What is included in a furnace inspection?
A real inspection measures things instead of just looking: gas pressure, combustion and carbon monoxide, temperature rise, the flame signal, blower amps, the safety switches, venting and the condensate system, plus a look at the heat exchanger and burners. With us it's part of the $129 furnace tune-up, or $219 with the AC.
Key points
- An inspection produces numbers: gas pressure, combustion, temperature rise, flame signal and amps.
- Each reading points to a specific failure, like a weak igniter, a coated sensor or a tired capacitor.
- No visual check sees every inch of a heat exchanger, so combustion and CO readings back it up.
- A furnace found unsafe can be shut off and tagged until it's repaired or replaced.
- Your own fall checklist is a separate list; do both.
A furnace inspection checks safety and performance with instruments: a gas leak check, combustion and carbon monoxide readings, venting and draft, the heat exchanger and burners, the safety switches, the igniter and flame signal, gas pressure, temperature rise, blower amps and the condensate drain. Each check produces a reading you can ask about. If all you got was a new filter and a sticker, you didn't get an inspection. Our fall furnace checklist covers the half you can do yourself: the filter, the vent pipes, the drain and your alarms. This post is the technician's half, and why each check matters.
The checklist at a glance
| Check | What gets measured or looked at | A bad result usually means |
|---|---|---|
| Gas leaks | Valve and fittings, with a detector or bubble solution | A leaking union or valve |
| Combustion | Carbon monoxide and oxygen in the flue | Dirty burners, wrong gas pressure or a venting problem |
| CO around the furnace | Air near the cabinet and at the supply | Exhaust escaping where it shouldn't |
| Venting | Draft on a chimney flue; PVC joints, slope and terminations | A blocked or poorly drafting vent |
| Heat exchanger and burners | Openings, flame behavior when the blower starts | Cracks, rust-through or soot |
| Safety switches | High limit, rollout and pressure switch response | A failed switch, or one tripping for a reason |
| Igniter | Condition and electrical check | A weak igniter heading for a January failure |
| Flame signal | Microamps against the manufacturer's minimum | A coated or failing flame sensor |
| Gas pressure | Manometer reading at the gas valve | Overfiring or underfiring |
| Temperature rise | Supply minus return, against the rating plate | An airflow problem |
| Blower | Amp draw, capacitor, blower wheel | A motor or capacitor wearing out, or a dirty wheel |
| Condensate | Trap, drain line and pump | A clog, or a line at risk of freezing |
What safety checks come first in a furnace inspection?
Gas leaks. The technician checks the gas valve and the fittings near the furnace with a leak detector or bubble solution. A small leak at a union or valve is easy to fix once someone finds it.
Combustion and carbon monoxide. An analyzer in the flue reads carbon monoxide and oxygen in the exhaust, which shows whether the burners are getting the right mix of gas and air. A second reading in the air around the furnace looks for CO escaping where it shouldn't. High flue CO usually points to dirty burners, bad gas pressure or a venting problem.
Venting. On an 80% furnace, that means the metal flue, the draft into the chimney, and whether a water heater sharing the chimney still drafts properly. If a furnace was converted to high efficiency years ago, the water heater may now be alone in a chimney that's too big for it, which can call for a liner. On a 90%+ furnace, it means the PVC intake and exhaust: joints, slope, and the terminations outside.
Heat exchanger and burners. The heat exchanger keeps combustion gases separate from the air you breathe. The technician looks through the burner openings and any accessible points for cracks, rust-through and soot, and watches the flame when the blower starts. A flame that jumps or changes when the blower kicks on is a warning sign. Our post on cracked heat exchanger symptoms goes deeper. No visual check can see every inch, which is why the CO readings matter.
Safety switches. The high-limit switch shuts the burners off if the furnace overheats, the rollout switch trips if flame escapes the burner area, and the pressure switch proves the inducer is venting before gas flows. A technician confirms they respond. A limit switch that trips repeatedly is a symptom, and our limit switch guide explains what usually causes it.
If a technician finds a cracked heat exchanger or dangerous carbon monoxide, the furnace can be shut off and tagged so no one runs it until it's repaired or replaced.
How are the igniter and flame checked during an inspection?
Igniter. Most furnaces use a hot-surface igniter, a brittle part that wears a little every time it heats. The technician looks for cracks and checks it electrically. A weak igniter that still lights in October is a common January no-heat call.
Flame sensor. The sensor rod proves the burners actually lit. The technician pulls it, cleans it and then measures the flame signal in microamps against the manufacturer's minimum. Cleaning without measuring is a guess. Our flame sensor post covers the homeowner version.
