Upstairs is hotter. Where is the fault actually living?

Air leaves the air handler cold and arrives upstairs warmer than it should, because it travelled through an attic that reaches well over 120°F on a July afternoon. Every leaking joint and every thin patch of insulation on that duct costs you twice — once in the cold air you lose and once in the hot attic air pulled in behind it.

• Measure what each upstairs register actually delivers. That number settles the argument.
• Seal and re-insulate the trunk and the branches before anybody talks about equipment.
• Check the return upstairs is big enough to take back what is being supplied.

When the rest of the floor is acceptable and one room is not, the problem is local: the run to that room, the damper feeding it, or the fact that the door is shut and the air has no way back out.

• Check the flex run for a crush, a kink or a disconnected boot at the ceiling.
• Check whether that room has any return path at all with the door closed — an undercut door or a transfer grille.
• Only then look at balancing dampers, and get the figures written down when they are set.

Running constantly on a hot afternoon is not automatically a fault — on a design day a correctly sized system runs nearly continuously. It becomes a fault when the house still loses ground. That is usually airflow and attic losses rather than capacity.

• Filter, coil and blower first. A loaded filter or a dirty evaporator ends this conversation cheaply.
• Then total airflow and static pressure, to find out whether the duct system can carry what the equipment makes.
• Then the attic: duct leakage, insulation depth and whether the attic itself is being vented.
• Capacity comes last, and only after a proper load calculation — not from the label on the old unit.

A stairwell is a chimney. Warm air rises up it all summer and collects at the top, which is why the landing is often the worst spot in the house and why the thermostat downstairs never hears about it.

• Check where the return is. A single return low downstairs leaves the stack unopposed; a high return on the landing pulls it away.
• Check where the thermostat is. If it sits downstairs in still air, it will satisfy long before upstairs does.
• Running the fan continuously mixes the floors and is worth trying for a week before spending anything.

Air conditioning removes moisture only while the coil is cold and air is passing over it. A system that reaches the setpoint in short bursts satisfies the thermostat without ever running long enough to dehumidify — which is exactly what an oversized system in a mild week does.

• Time how long it runs per cycle on a hot afternoon. Very short cycles point at sizing or airflow.
• Check the airflow setting on the equipment — pushing more air makes it colder-feeling and worse at drying.
• Check the drain is actually draining. Water that cannot leave ends up back in the air.
• Where the system is genuinely oversized, that is fixed at replacement, with a load calculation.

In winter the heat you pay for rises up the stairwell and sits upstairs, so the downstairs is the floor that stays cold — with the same single system trying to satisfy one thermostat for both.

• Balance for the season. Dampers set for August are wrong in January, and on most systems they can be adjusted twice a year.
• Check the return arrangement again — it is usually the same cause as the summer complaint, seen from the other end.
• If both floors genuinely need to be controlled separately, that is a zoning conversation, and it is the honest answer for some houses.