Why the tune-up happens before the cold, not during it
The first genuinely cold week in Sacramento is when every furnace in the region gets switched on at once, and it is also when the emergency schedule fills. A part that has to be ordered on a Tuesday in October is an appointment. The same part in the middle of a cold snap is several nights without heat.
There is a second reason, and it is the more important one. A tune-up in mild weather can be done properly — the technician can shut the appliance down, pull the burner assembly, take combustion readings at steady state and inspect the heat exchanger without a household standing there in coats waiting for the heat to come back on.
September through November is the window. If it is already cold and your furnace has not been looked at, call anyway — we would rather inspect it late than not at all.
What fails on a furnace that has sat unused all summer
Flame sensors. A thin oxide film builds on the sensing rod and the control board can no longer confirm a flame. The furnace lights, runs a few seconds, and shuts down — over and over. It is one of the most common no-heat calls of the season and it is cleaned in minutes during a tune-up.
Hot-surface igniters. Silicon carbide and nitride igniters are brittle and degrade with every cycle. Resistance drifts out of range long before the igniter cracks, which is why it gets measured rather than looked at.
Dust on the burners and heat exchanger. Eight months of settled dust is what produces the burning smell on the first cold night. A light smell for a few minutes is normal; a persistent one is not.
Rodent and insect damage. Wiring, insulation and inducer housings in attics and crawlspaces take a beating over a Sacramento summer, and nothing was running to reveal it.
Seized or noisy inducer and blower motors. Bearings that sat idle for months announce themselves at start-up, usually as a squeal or a hum with no rotation.
Clogged condensate drains on high-efficiency furnaces. The pressure switch reads the blocked drain and refuses to let the furnace fire at all.
Filters left in since spring. A loaded filter starves airflow, drives temperature rise above the rated range, and trips the high-limit switch — which looks to a homeowner like a furnace that keeps shutting itself off.
How to tell a system is not ready for winter
Turn it on before you need it. Pick a cool morning, set the thermostat above room temperature and let the furnace run a full cycle while you stand near it.
What you want is a quiet start, ignition within a few seconds, steady blue burner flame, warm air at the registers, and a shutdown that ends with the blower running on for a bit and then stopping. What you do not want is repeated ignition attempts, a boom at light-off, a smell that persists, a yellow flame, or a system that heats for a few minutes then quits.
Also check the obvious things: the filter, the vents that got closed off in summer, and the area around the furnace itself, which should not be storing paint, solvents or boxes.
If anything in that list is off, that is the call to make in October rather than January.
Maintenance versus repair
Maintenance is scheduled, documented and preventive: cleaning, measuring, testing safeties and recording readings so this year can be compared to last year. Repair is what happens when a component has already failed.
A tune-up frequently turns up something that needs correcting. When it does, we stop, explain what we found, and put the scope and price in writing before doing anything beyond the maintenance itself. If the furnace tests healthy, the report says it tests healthy and we leave.
If your furnace is already down, go to our furnace repair page instead — that is a diagnostic call, not a maintenance visit.