Inducer and pressure switch. The inducer motor clears the flue before ignition. A noisy bearing, a cracked hose or water in the pressure switch tubing all cause the "clicks but won't light" failure.
Gas pressure. A manometer on the gas valve shows whether the burners get the pressure the manufacturer specifies. Propane and natural gas settings are different, and a furnace moved from one fuel to the other needs the right conversion. Pressure that's too high overheats the heat exchanger; too low causes poor combustion and condensation where it doesn't belong.
Airflow, drains and controls
Temperature rise. The technician measures supply air temperature minus return air temperature and compares it with the range printed on the furnace's rating plate. Too high means the furnace isn't moving enough air, usually from a dirty filter, closed registers or undersized ductwork, and that stresses the heat exchanger and trips the limit switch.
Blower motor. Amp draw compared with the motor's rating, the run capacitor on older-style motors, and the blower wheel itself. A wheel caked with dust moves noticeably less air.
Filter and static pressure. On systems with airflow complaints, a static pressure reading shows whether the filter and ducts are choking the blower.
Drains, wiring and the thermostat. The technician checks the condensate trap, drain line and pump on a high-efficiency furnace, tightens loose low-voltage and line-voltage connections, reads any stored fault codes on the control board, and runs the thermostat through a full heating cycle. If there's a whole-home humidifier, the pad, water valve and drain get a look too.
What gets extra attention in Minnesota
Book the inspection in September or early October, before the furnace starts running for hours a day. A lot of what fails in the first cold week was already weak in the fall, and the readings above are how it gets caught.
A few checks carry more weight here than in a milder climate:
- Vent terminations. PVC intake and exhaust pipes have to clear the snow line and stay out of spots where drifts pile up or a roof edge drips. A termination that looks fine in October can drift shut in January.
- Condensate routing. A drain line or pump discharge that crosses a garage, a crawlspace or an outside wall is the one that freezes in a cold snap. Our post on a frozen furnace condensate line shows why.
- Chimneys in older city houses. Plenty of Minneapolis and Saint Paul houses still vent an 80% furnace or a water heater into a masonry chimney, so draft and the condition of the flue get a close look.
- Propane settings. Outside the town gas lines, gas pressure is checked against the propane spec, and the conversion is confirmed.
What should you get at the end of a furnace inspection?
You should leave with a short list: what measured within spec, what's wearing, and anything unsafe. If a repair is needed, you get a written, flat-rate price before any work starts, and completed repairs carry a 1-year parts-and-labor warranty. When a furnace is old and the repair is large, compare the repair against furnace replacement cost before deciding.
Some manufacturers' warranty terms mention regular maintenance, so keep your tune-up receipts with the furnace paperwork.
How much does a furnace inspection cost?
The inspection is part of our furnace tune-up: $129 for the furnace, or $219 with the air conditioner. The price is the same everywhere we work, from furnace maintenance in Minneapolis to maintenance in Saint Paul and the towns around them, and if you want the inspection done every year without having to remember, ask us about a maintenance plan. Before the visit, our guide to turning the furnace on for the first time this fall covers what to watch on the first cycle. If the furnace has already quit, a furnace repair call carries a $79 trip charge, waived when you approve the repair. Call 612-429-3465.
Questions
Is a furnace inspection the same as a tune-up?
They overlap. The inspection is the measuring and safety checking; the tune-up adds cleaning and adjusting, like cleaning the flame sensor or setting gas pressure to spec. We do both on the same visit. Our furnace tune-up is $129, or $219 when we do the furnace and air conditioner together.
Can an inspection find a cracked heat exchanger?
Often, but not always by sight alone. Much of a heat exchanger is hidden inside the furnace. A technician looks through the burner openings and any access points, watches the flame when the blower starts, and takes carbon monoxide and combustion readings. Together those catch most problems. If a crack or unsafe CO reading turns up, the furnace can be shut off until it's fixed.
What happens if the technician finds a problem?
You get an explanation of what was measured and what it means, and a written, flat-rate price for any repair before work starts. Completed repairs carry a 1-year parts-and-labor warranty. If the furnace is unsafe to run, the technician will shut it off and tell you why. Replacement is only a conversation when the repair doesn't make sense for the furnace's age.
Should I get a furnace inspection when buying a house?
It's a good idea on an older furnace. A general home inspector looks at the whole house, usually without combustion instruments. An HVAC technician can measure carbon monoxide, gas pressure and temperature rise and look closer at the heat exchanger, which gives you better information about what you're buying.