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Thread: AC Adequate?

  1. #1
    Join Date
    Sep 2008
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    AC Adequate?

    My wife and I toured a home for sale in the Dayton, OH area yesterday. It has all electric heating and cooling with a heat pump and electric backup (I think). It was sunny with a temperature about 80 degrees. We got there about 11 a.m.

    The outside unit was running and seldom cycled off while we were there (about an hour). Noticed the thermostat was set on 72. It was OK inside, but it did not feel like 72. Neither of us noticed what the temperature was reading on the stat. The model number on the outside unit indicated it was a 30K btu Rheem with a manufacturing date of 1991.

    The house is built on a slab and has high and/or cathedral ceilings in some places. The master bedroom has a tray ceiling about 2' higher in the middle of the room than surrounding walls.

    On further reflection after coming home I am wondering if the AC is up to the job on a really warm/hot day. The AC we have in our condo would not even be running by 11 a.m. on a day like that.

    Thanks in advance for your thoughts.

  2. #2

    HVAC ears

    30K is a 2 1/2 ton. A rule of thumb for sizing is 1 ton will cool approx 600 Sq ft. Windows and insulation are of course a factor. Air out of registers should be around 55 Degrees . Also if the duct runs are in the attic they should have blow in insulation over them.

  3. #3
    Join Date
    Sep 2008
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    58
    Thanks HVAC ears,

    I should have mentioned that the house in question is a 2000 square foot ranch with awnings on the west facing windows. From your 600 square feet per ton rule of thumb, the AC would be quite inadequate.

    I note, however, that the AC for our condo, which is a 2 stage, 2 ton unit easily cools our 1300 square feet on first stage only (unless we have to bring the temperature down following a setback). So there must be, as you noted, several variables to take into account besides square footage. Perhaps the AC at the toured house needs some checking for low freon, etc., because the air temperature at the registers didn't seem as cool as what the temperature at the registers at our condo feels like.

    I don't know if it makes a difference, but the house in question is built on a slab, and the supply ducts are under the slab. Return ducts are in the attic. I looked in the attic briefly, but did not have enough light to see if the ducts were covered with insulation.

  4. #4
    Join Date
    Jun 2005
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    SW Wisconsin
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    Quote Originally Posted by Dave66 View Post
    Thanks HVAC ears,

    I should have mentioned that the house in question is a 2000 square foot ranch with awnings on the west facing windows. From your 600 square feet per ton rule of thumb, the AC would be quite inadequate.

    I note, however, that the AC for our condo, which is a 2 stage, 2 ton unit easily cools our 1300 square feet on first stage only (unless we have to bring the temperature down following a setback). So there must be, as you noted, several variables to take into account besides square footage. Perhaps the AC at the toured house needs some checking for low freon, etc., because the air temperature at the registers didn't seem as cool as what the temperature at the registers at our condo feels like.

    I don't know if it makes a difference, but the house in question is built on a slab, and the supply ducts are under the slab. Return ducts are in the attic. I looked in the attic briefly, but did not have enough light to see if the ducts were covered with insulation.
    I am not a fan of ducts in or under the slab.
    I've seen those ducts fill with water, also potential mold problems.
    Always have that situation checked for mold & moisture problems.

    The attic Return-Air could be pulling in hot air from the attic.

    There are a score of things that can compromise A/C performance.

    That home may need better weatherization & reduced air-infiltration, which can be up to 50% of the heatload.

    There are many factors affecting correct sizing of the unit, 800-sq.ft. per-ton of cooling may be adequate for your climate & conditions.

    Click udarrell for more information...
    Last edited by udarrell; 07-18-2009 at 08:23 AM. Reason: Tonnage may be adequate...

  5. #5
    Lots of variables that have nothing to do with capacity of the system.

    If the house was unoccupied the AC might have been off upto 1/2 hour before the open house.

    And I don't know how many people where coming and going.

  6. #6
    Join Date
    Sep 2008
    Posts
    58
    Udarrel wrote:

    I am not a fan of ducts in or under the slab.
    I've seen those ducts fill with water, also potential mold problems.
    Always have that situation checked for mold & moisture problems.

    The attic Return-Air could be pulling in hot air from the attic.

    There are a score of things that can compromise A/C performance.

    That home may need better weatherization & reduced air-infiltration, which can be up to 50% of the heatload.

    There are many factors affecting correct sizing of the unit, 800-sq.ft. per-ton of cooling may be adequate for your climate & conditions.

    Click udarrell for more information...

    Thanks, Udarrel

    If our offer is accepted, we will have the possibility of leaking ducts checked as well as mold and moisture in the supply ducts during house inspection. If we buy the house, I will check for air leaks to minimize infiltration.

  7. #7
    Join Date
    Sep 2008
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    58
    WildBill99 wrote:

    Lots of variables that have nothing to do with capacity of the system.

    If the house was unoccupied the AC might have been off upto 1/2 hour before the open house.

    And I don't know how many people where coming and going

    Thanks, Wild Bill

    The subject house was occupied just before we toured it. The cloths dryer was running when we got there.

  8. #8
    Join Date
    Apr 2009
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    Pan Handle, Fl
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    If I did my math right it's an 18 year old unit. I'm not sure of the weather conditions in Ohio but that unit is probably close to the end of it's life expectancy/efficiency. There are several factors that determine the size unit for a house. If serious about buying house I would have contractor evaluate during inspection period. Can ask for money off cost to replace/repair.
    Insanity: Doing the same thing over and over again expecting a different outcome!

  9. #9
    Join Date
    Sep 2005
    Location
    Bay Area, CA
    Posts
    843
    If you feel its questionable you should have an hvac tech evaluate it for you, I wouldn't necessarily trust the home inspection report myself.
    Global Warming or: None like it hot
    No pleasure, no rapture, no exquisite sin greater... than central air. -Dogma

  10. #10
    Join Date
    Sep 2008
    Posts
    58
    Thanks Willserve and Green Jumper,

    I plan to have the HVAC inspection done by a (hopefully) qualified tech who will also evauate the overall condition of the equipment. Now all I have to do is find one in that area. I live about 150 miles from that house, so finding one will be a challenge. I've been reading this site for some time now, so I think I know what I have to do.

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