Guidance for homeowners and facility managers.

Heat rises, so the second floor starts the afternoon warmer than the first. That is physics. But when the gap is 6 to 8 degrees instead of 2 or 3, the cause is usually one of three things: a duct system that leaks or is undersized to the upstairs, a return that cannot pull hot air out of those rooms, or a single thermostat controlling two very different loads.

Before anyone talks about a bigger system, three measurements tell you which problem you have. First, static pressure at the air handler — high pressure means the blower is fighting a restrictive duct system. Second, supply airflow at the registers in the warm rooms. Third, the temperature difference between the return and supply at the unit itself, which shows whether the equipment is actually doing its job.

If the equipment is fine and the ductwork is the problem, balancing dampers, a corrected return, or zoning solves it for much less than a replacement. If the equipment is undersized and the ducts are also restrictive, both have to be addressed or the new unit will disappoint you the same way.

A tune-up that consists of a filter change and a pat on the compressor is not maintenance. A real seasonal visit measures and cleans, and it leaves you with findings.

On the cooling side that means washing the condenser coil, checking the evaporator coil condition, verifying refrigerant charge against the manufacturer's target, testing capacitors and contactors, checking the condensate drain and float switch, and confirming airflow. On the heating side it means inspecting the heat exchanger, checking burners and flame, testing safeties, and on a heat pump verifying the defrost cycle and reversing valve operation.

Ask for the numbers, not just a signature. Charge, temperature split and static pressure are the three that tell you whether the system is set up correctly rather than merely running.

The age of the equipment matters less than three questions. How much does the repair cost relative to a new system? Is the rest of the system — coil, air handler, ductwork — in condition to justify the repair? And is a second failure likely this season?

A 12-year-old system with a failed capacitor or blower motor is an easy repair. An 18-year-old system with a refrigerant leak that cannot be isolated economically, paired with failing ductwork, is usually a replacement conversation.

Ask for both paths with numbers. If a contractor will only quote one of them, that is information about the contractor.