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  1. #1
    Join Date
    Jul 2008
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    35

    Higher ceilings: more or less A/C usage?

    We have 11 ft ceilings in our 1 story house in Dallas (current temp: 105). My friend says that higher ceilings result in less A/C usage during the summer compared to 8 ft ceilings (assuming that everything else is the same). His theory is that with high ceilings, the hot air stays up top where you can't feel it (and where it can't trip the t-stat). So the A/C only runs to cool the bottom 6-8 ft of air in the house and the air above that stays warmer. I understand what he's saying, but I think the air is going to mix quite a bit, so while there might be a small temperature difference between the bottom and the top, its not going to be anything substantial (maybe a degree or two). And with higher ceilings, you have a larger air mass to cool, which means more A/C time.

    So the question is, if you had two identical houses next to each other with the same insulation, shade, windows, t-stat setting, and A/C system, but one house had higher ceilings, which one would use the A/C more?

  2. #2
    Join Date
    Mar 2005
    Location
    Suppy NC
    Posts
    4,516
    higher cielings imo have more load thus requiring a slighty larger unit in most cases. If the returns are high wall then the temp near the cirling will be the same as the floor. Reason is you are pulling the heat out

    your question will result in the house with the higher cielings useing more electric to cool if it can to the same temp.

  3. #3
    Join Date
    Feb 2005
    Location
    In a van by the river
    Posts
    621
    A higher heat gain would occur in a house with higher ceilings. If you were to do a manual J more cooling, slightly higher probably, would be required.
    ## + years in the field never made you a know-it-all This industry is far more diverse than you are

  4. #4
    Join Date
    Jan 2002
    Location
    Ft.Worth,Tx
    Posts
    4,584

    LOAD

    I don't care if ceilings are 25ft high and 11ft in every room, there is still load on that condition space and air changes per hour to remove the heat and humidity.

    So, more LOAD; more capacity needed to remove heat & humidity.

    Tell your guy to read up on load calculation, before he makes a blanket statement like that..
    "Everyday above ground, is a good day".
    "But everyday that you have made a difference in someones life, may insure you stay above ground a little longer".<aircooled>

  5. #5
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,031
    I agree with Tinknocker & git-r-dun, with perhaps a caveat or two.

    We will say you have an attic above the 11 or 12 foot ceiling that is not well insulated; then if the airflow design is setup to allow stratification, there will be less temperature difference between the attic & that warmer air.

    That would help slow the rate of the conduction transfer.
    Of course you have more wall area to account for.


    Yes, with no reduced conduction rate allowances, the manual J would show a higher heat-gain on the high ceilings.

    I prefer the standard 8' ceilings.
    I would like to see some hard research data on that comparison.

    Soupcxan, I do not know the right answer to your question! - Darrell

  6. #6
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,376
    Quote Originally Posted by soupcxan View Post
    We have 11 ft ceilings in our 1 story house in Dallas (current temp: 105). My friend says that higher ceilings result in less A/C usage during the summer compared to 8 ft ceilings (assuming that everything else is the same). His theory is that with high ceilings, the hot air stays up top where you can't feel it (and where it can't trip the t-stat). So the A/C only runs to cool the bottom 6-8 ft of air in the house and the air above that stays warmer. I understand what he's saying, but I think the air is going to mix quite a bit, so while there might be a small temperature difference between the bottom and the top, its not going to be anything substantial (maybe a degree or two). And with higher ceilings, you have a larger air mass to cool, which means more A/C time.

    So the question is, if you had two identical houses next to each other with the same insulation, shade, windows, t-stat setting, and A/C system, but one house had higher ceilings, which one would use the A/C more?
    I'm over in Fort Worth. My weather station is indicating 103.8 degrees outside. My entire 1,800 square foot home has eight foot ceilings. I just measured air temperature at ceiling level and at floor level, and had around a two degree gradient (76 at ceiling, 74 at floor level, 75 at t-stat...75 is setpoint). The house has round ceiling diffusers, which are excellent for cooling, not so hot for heating.

    Higher ceilings can only come at the price of more material to elevate them, hence more surface area for heat transfer (walls/windows, and if ceilings are sloped, they also have more surface area than a flat ceiling). Higher ceilings are popular because they give the occupants a sense that the space is larger...if the same floor plan had eight foot ceilings it would appear considerably smaller, with no change in actual square footage.

