Gas Furnace Troubleshooting and Repair - Aside from understanding the basics of electrical, electronics, and controls to troubleshoot a furnace one needs to understand the components of the furnace and the gas furnace sequence of operation. Therefore repair techniques for various furnaces will be different from one furnace to another furnace depending on what type of furnace you are repairing. This is a basic gas furnace repair article that simply covers the basics.

If you want to learn more about repairing furnaces, you can check out the recommended reading. Books are available on HVAC or Furnaces, or another option is to take a repair class at your local community college or trade school. These can help you learn gas furnace troubleshooting and repair. Of course, you can’t learn it all in a book nor taking a repair class. However, you can learn the basics of repairing furnaces by learning the basics of HVAC. After all, nothing beats experience, especially educated experience.

Gas Furnace Troubleshooting and Repair - The Thermostat

Always remember that when repairing any appliance and you are ever in any doubt, it is best to call professional. Gas Furnace Troubleshooting and Repair is recommended for someone with a technical background. If you ever smell gas, open your windows and leave. Do not turn any light switches on or off or light any matches or any fire. Go outside and call your local HVAC contractor or the utility company. Let them check everything out. Don’t take any chances.

The first thing I always do when arriving on a no heat repair call is to talk to the homeowner. I see if they have any input, such as describing any strange noises or anything else they want to offer — any unusual observation of the furnace. The next thing I do is to find the thermostat and make sure the setting is for heat, and the temperature is turned up as far as it will go.

If it is a mechanical thermostat, you can check to make sure the heating anticipator is intact by making a visual inspection of the thermostat. In many mechanical-type thermostats, you visually see the circuit for the heating anticipator.

It is also a good time to ask the homeowner about the service. Have they paid the gas bill? You can check the meter to make it is not locked out. There are instances where the homeowner paid the bill, but the utility company red-tagged the meter because they sensed a leak in the line near or at home. Occasionally utility companies will send out people with special combustible gas detectors to check for leaks in their lines and homes that are served by their lines.

Checking the Air Filter | Gas Furnace Troubleshooting and Repair

The next thing to do is to check the air filter, which should go near the thermostat somewhere. If the air filter is loaded up and has not seen a change for a long time. That can be the problem as the loaded up air filter will impede the airflow. That can set off the high limit switch in the heat exchanger.

This high limit should automatically reset itself, but I have seen them not reset themselves, and the high limit switch needs replacing with an exact replacement. Never jump it out and leave the job. That is very dangerous and highly discouraged.

Are Supply Vents Open? | Gas Furnace Troubleshooting and Repair

While you are at the filter and have the homeowner there, you can ask about the supply vents. Make sure they ARE ALL OPEN. I have found some situations where the homeowner closed off too many vents and it has the same effect as having a dirty clogged furnace air filter. I always get an “I don’t know” from the homeowner. That means I go around and visually check all the supply vents.

I have never failed to find at least one or two supply vents closed, and in a few cases, I found multiple supply vents closed, and that was the problem causing the furnace to malfunction. Open those supply vents to make sure that the furnace has the appropriate airflow.

Gas Furnace Troubleshooting and Repair - Blower Motor Check

We finally get to the furnace itself, and we begin making basic checks having eliminated all the problems mentioned above. We check to make sure the blower motor is functioning properly. To do this, we must determine how the blower motor turns on and off. Is it controlled by a fan limit control?

A Printed Circuit Board? Or a relay that links to another device? The blower motor can easily be checked by jumping the relay if a relay controls it. If a fan limit control controls it, we can manually adjust the fan limit control until the blower comes on.

With a printed circuit board, we can use a multimeter to check the voltage. The voltage from the fan terminal to make sure it is getting proper voltage to the fan motor. If it is getting the proper voltage, but the blower fan is not working, then we need to look at the blower fan motor. Lastly, check the motor run capacitor and other things with the fan. If we are not getting voltage to the fan motor, we need to find out why.

Determining Proper Air Flow in Gas Furnace Troubleshooting and Repair | Temperature Rise Method

As noted above, checking those basic things is important to make sure everything is functioning properly. However, how do you know exactly how much CFM’s or cubic feet per minute of airflow you have? That is important as the heat exchanger needs a specific amount of airflow across it to prevent overheating. A minimal amount of airflow across the heat exchanger will make the limit switches open for safety reasons.

For this, we use the delta T (∆) or the temperature across the heat exchanger and a formula. It is important to take these temperatures (in Fahrenheit) with good measurement instruments and to use best practices. Radiant heat from the heat exchanger can affect the measurement, so you must take these temperatures away from the heat exchanger so to prevent the radiant heat from affecting the reading. This method also requires a combustion analyzer to measure the steady-state thermal efficiency of the gas furnace.

Here is the formula for a Natural gas or Propane Gas Furnace:

CFM = (Input BTU x thermal efficiency) / (1.08 x ∆°F)

Input BTUs can be found on the side of the gas furnace (equipment nomenclature). A combustion analyzer determines thermal efficiency. ∆°F. is determined by measuring the temperature across the heat exchanger.

