What to know
Start with airflow, not the thermostat
A system that is short of air behaves almost exactly like one that is short of capacity. It runs constantly, the air at the register feels less cold than it should, and the house drifts up through the afternoon.
Because the symptoms overlap, it is worth ruling out the airflow causes first. They are cheaper, and they are the ones you can check yourself.
Work through it in this order
- The filter. If it is loaded, replace it and give the system a few hours before judging anything else.
- The outdoor unit. Clear anything packed against it and check the fins are not matted with cottonwood or grass.
- The registers. Open every one, including in rooms nobody uses, and clear furniture off the return grille.
- The doors. Bedrooms with no return and a closed door will not keep up, however well the system is running.
If the house improves after any of those, that was your answer. Give it a day before deciding.
When it is the ductwork
If air is arriving at the registers but the house still cannot hold temperature, there are two very different possibilities and it is worth separating them before spending anything. Either the air reaching the rooms is not actually cold, which is an equipment problem, or it is cold and not enough of it is arriving, which is a distribution problem.
The check is simple. Hold your hand at a register near the furnace with the system running and cooling. If that air is not noticeably cold, the problem is on the equipment side — refrigerant charge, the evaporator coil, the compressor or the controls — and that is an HVAC technician's job rather than ours. No amount of duct work will fix a system that is not producing cold air.
If that air is properly cold and the house still will not hold temperature, then the question moves to how much of it is arriving. Ducts leak at the joints and seams, and a run that has been crushed, kinked or pulled apart at a boot delivers a fraction of what it should. Build-up inside the runs narrows them further.
None of that is visible from inside the room, which is why guessing at it from the thermostat rarely gets anywhere.
What you can tell us that helps
- Whether it is the whole house or particular rooms.
- Whether it changed suddenly or drifted over a season.
- What the airflow feels like at a register near the furnace compared with one far from it.
- Whether any remodelling has happened since it last worked properly.
What we would do
Look at the accessible ductwork and the way the system was put together, and show you what is there. If the runs are carrying years of build-up, cleaning them is a real improvement to airflow and we will say so.
If the trouble is a disconnected run or a system that was undersized from the start, cleaning will not fix it, and you should hear that from us before you spend anything.