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Thread: Good answer?

  1. #1
    Plumber jimbo's Avatar
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    Default Good answer?

    My daughter had trouble with her central A/C during the recent hot spell. Daily high temps above 100, possibly as high as 110. Basically, could not get the house below about 85. It seemed to me the compressor was shutting down for periods of time. ( The condensor fan stayed on, and the inside blower fan stayed on ). The air would be recirculating, but you could feel at the duct it was just recirc, not cooled. Then after maybe an hour, the compressor would kick in again.

    They contacted their A/C contractor recently and he said " nobody's system works when it's 110".

    Is that a blow-off, or a reasonably reply.

    Since I already have a long winded post going here, let me elaborate on the system: 9 months ago, inconjunction with a 500 sq.ft. room addition, they had this contractor install a new horizontal Trane XL80 gas furnace in the attic, new coil, but kept the 3 ton Heil condensor, which is only about 6 years old. He said the system was big enough for the new sq.ft. With my just dangerous enough knowledge, I am suspecting a return air problem. Used to be a direct input into the botton of the old vertical furnace, through 2 large grills into the closet. Now, return air is a single 20X20 grill, connected to the unit with about 20 feet of 16" ID flex insulated duct. Could this be a problem, or is there a more basic issue?

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    I'm not an expert, either, but the outlet temperature from the duct should be pretty much the same regardless of the outside temperature. It's just that there isn't enough cold air, not that the outlet temperature is too warm.

    As I understand it, the system is designed to some load at which it can attain some desired temperature in the house. As the outside temperature exceeds that design load, it would gradulally fail to maintain the set temp. For example, if it was designed to be able to maintain the house at 73 with the 3T load at a 95-degree day, if it was 100, then the inside should only rise 5 degrees.

    It might not rise that much depending on how much of the house was cold-soaked, and how long it was that temperature. this all plays into the proper heat load analysis and how oversized, if any, the system is. For maximum comfort, it is better to not have it oversized, as running contantly handles the moisture and comfort levels better.

    If the ducts were blowing warm air, something is wrong. The compressor may have a leak, and lost some refrigerant. If the air flow isn't proper, things can ice up, restricting air flow.

    I think he was blowing smoke...
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    Plumber jimbo's Avatar
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    Thanks, Jim. It didn't seem right to me that the compressor turns off. I will tell my son in law to insist that the contractor do a more thorough check out of the system.

  4. #4
    DIY Senior Member SteveW's Avatar
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    I agree.

    We've had some scorchers here in the midwest, and certainly there have been days when my system couldn't get to the set point of 76 degrees, but instead may have only been able to pull the temp down to 78 (until the sun went down).

    But in that case, I can assure you the compressor was running the whole time, and cool air was coming out of the registers -

  5. #5

    Default

    My experience with compressors is that they don't shut down unless they are overheated--usually from some restriction in air flow, eg, icing, dirty coils or dirty filter. Even if most of the freon leaked out, the compressor should keep going.

    Fully charged, the air temp. coming out of the vents is approx. 20-25 degrees cooler than the temp. of the return air. Thus, as the house gets cooler, the temp. coming out of the vents gets cooler. This is similar to setting the A/C on recirc. in your car. The car cools down much faster when it is recirculating cooled air rather than fresh air.

    My apartment has only slumpblock for insulation. On very hot days, the slumpblock soaks up heat, and even though my A/C is fully charged, it won't cool it below 85-90 degrees until the sun sets. And that's with the compressor going all the time.

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