“Tune-up” covers an enormous range of work. At one end is a genuine diagnostic service that tests the components likely to fail and verifies combustion safety. At the other is twenty minutes, a new filter and a sticker on the furnace door.
Both get called a tune-up and both get invoiced. This is how to tell the difference.
The difference between looking and measuring
The single most useful distinction is whether the technician took readings.
Anyone can look at an igniter and say it looks fine. An igniter either works or it does not, and a cracked one frequently still works — right up until a cold night asks it to light thirty times. What tells you about its condition is its electrical resistance, measured and compared against the manufacturer's specification.
The same logic runs through the whole job. A capacitor that looks fine may be well down on its rated capacitance. A blower motor that sounds fine may be drawing more current than it should. A flame sensor that is passing today may be producing a signal barely above the control board's threshold.
Measurement is what turns a visit from an inspection into a prediction. That prediction is the entire product you are buying.
What should happen on a gas furnace
Combustion side
- Burners inspected and cleaned, flame pattern observed.
- Flame sensor cleaned, and its microamp signal measured.
- Igniter resistance measured against specification.
- Heat exchanger inspected for cracks and corrosion.
- Gas pressure measured at the appliance, not assumed.
- Combustion analysed — a meter in the flue reading carbon monoxide and oxygen or carbon dioxide.
- Venting and the outdoor termination checked, including draft.
Safety controls
- High limit switch tested.
- Pressure switch and its tubing checked.
- Rollout switches inspected.
- Flame failure response timed — the gas valve should close promptly when flame is lost.
Air side
- Blower wheel inspected and cleaned if dirty. A loaded blower wheel quietly costs a lot of airflow.
- Motor amp draw measured.
- Run capacitor tested.
- Temperature rise across the heat exchanger measured and compared with the specification on the data plate. This is one of the most informative single numbers available and it is skipped constantly.
- Filter replaced, and the right type specified for the system.
What should be on the invoice
Numbers. Specifically:
- Temperature rise, with the data plate's specified range.
- Flame sensor signal in microamps.
- Gas pressure, usually in inches of water column.
- Combustion readings, including carbon monoxide.
- Motor amp draws, against the plate ratings.
- Capacitor readings, against rated values.
- Static pressure, if measured.
An invoice reading “performed annual maintenance, system operating normally” documents nothing. It cannot tell you whether anything is drifting, and it is of no use at all if you later need to make a warranty claim — which most manufacturers condition on documented annual service.
You do not have to interpret the numbers. Their presence tells you the work was done, and comparing one year against the next is how you see a part heading for failure.
The part that is not about money
Combustion analysis is the item most often skipped and the one we would least want skipped in our own homes.
Any fuel-burning appliance — furnace, boiler, water heater — can produce carbon monoxide when combustion or venting goes wrong. It has no smell and no colour. A furnace can run, heat the house and seem entirely normal while venting badly.
A meter in the flue takes a few minutes and either confirms the appliance is burning cleanly or finds a problem nothing else would reveal. A heating service without combustion analysis has not checked the thing that matters most.
Independent of any service visit: carbon monoxide alarms on every level of the house, tested. They are inexpensive and they are the backstop.
Oil furnaces are a different list
Oil heat is still widely used in north-eastern and rural Pennsylvania, and oil equipment needs annual service more genuinely than gas does. Expect the nozzle replaced, electrodes cleaned and gapped, the oil filter changed, pump pressure measured, the cad cell flame detector checked and draft tested over the fire. Oil burners that go unserviced soot up, and soot is both an efficiency and a reliability problem.
On timing
Early autumn, before you need the heat. The reason is practical rather than promotional: a failing igniter found in October is a part ordered at your convenience. The same igniter found on the first properly cold night is an emergency call in a cold house, competing with everyone else whose igniter chose the same evening.
The repair is identical. Only the circumstances differ, and the circumstances are the expensive part.
Reasonable questions to ask
None of these are rude. A technician doing the work properly will be glad to answer them.
- What was the temperature rise, and what does the data plate call for?
- Did you run a combustion analysis? What were the readings?
- What is the flame sensor signal?
- Is anything measuring out of specification but still working?
- Can you put the readings on the invoice?
That last one matters most. It is the record that protects your warranty and gives next year's visit something to compare against.