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  1. #1

    AC run time to Cool 2 Degrees

    We have a newly built house that the AC is taking longer than I would expect to drop the temp 2 degrees. I'm wondering how long we should expect it to run under the conditions I've specified below. Any help would be really appreciated.

    House specs:
    1950 sq ft new construction house, single story + basement
    R60 Ceiling insulation
    R19 - 2x6 walls
    house faces north with AC unit at the front of the house
    there are large windows to the south of the house but no windows on the east and very few on the west
    the AC unit is a 3 ton 13 seer Carrier 24ABB

    Scenario:
    The house temp was maintained at 78 deg from 4:00pm to 9:00pm. At approx 9:00 pm the thermostat kicked down to 76 deg. The outside temp was 75 deg with the sun fully set but it was more humid than inside (not horribly) but enough that to sleep comfortably we wanted to keep the air on.
    The AC ran for 120 min straight never shutting off, to drop the temp to 76 deg.
    We also run a dehumidifier in the basement that keeps the humidity down there at approx 55%.

    This seemed excessive to me based on our past 15 year old house with a 15 year old AC unit. That would typically take 30 min maybe 45 min to kick the temp down 2 deg.

    I called our HVAC contractor and he said he checked everything out and it's running fine. He said that because we don't have any blinds (haven't gotten any yet, we just moved a month ago) the radiant heat coming from the windows was causing it to run this long. This seemed like a bad excuse to me. I understand the concept of radiant heat and do not believe this would be enough of a cause for the AC unit to run this long with the temp being that low outside and no sun.

    I'm just wondering what should be expected under these conditions. Thanks for any help you can give me. If you need any more info please let me know.

  2. #2
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Sounds like you have a properly sized system overall. Not having blinds will make a big difference. You may have a fair amount of air leaks. New home construction while well insulated is sometimes not all that tight, it depends on the builder and the specs.

    Is the furnace or air handler in the attic? If so, the attic temp is probably still almost 90F+ at 9PM at night if it wa sunny that day, so that will keep a large load on the unit.

    You last house likely had an oversized unit...most home do. With a properly sized unit, you may not want ot use as long of a setback, or have it start cooling down sooner.


    Again, the main thing to look for is air leak on the ductwork, especially if it's in the attic.

    Finally, if the ductwork was undersized or poorly run (not uncommon in most new construction I've seen), you may not be getting the 1200CFM's of airflow you need for a full 3 tons of capacity.


    One more thing... heat from the windows shouldn't be an issue at 9PM.

    Also, where do you live? it will help us understand what you design condition should be. 75F outside temp is quite cool in some areas and fairly warm in others. If you're in MI, you system isn't doing too bad. If you're in Southern Illinois, you may have some issues.

  3. #3
    Join Date
    Jan 2008
    Posts
    1,858
    I just skimmed your post, but it sounds like your old system was oversized, and your new one is doing well.
    A properly sized system should not pull the temp down quickly, no matter the outdoor temp.
    "Hey Lama, hey, how about a little something, you know, for the effort." And he says, "there won't be any money, but when you die, on your deathbed, you will receive total consciousness." So I got that goin' for me, which is nice. - Carl Spackler

  4. #4
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,086
    What do the humidity levels normally run in the house (not basement)?

    Do you have a lot of moisture generating things in the house, such as plants, pets, aquariums, indoor water features, etc?

    Do you normally use a lot of heat generating appliances, such as home theater, computers, etc?

    When your a/c guy said "everything's fine" did he give you any specifics as to why he reached that conclusion?
    • 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
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    3,948
    What do your attic temps look like?

  6. #6
    Join Date
    Aug 2011
    Location
    San Jose California
    Posts
    153
    an attic fan will do wonders. also on a 70 deg day with sun on the windows, the window temp can hit 105 deg. just like your car

  7. #7

    Additional Info

    Thanks for all of the info.

    Here is some additional:
    The furnace is in the basement
    I believe that the upstairs humidity is typically around 55%ish but I haven't checked that recently.
    We live in WI, where we've had some hot days here but I've been monitoring run time on the not so outrageous days as well (temps in the mid 80's) and it's still running for around 2 hours at night when it kicks down 2 deg.
    I do not know the attic temp, I can try to measure this.
    I know that the attic is a vented attic.
    We had the house energy star sealed and had an audit done by an independent home performance inspector who had them address a few items to seal up the house better but overall said it was a really good build

    When the HVAC guy said that everything is working as expected he checked to make sure that the Puron was fully loaded or charged (sorry, not an expert obviously) and that everything kicked on and off when he adjusted the temp.