    Higher ceilings also come at the price of more air volume. ALL air within a structure is subject to heating, and to absorbing humidity via infiltration. With many newer homes with high ceilings, the ceilings themselves are perforated like Swiss cheese to allow for recessed lighting, a/c vents, etc. In winter that sets the house up to act like a big chimney, allowing heated air to escape into the attic, causing cold air from outside to draw into the house, making it feel chilly. In summer, higher ceilings provide more area for heated air to rise and stagnate, and that air will mix with cold air emerging from the supply vents, diluting somewhat the cooling effect of the supply air. Although insulated walls generally do not add as much heat gain to a house as windows and ceilings do, a house with higher ceilings have higher walls (meaning more exposed surface area), and from what I've seen, many of these walls face west, with no shading, all to eager to load up with heat all afternoon during the worst heat and highest solar loading of the day.

    My idea of a house profile for the north Texas climate is similar to the architect's that designed my house, single story with wide overhangs completely around the house, deeply shaded entrance and patio, and a carport instead of garage so there's not a huge heat trap adjacent to two or more walls of the house (a closed garage with an uninsulated attic overhead will impose more of a heat load to an insulated wall adjoining the interior than outdoor air will due to it being considerably warmer than outdoors), eight to nine foot ceilings max, all glazing shaded majority of day, minimal glazing to east and west. Only fault I'd find with my house design is large patio glazing facing west, but it's partially mitigated by a patio cover. IMO no glass should face west unless it is deeply shaded. How often is that seen on newer two story houses that face west all over north Texas?

  7. #7
    Join Date
    Apr 2002
    Posts
    11,808
    higher ceilings also means less shade on the glass

    plus 8 foot doors and taller windows so it does not look dumb

    I always had a gut feeling that the cathedral ceilings were a medevil cooling system, for the few hours during the mass in the summer time a big pocket of hot air would start stratifying up there.

  8. #8
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,776
    A 3' higher ceiling means you have a 3' higher wall that gains heat also.

    So you need more cooling for a higher ceiling house under the circumstances you posted.
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  9. #9
    Join Date
    Jan 2008
    Location
    Lehighton, Pa.
    Posts
    178
    Speaking of high ceilings, I'm getting conflicting methods of how to run my ceiling fans in the summer in order to circulate the cooled air. I have heard that the ceiling fan should be set to force air down to the floor in the forward position and in the winter to reverse the flow. Yet, my neighbor is telling me that it is the opposite of what he was told.

    Could you pros kindly provide the proper airflow methodology for the fans. Thanks.

  10. #10
    Join Date
    Jul 2008
    Posts
    26
    Quote Originally Posted by jeepgrady View Post
    Speaking of high ceilings, I'm getting conflicting methods of how to run my ceiling fans in the summer in order to circulate the cooled air. I have heard that the ceiling fan should be set to force air down to the floor in the forward position and in the winter to reverse the flow. Yet, my neighbor is telling me that it is the opposite of what he was told.

    Could you pros kindly provide the proper airflow methodology for the fans. Thanks.
    What makes you think the Pros will agree on it? One of the biggest HVAC shops in my neck of the woods agrees with your neighbor based on the theory that you want to move the cool air up in the summer and the hot air down in the winter. They are wrong.

    The problem with their theory is the simple fact that you don't want room temperature air blowing on you in the winter because it feels cold. Moving air cools you by evaporating skin moisture. In the summer, when you are hot, it also feels cold, which is what you want. So it's up in the winter in order to mix the air, and it's down in the summer to provide evaporative cooling.

    Either way, the energy consumed by the fan's motor is added to the room's heat load.

  11. #11
    Join Date
    Jul 2008
    Posts
    35
    Quote Originally Posted by jeepgrady View Post
    Could you pros kindly provide the proper airflow methodology for the fans. Thanks.
    I'm no pro, but everything I've ever read says fans should rotate counter-clockwise in the summer and clockwise in the winter.

  12. #12
    Join Date
    Jan 2004
    Location
    PA
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    68,776
    Around here.
    Draw the cool air up in the summer.
    Push the warm air down in the winter.
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  13. #13
    Join Date
    Apr 2002
    Posts
    11,808
    push the air down to cool you by convection, air goes to floor over to wall and up wall

    pull up air from centre of room up to ceiling in heating mode, forces hot air along ceiling and then down the walls

    Were I come from ceiling fan blowing down in winter was good for nipplitus erectus

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