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  1. #1

    Expected time to maintain temp

    I've read the rules, and I'm not a HVAC tech.... but I'm going to go out on a limb and ask because I don't think this question crosses the "I'm not giving out free advice" line. Not looking for advice, or troubleshooting, or any "how-to's"

    I just purchased and installed a Nest thermostat. My previous thermostat... set it, forget it... end of discussion. Now, I've got all this data reporting about my energy use and the ability to check my unit at work and on the road, how long it takes to heat and cool, etc.

    Now that I've got all this information at my finger tips, one thing that really stands out. That being how poorly my unit does at cooling.... I think. This summer has been HOT, yesterday was 100.

    Yesterday I had the AC off for a while, inside temp got up to 88. I turned the AC on at 5:30 and at 10:00 when I went to bed, it was still running with a temp of 74. At night, the thermostat goes up to 78, at 5:15 it is set to go to 72. When I left the house at 6:30 the temperature was 75.

    My home is about 2,400 sq ft... with 9' ceilings... with a lot of windows. Its a new construction home in 2008... built by a "kit" home builder.. so I'm pretty sure they cheaped out on my HVAC system.

    So my question is, with temps doing a Riverdance on the 100 mark, is taking 4:30 to drop the temp 14 degrees BAD, or for that matter, 1:15 to drop 3 degrees BAD? I'm just not certain if I have a problem with my system, if its just simply under powered, or I'm just not being realistic with my expectations.

  2. #2
    Join Date
    Oct 2004
    Location
    Central WA
    Posts
    1,642
    It's probably ok. If the unit was properly sized for your construction and area, and 100 degrees is not a typical temp for your area it will have to work very hard to pull down to or even maintain your setpoint. This is done on purpose as an oversized system can cause other problems when the temperature outside is "normal" (short cycling, high humidity, etc.).

    Keep that filter changed, and have the system serviced once or twice a year.

    Why do you have the temp go up to 78 at night?

  3. #3
    Quote Originally Posted by cjpwalker View Post
    Why do you have the temp go up to 78 at night?
    Because that's what my wife made me set it at, and she still puts out.

    Thanks for the reply.

  4. #4
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,376
    Don't turn the system completely off when you know outdoor temperatures will be high.

    Overall I'd say your system is kicking butt considering the pulldown times and ranges you posted.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  5. #5
    Thanks again for the replies...

    This kind of raises another question then. What do you professionals recommend for temperatures? I've read the Energy Star recommendations and 78 in the summer with away temps +7 deg is absolute BS. I work too hard to not be comfortable in my own home. With that being said, I am flexible.

    Like I said, I've just never paid any attention to my temperatures... I've always programmed the thermostat and forgot about it. If I get too hot, I'll adjust it down, if my wife gets too cold, I'll adjust it up.

  6. #6
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
    Posts
    5,062
    Quote Originally Posted by Boomn4x4 View Post
    I've read the rules, and I'm not a HVAC tech.... but I'm going to go out on a limb and ask because I don't think this question crosses the "I'm not giving out free advice" line. Not looking for advice, or troubleshooting, or any "how-to's"

    I just purchased and installed a Nest thermostat. My previous thermostat... set it, forget it... end of discussion. Now, I've got all this data reporting about my energy use and the ability to check my unit at work and on the road, how long it takes to heat and cool, etc.

    Now that I've got all this information at my finger tips, one thing that really stands out. That being how poorly my unit does at cooling.... I think. This summer has been HOT, yesterday was 100.

    Yesterday I had the AC off for a while, inside temp got up to 88. I turned the AC on at 5:30 and at 10:00 when I went to bed, it was still running with a temp of 74. At night, the thermostat goes up to 78, at 5:15 it is set to go to 72. When I left the house at 6:30 the temperature was 75.

    My home is about 2,400 sq ft... with 9' ceilings... with a lot of windows. Its a new construction home in 2008... built by a "kit" home builder.. so I'm pretty sure they cheaped out on my HVAC system.

    So my question is, with temps doing a Riverdance on the 100 mark, is taking 4:30 to drop the temp 14 degrees BAD, or for that matter, 1:15 to drop 3 degrees BAD? I'm just not certain if I have a problem with my system, if its just simply under powered, or I'm just not being realistic with my expectations.
    Humidity level in the home could be the reason for long run time versus temperature drop. Set it at what you feel comfortable with and see if it maintains temperature.
    Where are you located and I will post your design temperature?

  7. #7
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Don't use setbacks with AC or heat pumps. They don't save any energy, and if the equipment is sized correctly, it will take all evening to "catch-up" when it's at or above design conditions. Except for the desert and most of Texas, if it's 100F outside, the system if sized correctly should be falling behind around 3-6PM and start catching up after that.

    WIth a setback, or turnign the systme off all afternoon, you are saving some energy initially by allowing hte inside temp to get closer to outdoor conditions. That is fine if you work 2nd shift and wait un 11PM to turn the system back on. However if you switch ot bakc on at 4-5PM, it's the least efficient it will be all day. IT has the least reserve capaicty as well. AC equipment is more efficient in cooler outdoor temrpatures. With a 2 stage system, the results are even more dramatic since you are deferring 1st stage operation for more 2nd stage operation trying to catch up in the hotest weather.


    Energystar is full of BS... and you can see above. They want you to try and sleep with the thermostat set to 83F at night. Who the he** can sleep in that?

    Upstairs I set mine to 74F all afternoon and evening, then at 9PM drop it to 73F at night where I'm comfortable then return to 74F around 9AM when it start warming up outside and the AC is less efficient. Downstairs, since it's less humid due to reverse stack effect (air leaks in upstairs and leaks out downstairs) and because my house is well... special... its' a long story... I set it to 77F all afternoon, drop it to 76F at 8PM when we sit down to relax after our toddler is down for bedtime, then at 10PM it goes to 80F since it's unoccupied, then at 5AM I dorp it to 76F to take advantage of cool outside temepratures.

    In winter I do use setbacks, but only 3-5F mostly for comfort. With properly sized equipment you can't recover more than 1F per hour anyway.

    IT makes a big difference too how much thermal mass you have. You can even undersize a system slightly if you have for example a real brick load bearing walls, tiles floors, plaster walls (plaster is light concerte, its' very dense and stores energy... don't get me started on all the ways plaster and old building materials are superior), real stucco exterior (not that foam board crap) and so on. Larger homes also change tmerpature a lot slwoer than smalelr ones and cna have proportionally much smaller HVAC equipment.

    AC is about comfort 1st, saving money 2nd. A well designed house doesn't need AC anyway. Humans lived without it for a very long time even in the record heat waves of the 1930's. Records that where I am, still haven't been broken. We haven't even gotten close this year.

  8. #8
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,376
    Whatever temperature you and your wife can both live with when you're both home, I would recommend no more than two to three degrees higher setback when you're away. So if you both have settled on 75 degrees as a happy compromise, setting it back to 78 when you're away will probably work better than 78 + 7 (85?!) as for pulling the house back down on a hot day and using less energy to get you back into a comfortable range.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

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