    I checked the temp of the air coming out of the ducts when the AC is running and it's around 64deg.

    Thoughts? Is this just normal? I hate to hear the AC run so much, it drives me nuts, and our last bill seemed very high during June when we didn't run the AC 1/2 of the time. I'm dreading what it's going to be this next month.

    motoguy128, How would I be able to tell if I'm getting the proper airflow needed?

  8. #8
    Join Date
    Dec 2005
    Posts
    1,823
    2 hours of run time to cool the house down 2 degrees in the evening in WI. Starting from 9:00PM at night to 11 PM at night to cool 2 degrees

    AND approximately 14 degrees split on the system.

    AND we say properly size ?? Ouch !

    Get the contractor out please.

  9. #9
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,396
    3 tons for well insulated new construction in WI sounds big to me. Here, further south, a 20 year old house that size with less insulation would be 2.5 tons. Something fishy somewhere. 64 out the vents is really high for a 76 degree house.

  10. #10
    Join Date
    Dec 2010
    Location
    Down by the river
    Posts
    1,579
    Quote Originally Posted by BaldLoonie View Post
    3 tons for well insulated new construction in WI sounds big to me. Here, further south, a 20 year old house that size with less insulation would be 2.5 tons. Something fishy somewhere. 64 out the vents is really high for a 76 degree house.
    I agree, I smell a fish
    It's hard to stop a Trane. but I have made one helluva living keeping them going.

  11. #11
    Join Date
    Aug 2011
    Posts
    568

    Wink

    Quote Originally Posted by jlbartley View Post
    We have a newly built house that the AC is taking longer than I would expect to drop the temp 2 degrees. I'm wondering how long we should expect it to run under the conditions I've specified below. Any help would be really appreciated.

    House specs:
    1950 sq ft new construction house, single story + basement
    R60 Ceiling insulation
    R19 - 2x6 walls
    house faces north with AC unit at the front of the house
    there are large windows to the south of the house but no windows on the east and very few on the west
    the AC unit is a 3 ton 13 seer Carrier 24ABB

    Scenario:
    The house temp was maintained at 78 deg from 4:00pm to 9:00pm. At approx 9:00 pm the thermostat kicked down to 76 deg. The outside temp was 75 deg with the sun fully set but it was more humid than inside (not horribly) but enough that to sleep comfortably we wanted to keep the air on.
    The AC ran for 120 min straight never shutting off, to drop the temp to 76 deg.
    We also run a dehumidifier in the basement that keeps the humidity down there at approx 55%.

    This seemed excessive to me based on our past 15 year old house with a 15 year old AC unit. That would typically take 30 min maybe 45 min to kick the temp down 2 deg.

    I called our HVAC contractor and he said he checked everything out and it's running fine. He said that because we don't have any blinds (haven't gotten any yet, we just moved a month ago) the radiant heat coming from the windows was causing it to run this long. This seemed like a bad excuse to me. I understand the concept of radiant heat and do not believe this would be enough of a cause for the AC unit to run this long with the temp being that low outside and no sun.
    on a hot humid day you unit starts to dry out your house then starts to
    I'm just wondering what should be expected under these conditions. Thanks for any help you can give me. If you need any more info please let me know.
    a lot of possible vriables, ex. on a 92 degree day when do you put the a/c on? when you get home at 4PM? during the day when system isn't on are the windows open? some people on a super high temp day, leave system off get home on a 92 degree hot/humid day, turn unit on and expect system to cool down quick! on a super hot day your unit starts to dry out your house then start to pull the temp down!

  12. #12
    Join Date
    Dec 2010
    Location
    Palm Beach,Fl.
    Posts
    920
    Something is indeed rotten in Denmark. There are a plethora of factors that can contribute to this problem. I would get a 2nd opinion.
    Quote Originally Posted by k-fridge View Post
    The laws of physics know no brand names.

  13. #13
    Join Date
    Aug 2011
    Posts
    568

    Exclamation

    Quote Originally Posted by SoFlaDave View Post
    Something is indeed rotten in Denmark. There are a plethora of factors that can contribute to this problem. I would get a 2nd opinion.
    what exactly is a plethora of factors mean?

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