Other Furnace Component Checks | Gas Furnace Troubleshooting and Repair

There are other components of the furnace we need a check including the furnace igniter, the gas furnace ignition controls, and making sure the valve is opening as it should. Inside the furnace itself, multiple safeties need to be checked and verified. The safety circuit for the furnace is typically 24 volts. That will require us to check the transformer inside the furnace.

Other things in the process of troubleshooting a furnace include the ignition process, the safety circuit, the forced draft or induced draft motor. Additionally, we need to check the flame sensor and check the exhaust system or flue to make sure the furnace properly exhausts the gases. Furthermore, some gas furnaces have diagnostic controls that will give you a flash code for troubleshooting.

Safety Circuits | Is there a Furnace Reset Button? | Gas Furnace Troubleshooting and Repair

Parts that typically fail in a gas furnace can include:

1. Limit Switches for Internal Furnace Temperature
2. Pressure Switch for Furnace Heat Exchanger and Exhaust Pressure

Limit Switches

Manual Reset Limit Switch

1Limits switches - these are located in the safety circuit. Limit switches can fail but never assume they are simply bad. Too many techs jump them out without finding out why they tripped (or opened). I had an atmospherically vented boiler one time. The homeowner had another tech over, and that tech jumped out a limit on the draft diverter, thinking the limit was bad. It was a freezing night, and the boiler was running to keep up with demand.

Luckily this homeowner had a CO detector. The flue was plugged up, and the boiler was venting to the basement, and eventually, the CO built up enough to set the CO detector off on the first floor. Jumping out limit switches is okay when troubleshooting but never leave it that way. Always find out why it is tripping before making any assumptions. Some manufacturers put limit switches throughout the unit including near the blower fan.

Pressure Switches

2Pressure switches - this can be the result of several issues from a bad blower wheel in the induced/forced draft motor or even a bad draft motor fan. The tubing could have holes in it or even moisture in it. Rodents like chewing on the hoses and some insects can cause blockages in the hose.

You test the proper pressure with a manometer and also using a multi-meter to make sure the bellows are functioning properly and making proper contact. Lastly, check the manufacturer’s literature to make sure you are testing for the proper pressure for the pressure switch.

Furnace Safe Ignition | Gas Furnace Troubleshooting and Repair

Notice the burn marks on this control board

Following out the proper gas furnace sequence of operation is always a good practice when troubleshooting a gas furnace. A standing pilot gas furnace will have a different sequence of operations than a furnace equipped with electronic ignition. Safe ignition is simple for a standing pilot gas furnace, whereas electronic ignition gas furnaces can be complex depending on the control or control module.

With a standing pilot system, as long as the pilot light is lit and properly position next to the gas furnace burners, safe ignition is a given. Electronic ignition is different, though, since there are different types of electronic ignition systems, and each different manufacturer uses a different control method. However, following the basics can help you solve the problem:

Problems That Can Result in No Heat - Checklist

1. Fuel - and Delivery Pressure
2. Furnace Circuit Boards and Control Modules
3. Proper Ignition
4. Flame Sensor

1

Fuel - and Delivery Pressure | How Do You Troubleshoot a Propane or Natural Gas Furnace

As noted above, make sure you have a good gas supply and the proper manifold pressures using a manometer. Natural gas: Any pressure over 14″ WC can damage the gas valve. That is the reason why the piping is not connected to the gas valve when pressure testing the gas line for leaks. The manifold gas pressures should have a listing on the data plate of the gas furnace. If not listed there or unreadable, then you will have to find the proper pressures from the manufacturer’s literature that came with the gas furnace.

Remember, gas manifold pressures will be less for natural gas than they will be for propane. Additionally, there will be a pressure drop across the valve and should be within the specifications of the manufacturer. Improper pressures can result in the furnace not working. Be aware that there is a difference in manifold pressures from a propane furnace to a natural gas furnace. The operation of the furnaces is the same but the delivery pressures differ because of the properties of the different gases.
2

Furnace Circuit Boards and Control Modules

Circuits boards and control modules need to be check for proper input voltage. A good reference for troubleshooting circuit boards can be found here. That article is applicable to air conditioner circuit board troubleshooting however it really can be applied to any circuit board.
3

Proper Ignition | Gas Furnace Not Igniting

Proper spark ignition or hot surface ignition. See our menu to the right about these types of igniters.
4

Gas Furnace Troubleshooting Flame Sensor

A good clean flame sensor. Contrary to popular belief, the flame sensor does not produce microamps or millivolts. It is simply a sensor that detects microamps. If it is not clean, it will not send the proper signal to the ignition board or ignition module.

Conclusion

As stated above, there are many different types of furnaces, and each furnace will require different methods of troubleshooting. A trained HVAC technician can properly, safely, and efficiently troubleshoot your furnace. Always make sure as a homeowner that you get regular annual furnace preventive maintenance on your furnace. Regular maintenance will likely prevent anyone from having to troubleshoot your furnace. Consequently, because it is properly maintained.

Gas Furnace Troubleshooting – Furnace Repair

Resource: The Home Comfort Book: The ultimate guide to creating a comfortable, healthy, long lasting, and efficient home