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AC Cycle time question

My home's 2-ton central AC was just tuned up. During today's hot weather - 90* (Boston) our system was running non-stop. Is this normal? The house has cathedral ceilings, so despite the insulation, there is a lot of heat gain.

I my system undersized, or is this the way it should be on a hot day? We keep the t-stat at 73*.


Steve from Newton, MA


  • Wethead7Wethead7 Member Posts: 170
    Run time

    I do not know the design conditions for Boston, but 90f has to near or above it. At that temp you should expect your unit to run continuously. You may have a unit that runs at a low speed and then this might be normal at even lower temps. Your home may also have moisture that needs removed. This tends to limit the sensible cooling you have avaible.
  • Jean-David BeyerJean-David Beyer Member Posts: 2,663
    It is suggest to keep thermostat higher.

    Environmentalists and penny-pinchers suggest keeping the air-conditioning thermostat set to something like 78F instead of 73F. Saves energy. Whether those are concerns of yours or not, I found (when I had an air conditioner) that by setting it to the higher temperature, I did not get as bad a "temperature shock" when I walked out of a cooled house into a 95F 90% R.H. world.
  • Jean-David BeyerJean-David Beyer Member Posts: 2,663
    edited August 2009
    Many years ago.

    Many years ago I passed through the brand new Amtrak railroad station in Chicago. You could say that its main waiting room had cathedral ceilings. It was so hot in there I could not stand it. "How could they put up a large new building in Chicago with no air conditioning?" I asked myself.

    Well it turns out the building was air conditioned, the air conditioner was working as designed, but it could not deal with the 110F or so outside temperatures. IIRC, 100s or perhaps a thousand people died from the heat in Chicago that week.

    Whatever capacity air conditioner one puts into a building, the weather can always exceed it. At least for whatever capacity people can reasonably afford. At least in Chicago.

    It was pretty bad. Here is a link with this heat described:
  • Empire_2Empire_2 Member Posts: 2,343
    On the right track

    Boston is similar to Buffalo, which has a design temp of 85*.  It is not out of the norm to find that your A/C is running all day at 95* outside temp.  It's always a good Idea to have the unit serviced to make sure the refrigerant charge is where it should be.  I find in some rare instances I will bump up the A/C size by 1/2 ton or in high humidity conditions which we are experiencing in the last few years, I will use a slightly larger A coil to air in more moisture removal. 

    My .02

    Mike T.
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