What it usually covers
- Checking the filter and airflow
- Inspecting the burners, igniter and flame sensor
- Inspecting the heat exchanger for cracks, rust or soot
- Checking the venting and, on high-efficiency furnaces, the condensate drain
- Testing the safety switches and limit controls
- Checking the blower motor and electrical connections
- Watching a full heating cycle
Why it is worth doing
A furnace burns fuel inside the house. Checking that combustion is clean and exhaust leaves through the vent protects against carbon monoxide as well as breakdowns. Dirty flame sensors and worn igniters are common reasons a furnace will not light on the first cold night, and both are found easily at a tune-up.
When to schedule it
Autumn, before heating season starts, is the usual time. Electric furnaces and boilers benefit from a yearly check too, though what is checked differs.