Furnace Repair

Basic Forced Air Furnace Trouble Shooting Guide for Maintenance Technicians

By Darrin Card

NOTE:  This is a basic guide for informational purposes only.  This guide will not cover all furnace makes and models.  Only trained professionals should work on furnaces, the author of this guide is not liable for any damage or injuries caused by the improper use of this guide.

One of the most frustrating things for a new comer to the maintenance field can be troubleshooting a forced air furnace.  At first glance a furnace can seem daunting and even dangerous, however once you understand the basic operation of a furnace the task of troubleshooting and repairing can be a fairly easy process.

The majority of new furnaces have self-diagnostic tools built into them.  Be sure to read through the manufacturer’s manual as these tools can save a lot of time and money.  However, they might not always tell the whole story, that’s where understanding the inner workings of the furnace can come in handy.

Familiarizing yourself with the nomenclature of parts in a furnace is the first step to understanding the operation of the furnace itself.  Below is a basic guide to the parts to the furnace.  

There is one part that isn’t labeled on the drawing that I would like to point out, as it plays an important role in diagnosing a broken furnace.  Above the gas control valve and below the draft hood there is a small gray box set on the back of the cabinet.  This part is called a limit control.  There are also a few other parts not depicted in the above schematic.  Those are ignition module (a circuit board), a transformer, inducer motor, pressure switch and a run capacitor.  I will provide pictures of these parts below.

Once you understand that a furnace operates in a fairly lineal process the task of diagnosing becomes much easier to understand.  A furnace goes through many different tasks before it will actually supply heat however most tasks won’t begin until a goal has been met.  For example, the furnace won’t start to run until the thermostat calls for heat, the thermostat won’t call for heat unless the temperature in the room dictates it.  That is a lineal process (in reverse).  I have included a diagram below explaining the steps to the process a furnace goes through in order to supply heat.  Once you understand these steps we can expand on them to include multiple options for repair.  The instructions below are based on a common furnace but there may be minor changes in the operation depending on the model of furnace you are working on.  I will expand on these later in the article however reading through the manufacturers literature for your specific model is always recommended.

Furnace Operation

1)      Thermostat will call for heat, meaning it will send a signal to the ignition control module

2)      In order for the thermostat it must get power from a transformer which lowers the voltage from 120V to 24V

3)      The furnace will get the 24v signal from the thermostat and start by kicking on the inducer motor (if present)

4)      The inducer motor starts a draft to expel the gasses from the furnace

5)      A pressure switch will read when the draft reaches the proper cfm

6)      The pressure switch sends a signal to the ignition board which in turn sends a signal to the igniter

7)      The igniter will heat up and send a signal to the gas valve to open (Sometimes through the ignition control module)

8)      Once the gas valve opens the igniter will ignite the gas and the flame should spread through the burner tubes and reach a flame sensor.

9)      The flame sensor will reach the set temperature and send a signal to the fan to start blowing.

10)   The fan will start with help from the capacitor and blow the unconditioned air over the heat exchanger and out the vents.

11)   A limit control will read that the temperature of the air blowing past the heat exchanger is within a set temperature range.  If the temperature is outside of the range it will shut the whole process down, if it is within the range the furnace will operate normally.

12)   Once the set temperature is met the thermostat will cut off the signal to the ignition control module and the furnace will start to shut down.

13)   The gas valve will shut down but the fan will stay running to expel any heat left in the heat exchanger.

Once a basic understanding of the operation of the furnace is achieved we can now concentrate on the diagnostic process involved in the repair of a furnace.   The first step in diagnostics is running the furnace through a cycle.  To do this you must make sure the furnace is reset and the thermostat is turned up so that it will be calling for heat.  Make sure power is off to the furnace by turning off the power switch or the breaker supplying power to the furnace.  Next check the ambient (air) temperature of the room itself; this can be done simply by checking the thermostat however a bad thermostat may give a false reading.  I always carry a small, digital thermostat in my tool bag for use in situations such as this.  I stick the probe of the thermometer in a supply register and wait for an accurate reading.  Then be sure to set the thermostat to a temperature at least 5 degrees above the room temperature.  Most thermostats have a differential of 1-3 degrees, meaning they won’t call for heat unless the temperature is 1-3 degrees below the set temperature and they will run until the room temperature is 1-3 degrees above the set temperature.  The differential can be adjusted on the thermostat however I normally leave them at the factory setting.

