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  • Determine A/C efficiency in cooler weather when not operational

    Originally posted 11-14-2018 11:34 AM
    By: bulgin


    Hello.

    I am a real estate broker selling a home with 9000 s.f of interior space (floor space). About 3,000 s.f. floor space on each of three floors. All floors except the fully finished basement have high ceilings. The buyer and I are trying to determine if the installed central A/C is sufficient to cool the entire home in the hottest weather in the Hudson Valley of New York. Suburbs of NYC. An building (not a/c) engineer looked at the system and stated the following:

    Central air conditioning systems are switched on via the thermostat(s), if outdoor temperatures permit. Thermostats are not checked for calibration or timed functions. Adequacy, efficiency or the even distribution of air throughout a building cannot be addressed by a visual inspection. The inspector does not perform pressure tests on coolant systems, therefore no representation is made regarding coolant charge or line integrity. Subjective judgment of system capacity is not a part of the inspection.

    The buyer called around to the A/C folks who said they could not test the system unless it was cooling season. Now, my question is this:

    Given that the a/c compressors and all destails of the system are known, including volume of air that needs to be cooled via calculations of space, etc. can't they do at least a simple calculation to determine if the installed compressors and air handlers are sufficient?

    I'm trying to upload an image of the compressor units but my existing photos from a previous topic has images and I cannot delete them, having exceeded my quota. it's not apparent how to delete photos from a previous topic.

    Thank you.

  • #2
    Originally posted 11-14-2018 1:23 PM

    Before installing a HVAC system, a Manual J calculation should be performed. This tells you what size system needs to be installed. However, in order to do this calculation you need to know the following:

    Floor area of bottom floor and whether that floor's perimeter is insulated or at least 5' below grade.
    The perimeter wall area of each floor and the insulation in those walls
    The window area in those perimeter and the U value of the windows and whether they are low-e or not
    The roof area and the insulation in the roof.
    Any skylights in that roof and their U value and whether they are low-e
    How well sealed is the building (air leakage), and is there a heat recovery ventilator


    So yes, there is a calculation you can make. Do you have decent answers to the questions above? If not you can guess and maybe go worst case to see what the required cooling and heating requirement is. This calculation is not difficult, but it is tedious and getting accurate data after the fact can be difficult. Also, many installers do not do this calculation, they just guess by looking at the building. If it is a recent building, a Manual J calculation may have been done.

    Once you know the cooling required in BTU-Hr, the installed HVAC system needs to be that size or a touch larger. In residential air conditioners, there is usually a suffix on the compressor model number like 024 or 060. The 024 would be 24,000 BTU-HR. The 060 would be 60,000 BTU-Hr.

    If the installed system is large enough, this does not mean the duct work is effective or it is performing to rated capacity.
    Mark
    Kent, WA

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    • #3
      Originally posted 11-14-2018 7:47 PM
      By: bulgin


      thank you! This is extremely useful. I will see if I can get answers to those questions. Very helpful indeed.

      Thanks!
      Mark
      Kent, WA

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