Wind speed has a LOT to do with load, especially on the heating side. 20F outdoor with no wind vs. a 15MPH wind makes a HUGE difference in heat loss. Did you account for wind speed when you were plotting your heat loss?
Thermal mass of the house is another thing that needs to be accounted for. If outdoor tempatures change quickly it can take up to 24hrs before the house feels the full effect of thermal load. This is why you don't need to size AC units for peak load with high mass homes.
Last edited by Servicetech; 11-23-13 at 08:01 AM..
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