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  1. #1

    Central air keeps on running when the sun is out & temps are in the high 80's -90's

    Stat finally gets satisfied when the sun starts to drop off (7 pm ). I have cathedral ceilings (15'-6")in living room, din room & east exposure.
    The house Is about 50 years old. Living room& din room is apprx 450 sq ft and I'm dumping in about 750 cfm's. supply wall grilles(3) are at 9'-0 & return is a sidewall about 11'-0 center line(16 x 30 ). I also have a 30 x14 return ceiling grille in the center hallway on a 8'-0 ceiling. House floor plan is about 1100 sq ft. Why does this unit keep running during these high heat days ? I was thinking is it the cathedral ceilings with very old insulation?
    Please help

  2. #2
    Join Date
    Aug 2012
    Posts
    2,594
    First what temp is it set at?
    Second sounds like a perfectly sized system. Welcome to the real world.
    My name is TooCoolforschool and I am a chronic over charger.

  3. #3
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    6,453
    That's what it's supposed to do.

  4. #4
    Join Date
    May 2004
    Location
    south louisiana
    Posts
    3,139
    insulation doesn't stop performing as it ages.
    50 years ago we didn't invest a lot in insulation, so it
    may not be recommended R-values for today.

    long run times of hvac isn't a bad thing, it is the short
    cycling that cost you to operate the system, lessens
    dehumidification & shortens life of the system.

    here in hot humid La. correctly sized units can run pretty
    much non stop from 2 pm until 9 pm.

    just fyi.

    is there a comfort or high utility cost problem??
    750 cfm for 450 sq ft area is a lot of cfm...almost 2 tons.
    do you need that much air to cool that area<<
    what size unit for 1100 sq ft?
    what location?

    best of luck.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  5. #5
    Quote Originally Posted by energy_rater_La View Post
    insulation doesn't stop performing as it ages.
    50 years ago we didn't invest a lot in insulation, so it
    may not be recommended R-values for today.

    long run times of hvac isn't a bad thing, it is the short
    cycling that cost you to operate the system, lessens
    dehumidification & shortens life of the system.

    here in hot humid La. correctly sized units can run pretty
    much non stop from 2 pm until 9 pm.

    just fyi.

    is there a comfort or high utility cost problem??
    750 cfm for 450 sq ft area is a lot of cfm...almost 2 tons.
    do you need that much air to cool that area<<
    what size unit for 1100 sq ft?
    what location?

    best of luck.
    Here in NY , Just for comfort. It's a 3 ton unit. I truly believe there is not enough insulation in the cathedral area to keep the suns heat out. At nite when the sun is gone I could drop the temp with no problem. Is there a way to do a test on cathedral ?

  6. #6
    Join Date
    Aug 2012
    Posts
    2,594
    The closer to the sun you get without insulation the more heat transfer there will be. Just be glad it cycles at night.
    My name is TooCoolforschool and I am a chronic over charger.

  7. #7
    Join Date
    Dec 2010
    Location
    Down by the river
    Posts
    1,623
    Its not that big of an area. How about ripping the ceiling out and spray foam it.

  8. #8
    Quote Originally Posted by Southern Mech View Post
    Its not that big of an area. How about ripping the ceiling out and spray foam it.
    Was thinking about that. Not sure what to do maybe add to rafters and make the 2x6 into 2x8's. or use spray foam. I think 2x8's with a air space and R19 would get a better R value than compacted f spray foam ?

  9. #9
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,248
    Options:
    1. Running
    2. NOT running
    3. Set t-stat = 82'F from 10:50 AM to 6:40 PM.
    4. Have the sun not visit at all during July & August.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  10. #10
    Quote Originally Posted by dan sw fl View Post
    Options:
    1. Running
    2. NOT running
    3. Set t-stat = 82'F from 10:50 AM to 6:40 PM.
    4. Have the sun not visit at all during July & August.
    Dan sw
    Don't quite get your joke ! I'm looking for answers

  11. #11
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,900
    With high ceilings the main thing is to NOT disturb the air above 8 feet; because air above that point acts as insulation, as heat rises, so the lower temp difference between the warmer ceiling air area & the attic or roof areas is left alone.

    You have a High Return that should NOT be there for either cooling or heating purposes, also supply diffusers should be lowered; could even be at the baseboards' area; the throw should be at the occupant level area but usually not directly at areas where occupants will be seated.

    The idea is to just cool &/or heat the air that is at the level of the occupants; therefore, all the air above say 8' is left undisturbed as a barrier to temp-difference between the ceiling & the interior; you absolutely do NOT want to be cooling that upper area so that there won't be a large temp-difference which increases the rate of heat transfer.

    That alone could make a big difference in your cooling load. Why cool all the warm insulation air nearest the ceiling you don't live in that area; or how is the rest of your home designed? That will affect what you do...

  12. #12
    Join Date
    Mar 2009
    Location
    Mount Holly, NC
    Posts
    3,024
    the problem is that often, cathedral ceilings are adjacent to balcony upper floors... which do need cooling, so the cathedral also needs to be cooled. is this the case with your house as well?
    The TRUE highest cost system is the system not installed properly...
    The three big summer hearththrobs...
    Mel Gibson
    Dwane Johnson
    The A/C repairman

  13. #13
    No just a roof above this problem area. Sheetrock on the inside

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