The state of Florida mandates that the so-called Manual J calculation be done for all new construction. Manual J is a heat gain/heat loss calculation for the entire home and takes into account such things as the size of the home, orientation, amount of window and the quality of the windows, the insulation in the home, etc. The calculation will essentially come up with the required number of BTUs to maintain a certain indoor temperature and humidity assuming a certain outdoor temperature and humidity. The actual equipment sizing in BTUs will typically be a bit larger than the Manual J calculation since the nominal performance of the equipment as measured in a lab is somewhat greater than when it is installed in a house plus other inefficiencies such as heat gain through the ducts in the house. Equipment sizing is typically on half ton increments. On days where the outside temperature and humidity exceed the figures used in the Manual J calculation and/or your desired indoor temperature is below the Manual J design point, then it is possible that the system will not be able to maintain your desired indoor temperature. The Manual J calculation typically won't be based on outdoor temperature and humidity values that rarely occur. There are other possible issues such as the quality of the duct installation.
Your Manual J calculation for your home is available on-line. I can't remember the exact county website but with some googling you should be able to find it. You can check the indoor and outdoor temperature and humidity assumptions and other inputs that went into the calculation of the number of tons of cooling you need. By the way, a ton is 12,000 BTUs. The size of your system is 4 tons, not 4000 tons.
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Originally Posted by Brandigirl
Had the same problem with our brand new home. We expanded our home and I told them to make sure we had the correct tonnage (BTU's ) for expanding the home. They put in a 4,000 ton when I think it should have been 5,000. Was told by the A/C people the same thing that GONAVY said in his post. The A/C guy said that The Villages has a calculation they use to determine this. He said someone took them to court on this and LOST. We are in the same boat. Can't get the temp below 75-76 degrees when it is extremely hot outside.
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