A common mistake in heating and cooling design is oversizing—building in more cooling or heating capacity than the house needs, and causing the system to run inefficiently. Of course, under-sizing the system has its own set of drawbacks: nobody wants a house that they can’t heat or cool when the weather outside is at its worst. So “rightsizing” the HVAC, using modern software and following the industry standard Manual J, is one of the signs of a truly professional HVAC contractor.

But even if the system as a whole is perfectly matched to the total load, there’s another design mistake to look out for: a mismatch between the supply and the return sides of the system. This can make individual rooms too hot or too cold, and it can cause leakage of conditioned air to the attic or the outdoors—and yes, there’s an efficiency penalty, too. So here’s some advice on the final step in HVAC design: balancing the system.