What it is

A heating PM on a gas furnace is the fall visit that makes sure the unit lights reliably, burns cleanly, moves the right amount of air, and — most important — isn't leaking combustion products into the home. Gas heat adds a safety dimension a cooling PM doesn't have: you're dealing with combustion, carbon monoxide, and a heat exchanger that separates breathing air from flue gas. This is a safety inspection wearing a maintenance hat.

How it works

A furnace fails in season for predictable reasons: a dirty flame sensor that won't prove flame, a weak igniter, a clogged condensate path on a high-efficiency unit, a flame-rollout or limit trip from poor airflow, or a cracked heat exchanger. Most of those start small. A PM catches the weak igniter before it leaves a family cold, and — critically — a combustion check and heat-exchanger inspection catch a CO problem before it hurts someone. The stakes are higher than cooling, so the discipline has to be higher too.

In the field

  1. Interview the customer. Any short rooms, odd smells, soot, or a furnace that "takes a few tries" to light? Headaches in the house? That last one is a CO red flag — take it seriously.
  1. Change/check the filter and confirm airflow. Low airflow on a furnace isn't just comfort — it overheats the heat exchanger and trips limits, and over time it cracks exchangers. Air side first, like always.
  1. Inspect the burners and flame. Look for clean, stable blue flames. Lazy yellow, lifting, or rolling flames mean a combustion or airflow problem. Clean burners if they're rusty or dirty.
  1. Clean the flame sensor. A flame sensor coated in oxide reads a weak microamp signal and the board shuts the gas off as a safety. A light cleaning with a non-abrasive pad restores the signal. This one cheap step prevents a huge share of nuisance no-heat calls.
  1. Check ignition. Hot-surface igniter resistance and condition, or spark/pilot operation. A weak HSI is a no-light waiting to happen — catch it now.
  1. Inspect the inducer and pressure switches. Confirm the inducer runs free and the pressure switch proves draft. On condensing furnaces, check and clear the condensate trap and lines — a plugged trap trips the pressure switch and locks the unit out.
  1. Test the safeties. High-limit, rollout switches, and pressure switches must actually function. These protect the family; confirm they're in place and working.
  1. Inspect the heat exchanger. Visually inspect for cracks, rust-through, or separation. This is the wall between flue gas and breathing air — a breach is a CO hazard and a condemn-the-unit call.
  1. Run a combustion analysis. Put the analyzer in the flue and read CO, O2, and (where applicable) draft and temperature rise. This is how you prove the furnace is burning safely and efficiently, not just assume it.
  1. Verify temperature rise. Measure return and supply temps and confirm the rise falls within the range on the data plate. Too high = airflow problem; too low = often an oversize/airflow or gas-input issue.
  1. Confirm gas and electrical. Manifold pressure to spec, tight electrical connections, proper voltage, correct draw on the blower.
  1. Document and report honestly — including any safety concern, which gets communicated clearly and in writing.

Normal values & targets

Residential gas-furnace ballparks — always defer to the data plate:

  • Temperature rise: stamped on the furnace, commonly a 35–65°F window; stay inside the labeled range.
  • Flame-sense microamps: typically a few microamps minimum to keep running (often ~1–2 µA floor, healthy several µA); compare to the manufacturer's spec.
  • Hot-surface igniter resistance: varies by type — silicon nitride and silicon carbide read very differently; check the spec, not a universal number.
  • Manifold gas pressure: commonly ~3.5" w.c. for natural gas and ~10–11" w.c. for propane, but set per the rating plate.
  • Flue CO: as low as practical; an elevated, rising CO reading is a problem to investigate, not to ignore. (Use the analyzer manufacturer's and the appliance's guidance for acceptable air-free CO.)

Common faults & what they mean

  • Lights then drops out after a few seconds: classic dirty/failing flame sensor not proving flame.
  • Clicks/glows but won't light: weak or cracked igniter, gas-supply, or valve issue.
  • Inducer runs, no ignition, locks out: pressure switch not proving — blocked flue, condensate-plugged trap, or a stuck switch.
  • Limit trips, cycles on high temp: low airflow — dirty filter, blower, or restricted ducts; can crack the exchanger over time.
  • Yellow/sooty flames, high CO: combustion problem — dirty burners, poor air, or a venting issue. Stop and diagnose; this is a safety call.

Tech tips & gotchas

Cleaning the flame sensor is the highest-value routine step on a furnace PM, just like the capacitor is on cooling. A two-minute cleaning prevents the single most common nuisance no-heat — the furnace that lights, runs a few seconds, then quits and locks out.

A combustion analyzer is not optional on a real heating PM. "The flames look blue" is not a CO measurement. The analyzer is how you actually know the furnace is safe and efficient — and it's your documented proof if a question ever comes up.

Take heat-exchanger inspection seriously. If you find a crack or you're suspicious, communicate it clearly and don't hand-wave it. A breached exchanger can put CO into the living space. When you can't see it well, the combustion numbers and CO behavior during blower cycling give you supporting evidence.

Airflow drives furnace longevity. Most cracked heat exchangers trace back to chronic low airflow overheating the metal through thousands of cycles. Fixing a dirty filter or an undersized return is real preventive maintenance, not a comfort upsell.

Confirm the CO alarm situation in the home. If there's no working CO detector, that's a worth-mentioning safety gap — gas heat plus no CO alarm is a real risk you can flag honestly.

Safety / code notes

  • Carbon monoxide is the headline risk. A cracked heat exchanger or bad combustion can introduce CO into breathing air — treat any CO concern as a safety stop. Recommend working CO alarms.
  • Combustion-air supply must be adequate; sizing is governed by the fuel-gas code (IFGC §304; Indiana enforces the IFGC). Inadequate combustion air causes poor burns and CO.
  • Venting connector clearances, slope, and termination are code-governed (IFGC venting provisions and the appliance listing). Confirm the vent is intact and unobstructed.
  • Manifold pressure and gas-train work fall under the fuel-gas code and the appliance rating plate — set to spec, leak-check joints with proper solution (never a flame).
  • Lock out electrical and shut off gas before service where appropriate; prove circuits dead with a verified meter.