Five things to look at before you call anybody

What you found

Nothing yet. Answer the first question and this fills in as you go.

• Filter is clean. Good — that rules out the single most common cause of a system that will not keep up, and it is worth saying when you ring.
• Change the filter first. A loaded filter starves the system, shortens cycles, and on a furnace can trip the high limit. Change it, run the system for a day and see whether the complaint goes away. This is free and it works more often than anything else on this page.
• No filter fitted. Everything the air carries has been going straight onto the coil and the blower. Fit the right size, and expect us to want to look at the coil — that is where it all went.
• Registers all open. Good. Airflow restriction at the grilles is ruled out.
• Open everything and move the furniture. Closing vents to save an unused room raises pressure across the whole system and usually makes every other room worse. A blocked return does the same thing and is easier to miss.
• Almost no air anywhere is a system-level problem. Blower, a badly restricted filter, or a duct that has come apart. Worth a visit rather than more looking.
• Thermostat is well placed. Good — that rules out the system satisfying early on a false reading.
• The thermostat is reading the wrong thing. Sun on the wall, a supply register blowing at it, or a cold outside wall behind it will all make it satisfy and call again too quickly. Moving it is a real fix rather than a workaround.
• A blank thermostat is usually power, not the thermostat. Batteries if it takes them, otherwise a tripped safety or a switch somewhere upstream — which is what step five is about.
• Outdoor unit is clear and running. Good, and worth mentioning — it narrows things considerably.
• Clear two feet all round it. A coil packed with seed, leaves or snow cannot reject heat, so the system runs longer and works harder for the same result. Rinsing gently with a hose from the inside out is safe; poking anything into the fins is not.
• An outdoor unit doing nothing while the thermostat calls needs a visit. Check step five first — a breaker or the switch beside it accounts for a good share of these — but if both are on, something is not starting.
• Power is fine. Good. That takes the cheapest explanation off the table and makes everything else easier to narrow.
• One reset is reasonable. Watch it. A safety device tripped for a reason, and if it goes again, the reason is still there.
• Stop resetting it. A breaker that trips repeatedly is telling you about a fault, and each reset puts the equipment through the start it has already failed. Leave it off and ring somebody today.

That is all five. If the list above is all “good” and the problem is still there, it is a real fault and worth a visit — and you will have saved the first fifteen minutes of it. If something on the list was wrong, fix that first and give it a day.