Is this a duct problem or an equipment problem?

Nine times out of ten, it's the duct system. We confirm with a static-pressure reading and room-by-room airflow measurements. If the equipment can move the air but the ducts can't carry it to the rooms that need it, the equipment isn't the problem.

Will adding a second AC fix it?

Sometimes, though adding equipment often papers over a duct problem that comes right back. We don't recommend adding a second system until we've measured what the existing one is actually doing and ruled out the cheaper fixes.

Manual or motorized dampers can rebalance airflow if the system has the static-pressure headroom to handle them. On a system that's already restricted, dampers make the imbalance worse. Measurement tells us which case you're in.

How do you actually diagnose it?

Static pressure with a manometer at the air handler. Total system airflow with a TrueFlow grid. Temperature split across the coil. The diagnosis falls out of the readings, not out of a hunch.

Will a bigger system help?

Usually not, and often it makes things worse. An oversized system short-cycles, never pulls humidity out, and still can't push air through a duct system that's restricting it. Sizing should follow the duct work, not lead it.

HVAC Answers

6 min read·Russell

Why is my upstairs hotter than downstairs?

Almost always it's the duct system rather than the equipment. Heat rises, the upstairs picks up sun load through the roof, and the duct system that was supposed to compensate for both is either undersized, leaky, or starved for return air. Adding a bigger AC rarely fixes it. Measuring static pressure and airflow at the upstairs registers usually tells you exactly what to do, and it's a sixty-second test most companies skip.

The five things we actually find

1. Undersized supply ducts upstairs. The original builder used the same trunk-and-branch design for both floors, and the upstairs branches can't deliver the airflow the upstairs needs in July. We measure static pressure at the air handler and total system airflow with a TrueFlow grid, then compare what the system can actually deliver to what the load calc says the upstairs needs. The mismatch is usually obvious.

2. Not enough return air upstairs. Conditioned air gets pushed up but has nowhere to come back from, so the system goes pressure-positive upstairs and the air just sits there. A single upstairs return can fix this, but you have to size it for the airflow instead of just cutting a hole.

3. High static pressure on the system overall. Equipment rated for 0.50″ wc that's actually running at 0.95″ is moving a fraction of the air it was designed to. The upstairs feels it first because it was already at the end of the line. The manometer reading tells us this in a minute.

4. One system, two floors. A single-zone system in a two-story home was always going to fight gravity. Sometimes a bypass damper or a zoning retrofit is the answer; sometimes it's a small dedicated upstairs system. Frequently, fixing the duct problems above gets you 80% of the way there without adding any equipment.

5. Attic heat infiltration. Hot attic air leaks into upstairs ducts through bad joints and uninsulated runs. Supply air that arrives upstairs warmer than it left the air handler confirms it. The fix is in the attic, not at the thermostat.

What won't fix it

A bigger outdoor unit. A smart thermostat. Closing downstairs vents (this makes static pressure worse, not better). Cranking the thermostat lower. Each of these is what we hear from homeowners who've been told to try them, and each one leaves the actual cause untouched.

Common questions

Why is my upstairs hotter than downstairs?

The five things we actually find

What won't fix it

More questions, answered straight.

Start with a measurement.