Gas Furnace Diagnosis and Tune-Ups

A furnace tells you what is wrong by where it stops. The trick is to watch the sequence, read the code and test the one part in question rather than swap parts until the heat comes on. This guide covers how to diagnose the failures that make up most no-heat calls, how to run an annual tune-up and when a furnace must not stay on. It assumes you know the normal sequence and parts. Gas, combustion and line-voltage work is for trained technicians.

What You Will Be Able to Do

• Sort a no-heat call by where the sequence stops.
• Test a flame sensor, ignitor and pressure switch.
• Find the cause of high-limit and rollout trips.
• Measure and set manifold gas pressure.
• Carry out a tune-up in order.
• Decide when to shut a furnace down.

Before You Touch Anything

1. Check your personal CO monitor when you enter the home.
2. Sniff for gas, and use a gas detector.
3. Read the code on the board before anyone cycles the power.
4. Check the thermostat, filter, power switch and gas valve.

Warning: If you smell strong gas, do not touch any switch or use a phone inside. Shut off gas from outside if you can do so safely, get everyone out, and call the gas utility from a safe place outside.

Where the Sequence Stops

Flame Sensor Problems

When a furnace fires for a few seconds, shuts down and repeats until it locks out, start at the flame sensor. It is the most common cause of a no-heat call.

1. Measure the signal with a meter on the microamp DC scale, in series with the sensor. A healthy signal is commonly 1 to 6 microamps.
2. If the reading is under about 1 microamp, clean the rod.
3. Lock out power and remove the sensor. Handle it carefully, because the ceramic is brittle.
4. Clean with a fine abrasive pad such as Scotch-Brite, or with 000 steel wool. Do not use sandpaper or emery cloth, because they scratch the rod and leave grit behind.
5. Reinstall it and test again. Replace it if the signal stays below about 0.7 microamps.

If a clean rod still reads low, check the ground and the burner, as well as the rod's position in the flame. A cracked ceramic or poor ground can also cause low signal.

Ignitor Problems

Hot surface ignition is the most common system on modern furnaces. Ignitors usually fail by cracking after thousands of heating cycles, or after a voltage surge.

1. With power locked out, unplug the ignitor.
2. Read cold resistance. Silicon carbide ignitors are typically 40 to 90 ohms, and silicon nitride ignitors are typically 11 to 18 ohms. Check the manufacturer's value.
3. An open reading means the ignitor has failed.
4. During a call for heat, a qualified tech can check for about 80 to 120 volts at the plug. If voltage is there and the ignitor stays dark, replace it. If no voltage reaches it, look at the board and the steps before it.

Do not touch a silicon carbide element with bare fingers. Skin oil creates a weak spot that can crack it early. Handle the part by its base.

Pressure Switch Problems

If the inducer runs but the pressure switch will not close, the furnace will not move on to ignition. A dirty flame sensor cannot cause this, because the flame circuit matters only after the gas valve opens.

Check these in order:

1. Vent and intake are clear of nests, ice and debris.
2. On a condensing furnace, the trap and drain are clear.
3. The pressure tube is clear of water, cracks and loose ends.
4. The inducer wheel is clean, and the motor spins at full speed.
5. Measure the pressure with a manometer, and compare it with the rating on the switch.
6. Only then, replace the switch.

Warning: Never jumper a pressure switch to run a furnace. A switch that stays open can be telling you that flue gases have nowhere to go.

Condensate Problems on 90%+ Furnaces

The trap and drain must stay clean and unfrozen. A plugged or frozen drain backs up into the inducer area. That causes pressure switch faults, and it can damage the secondary heat exchanger. Clean the trap every year, and protect drain lines from freezing.

A vent must slope back toward the furnace so condensate drains out. A sag in the pipe holds water, and that blocks flue gases.

High-Limit Trips

If a furnace heats briefly, then shuts down, and repeats, the problem is usually airflow.

1. Check the filter.
2. Check that registers are open.
3. Test the blower motor and capacitor. A failed capacitor on a PSC motor is a common cold-morning failure.
4. Check the blower wheel and coil for dirt.
5. Measure temperature rise and static pressure.
6. Check gas pressure, because overfiring also runs a furnace hot.

A cracked plastic vent elbow is a condensate and pressure problem, so it does not cause a limit trip.

Rollout Trips

A rollout switch opens when flame leaves the burner area. This usually means a cracked heat exchanger or a severely blocked flue. Do not simply reset it. Find the cause before you fire the furnace.

Gas Pressure

Manifold pressure is measured at the outlet tap of the gas valve, with a manometer.

Low pressure means poor combustion and low heat. High pressure overfires the furnace and overheats the heat exchanger. Follow the rating plate, and adjust only if you are qualified to.

Cracked Heat Exchangers

A crack can let carbon monoxide and other combustion gases into the home's air. Look for:

• Rust, soot or visible cracks, using a light, mirror and camera.
• Flame that shifts or wavers when the blower starts.
• A rise in CO or oxygen on the analyzer when the blower starts.
• CO in the supply air.

If you confirm a crack, shut the furnace down, close the gas valve, tag the furnace and tell the customer in writing. Do not re-fire it. Follow your company's process and local code.

Older Furnaces With a Standing Pilot

Atmospheric furnaces use a standing pilot and a thermocouple. If the pilot will not stay lit, check the thermocouple and the pilot orifice. Natural-draft furnaces run about 60 to 70% AFUE. You can check for spillage at the draft hood with a smoke source. Smoke should pull up into the flue and not back into the room.

Annual Tune-Up in Order

1. Ask about complaints, odors and any CO alarm events.
2. Check for gas and CO on arrival.
3. Lock out power, and close the gas valve to the unit.
4. Inspect the heat exchanger.
5. Vacuum the burner compartment, because dust clogs air ports and raises CO.
6. Clean the burners and flame sensor if needed.
7. Check ignitor resistance.
8. Inspect the pressure tube and vent.
9. Clean the condensate trap on a 90%+ furnace.
10. Open the gas valve, restore power and leak check.
11. Measure manifold pressure.
12. Measure static pressure and temperature rise.
13. Run a combustion test once the furnace has stabilized.
14. Replace the filter.
15. Check CO alarms in the home.
16. Run a full cycle and record your readings.

A home with no working CO alarm. Many companies require you to install one, or to leave the furnace off until one is installed. Follow your company's policy and your state's rules. Either way, tell the customer clearly and write it down.

When to Shut It Down

• Cracked heat exchanger.
• High CO that you cannot correct.
• Gas leak.
• Rollout with no clear cause.
• Blocked or disconnected vent.

Explain it in plain words, tag the unit and write the reason on the invoice.

Common Mistakes

Check Your Understanding

The inducer runs and the pressure switch stays open. The drain trap is full of sludge. What should you do first? Clean the trap and drain, then retest.

The flame signal reads 0.5 microamps after you clean the rod. What next? Check the ground and burner, and replace the sensor if the signal stays low.

A 90% furnace shows a high-limit code. The filter is packed solid. What should you do? Replace the filter, then check rise and static pressure to prove the airflow is right.

Manifold pressure on a natural gas furnace reads 2.5 in. w.c. Is that a problem? Yes. It is low compared with a typical 3.5 in. w.c., and it can cause poor combustion.

Key Takeaways

• Sort every no-heat call by where the sequence stops.
• Clean a flame sensor with a fine pad or 000 steel wool, never sandpaper.
• Pressure switch faults are usually vent or drain problems.
• High-limit trips are usually airflow problems.
• Natural gas manifold pressure is typically 3.5 in. w.c.
• Never bypass a safety, and shut down any unsafe furnace.