What the visit should include
The U.S. Department of Energy's HVAC guide for homeowners lists these furnace tune-up tasks:
- Test for flue gas spillage (exhaust coming back into the room instead of going up the vent) and fix it.
- Check the condition of the vent connector and chimney.
- Check the heat exchanger for cracks or leaks.
- Adjust the temperature controls.
- Adjust the blower control and supply-air temperature.
- Clean and oil the blower motor if it needs oil; the guide notes most mid- and high-efficiency blower motors are permanently lubricated.
- Remove dirt, soot and corrosion from the furnace.
- Check fuel input and flame characteristics, and adjust if needed.
- Seal the connections between the furnace and the main ducts.
The Consumer Product Safety Commission recommends having the heating system professionally inspected and serviced every year, and says the inspector should also check chimneys and flues for blockages, corrosion, and partial, complete or loose disconnections.
Checks that depend on your furnace type
- Older or standard-efficiency furnaces that pull combustion air from the basement: the Building America Solution Center says these can backdraft if the room is depressurized. Its combustion safety test procedure checks ignition and controls, draft with a smoke pen, fuel input for over-firing, and carbon monoxide in the flue, then spillage at the draft hood with dryers and exhaust fans running.
- High-efficiency condensing furnaces: BASC says the condensate is acidic, must drain properly and must be protected from freezing. Ask whether the drain, trap and any condensate pump were checked.
- Sealed-combustion (direct-vent) furnaces: properly installed ones draw air from outdoors and can't backdraft into the house, so a draft-hood spillage test may not apply. Ask what was done instead.
If you've added exhaust fans, a new dryer, or finished or sealed up the basement since the last visit, say so. BASC says household exhaust can affect venting on furnaces that use indoor air, and it calls for a service technician when a furnace fails spillage or combustion tests with the fans off.
What the report should say
Ask for a report, on paper or by email, that includes:
- Date, technician, company, furnace make and model, and fuel type.
- Each task marked done, adjusted, not applicable, or not done, with a reason for anything skipped.
- Readings, with units, for any tests run, such as temperature rise, gas pressure or fuel input, draft, and carbon monoxide in parts per million (ppm). DOE's furnace installation checklist treats airflow, firing rate and temperature rise as numbers to verify against the manufacturer's specifications, so ask whether each reading was inside that range.
- Where each carbon monoxide reading was taken. A reading in the flue pipe and a reading in room air are different measurements.
- Before and after readings for anything adjusted.
- Any defect found: where it is, what was seen (photos help), the proposed fix, and whether it is urgent.
Not every company tests everything on every visit, and there's no single national report form. The point is to know which checks were actually done.
Your part before and after the visit
- Have past service records ready, and note any noises, odors, soot or water around the furnace and when they happen. Look, don't open panels; CPSC warns against servicing fuel-burning appliances without the right knowledge, skill and tools.
- Change the filter on schedule between visits; see how often a furnace filter should be changed.
- If the furnace starts and stops every few minutes, mention it; short cycling has several possible causes.
- Before the tech leaves, ask which findings mean the furnace shouldn't run, which need prompt repair, and which are optional. If the furnace is shut off as unsafe, get the reason in writing and don't restart it until it's repaired.
If a CO alarm goes off
A tune-up is not an emergency call. CPSC says if a carbon monoxide alarm sounds, go outside right away, call 911 or the fire department, and don't go back in until responders say so. If a furnace was the source, don't run it until trained personnel service it. More on when a CO alarm means leave the house.