Say which room. The plan says which shortcut

The room at the end of the longest branch gets the least air, and in a house where that branch is flexible duct pulled around two corners, it gets considerably less than the drawing intended.

• Measure what the register actually delivers. That number ends the argument in about ten minutes.
• Look for the flex run stretched, kinked or looped over a truss. Straightening and re-supporting it is cheap and often enough.
• Check the room has a way for air to get back out with the door closed — an undercut door or a transfer grille.
• Balancing dampers come last, and the settings should be written down when they are made.

With a single return, every bedroom is pressurised the moment its door shuts: supply air goes in and has nowhere to go. The room drifts, and the hall the thermostat sits in stays comfortable.

• The test is free — leave the door open overnight and see whether the complaint goes away.
• If it does, the fix is a return path: a transfer grille, a jump duct or an undercut door.
• A second return in the primary bedroom is the proper answer where the ceiling allows it.
• Nothing here requires new equipment, and a larger system would make it slightly worse.

An open kitchen at the end of the house adds a large, sudden load that no thermostat in a hall is going to notice quickly. Where the extract is a recirculating hood rather than one ducted outside, none of that heat or moisture actually leaves.

• Establish whether the hood vents outside or recirculates. Many here do not vent outside at all.
• Where it does vent, check the damper actually opens and the duct is not crushed in the attic.
• A supply register position that works when the room is empty may be wrong when somebody is standing at the range.
• Ducting a hood outside is real work and it changes the kitchen more than any thermostat setting will.

A garage in July is hotter than outside, and the wall between it and the next room is usually insulated to the minimum. If that room also sits at the end of a duct run, it gets the worst of both.

• Check the duct serving that room first — it is the part you can change.
• Check whether the wall and the ceiling above it are actually insulated. Sometimes they are not.
• Weatherstripping the door between the house and the garage is cheap and does more than people expect.
• Where the room has been converted to an office or a gym, it may genuinely need its own head. That is a real answer, not a fallback.

When everything struggles at once, the question is whether the equipment is making its capacity and whether the ducts can carry it. Both are measurable and neither is answered by the size on the nameplate.

• Coil condition and refrigerant behaviour first, because a system that is not making its capacity explains everything else.
• Then static pressure and total airflow, which tell you whether the duct system can carry what the equipment makes.
• Then duct leakage in the attic, which is paid for twice in this climate.
• Capacity comes last, from a load calculation for your plan and your lot — not from the label on what is already there.

Noise at the return grille is air being pulled through an opening that is not big enough for it. It is the audible version of the same problem that starves the far rooms, and it is hard on the equipment as well as on the household.

• Check the filter first — a loaded filter makes exactly this noise and is free to rule out.
• Have static pressure measured. That single reading tells you whether the return is undersized or merely dirty.
• A larger return grille, a second return, or a filter with more face area are the usual fixes, in that order of cost.
• Turning the fan speed down quietens it and makes the rooms worse. It is a workaround, not a repair.