The problem of sizing HVAC equipment can be a particularly tortuous. It should involve some sort of a loads calculation to determine the amount of heating and cooling necessary over each hour of the year (although in homes the calculations may be done using rules of thumb based on calculations made in the distant past). (1) A key part of a loads calculation is specifying activities that will take place in the building and the desired indoor environmental conditions. Sizing doesn't seem so difficult. Find the worst hour of the year, add a safety factor and you have the size of the equipment needed. Unfortunately, a few problems exist with that approach. One problem is cost; the larger the capacity of the equipment, the more it costs. Oddly, no one wants to pay more than they have to for a basic function, so there is always the pressure to push down first costs.