too big, so setback for now and replace with smaller equipment when the time comes.

    Theory 2: More utilities are going to time of use rates where power is more expensive during times of peak demand. In our area going warmer from 2-7 weekdays will has the biggest effect on your bill since that's when power is most expensive.
    +1 on both.

    I'd also add, that if you do the math, that setback only saved you roughly 20% on your heat gain for that period. The "cost" of that setback instead of running your ac all afternoon at an average temprature of lets say 85F, is now running it during the hotest part of the day when it's least efficient so it's running at least 20% less efficient than if it had cycled all day.

    Geothermal...yes, expecially one sized for heating load, absolutely use as much as your system can recover from in about 2 hours... although I'd monitor the ground water tmeps to see how much they rise during that time.

    Furnace, yes, as much as it can recover from in 2-3 hours.

    If you have a 2 stage AC, absolutely do not use setbacks. Cycling in low stage all afternoon is a LOT more efficeint than running for 1-2 hours straight on high stage in the heat of the day.


    Personally, I setback just 1 F around 8AM, then drop the temp 1F at 9PM for sleeping. At night I setbakc my downstairs from 10PM to 5:30AM by 5F, then recover back to 76F, then set to 77F at 8AM, then 76F at 8PM again for relaxing and watching TV.

    For my furnace in winter I setback downstairs at night by 8F (downstairs is way oversized) and upstairs by 2F (more for comfort). In the afternoon both are setback by only 3-4F.


    Now just ot complicate matters further. Larger homes and expecially those with a lot fo thermal mass, often heat and cool off very slowly. In my house even when 100F outside will only gain about 3F upstairs from 8AM to 5PM. But it will also take 6-8 hours to recover. Heck we lost power for just 45 minutes last week and it took it 4 hours to recover the 1 F lost since it was well above design temperatures outside.

  8. #8
    Join Date
    Jul 2009
    Location
    Kansas
    Posts
    382
    Simplest way for them to find out if it's cost effective is to try it and log there electricity usage with setback and without setback. Pretty good programs out there for smartphone or computers that let you type your meter numbers in and get a daily report of how many KWH your using. I was just having my stat set at 76 all day long and my average KWH was 4.5 KWH and with setback I've dropped it to 2.8 KWH. BIG Difference considering I only use a degree or two setback, In the morning 7am I raise the temp to 77*; 10:30AM 78*; 7:30PM 77*; 9:30PM 76*

    These numbers are 95+ to 105+ Degree days, haven't had any days lower than that lately. A/C use to run Non-Stop during the day at 76* but cycles decently at 78*

  9. #9
    Join Date
    Jul 2010
    Location
    nebraska
    Posts
    1,629
    I'm in with the a degree or two crowd. Reason being an average to tight home isn't going change temp more than 3-4 degrees in a 8-10 hour period even with the system off. A loose/poorly insulated house on the other hand will have wild swings and when you're at or near design temp it's not going to recover from that setback for many,many hours.

  10. #10
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,429
    I have the meter connect program and have done it both ways. Setback saves money for us. The loss of efficiency by running more during the hot part of the day is more than offset by the long run cycle time. The short cycles by leaving the A/C on all day cost more than the higher temperatures during the recovery period.

  11. #11
    Join Date
    Nov 2011
    Location
    Phoenix,AZ
    Posts
    336
    unless the system is way oversized setback does not work here

    A couple of degrees is okay but if you turn it up to 85 during the day the unit will still run and not recover till probably midnight.

    However setback works great with a furnace here because they are always oversized. I usually just leave mine off then heat the house up to 82 before bed and then shut the thing off the rest of the night

  12. #12
    Join Date
    Jul 2011
    Location
    Arkansas
    Posts
    81
    Mine doesnt catch up till after I go to sleep... but Idc cuz by the time it brings it down from 85 to 80 it feels good. Then it spends the rest of the evening bringing it down to 68.

  13. #13
    Join Date
    Feb 2004
    Location
    Midwest
    Posts
    9,932
    And with most programmable stats, there's the variable of whether or not you have the stat set to "automatically recover" from the setback, i.e. the "energy management" algorithm, where the stat decides how early the system must come back on to cool the house down to the programmed temp at the programmed time.
    That feature is selected by default on the stats I sell. But it can be deselected.

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