Once the temperature is accurately set you can then turn the power back on to the furnace.  Be sure to pay careful attention to the operation of the furnace at this point.  The furnace should start to go through the steps above and, if it is not working correctly will stop at some point within those steps.  Knowing where the furnace is stopping is critical to diagnosis the furnace.

Once the thermostat calls for heat the inducer motor should kick in.  The diagram above doesn’t include an inducer motor as not all furnaces have them, especially older models.  The inducer motor will look like the one pictured below:

 

If the inducer motor doesn’t kick in turn the power off to the furnace and try and spin the blades by hand.  The bearings may have gone out of the motor itself, in which case you need to replace it.  If the blades spin freely you may have a problem with the transformer, thermostat, or ignition control module.  Start with the easiest to access piece, the thermostat.  Check for a 24v supply to the thermostat using a multi-meter to be sure the transformer is in working order.  Change the batteries in the thermostat and then try “jumping” the thermostat itself (bypassing it by directly connecting the power supply and the heat wire.  Do not do this unless properly trained as connecting the wrong wires may cause more damage to the unit).  If the inducer motor is still not running you may have a problem with the ignition control module or the motor itself.

Once the inducer motor is running you will want to listen for a very faint “click sound” coming from the pressure switch.  The pressure switch should be located close to the inducer motor and look something like this:

 

It should have two tubes running to it.  To check if the pressure switch is working shut down the furnace, disconnect one of the tubes and blow into the inlet tube of the pressure switch.  You should hear a clicking sound.  If you don’t hear the clicking sound the diaphragm is more than likely bad and you should replace the switch.

Once the pressure switch detects the inducer motor is working it will send a signal to the ignition control module which in turn sends a signal to the igniter.

There are a few different types of igniters in furnaces: a glow plug style, a spark igniter, and a standing pilot.  Here are pictures of each:

Glow Plug Igniter

Spark Igniter

Standing Pilot

Glow Plug Igniter

The glow plug igniter should turn a bright orange as it heats up.  If it is not heating up shut down the furnace and look at the igniter.  It may have a break in it or the wires may be charred in which case it will need to be replaced.  The gas valve will not open unless it detects the glow plug is heated up to the proper temp in this type of ignition.

Spark Igniter

A spark igniter has two parts to it, the distributer and the spark igniter.  The distributer is the rectangle looking part in the picture.  These can get plugged up and must be cleaned to allow the gas to pass through.  Be careful when disconnecting a spark igniter assembly as there may be an orifice below the distributer.  Once the inducer motor is detected the gas valve will open the pilot supply, allowing a small amount of gas to the ignition assembly itself.  At this point you should see the sparks coming from the spark igniter and the gas catch on fire.  If you are not seeing sparks replace the spark igniter.  Once the gas is ignited the gas valve will fully open.

Standing Pilot

A standing pilot will always have a flame, even when the furnace is not calling for heat.  The flame should be small and blue little or no orange should be detected in the flame.  The thermocouple on the standing pilot assembly sends a signal to the ignition control module if there is a flame present.  If there is not a flame present the gas valve will shut down the supply to the assembly.  Drafts can blow out standing pilots so be sure to try and re-light a pilot light that is out before replacing the thermocouple.  If the pilot lights but goes out quickly the thermocouple is bad and should be replaced.   If the furnace is calling for heat it will open the gas valve to the burners and the standing pilot will ignite that gas.

 

Once the ignition procedure is confirmed the gas valve should open and send gas through the burners.  There are many different styles of burners which all need to be kept clean and free of debris.  I would recommend hiring a professional to clean burners or train staff on how to clean burners before attempting it on your own.  Below is a type of common burner assembly, however burners can vary greatly.

The flame should spread throughout the burners and should be evenly distributed through the assembly.  If you see a larger flame on one side than the other the assembly may need to be cleaned or replaced.

If there is no gas at all you may have a bad gas valve.  Replacement is the only option here.  Be sure to re-seal all joints with Teflon tape or paste properly rated for gas.  

At the far end of the assembly there should be a flame sensor.  This detects the heat from the flame and sends a signal back to the ignition control module and/or gas valve to let it know things are operating normally.  If the flame sensor doesn’t detect heat it will tell the gas valve to shut down.

 

Flame sensors can become dirty over time and, while they can be cleaned, should be replaced if there is a flame present and yet the gas valve shuts down after only a couple seconds of operation.

Once the flame is detected the ignition control module will send a signal to the fan to turn on and start blowing non-conditioned air over the heat exchanger.  The fan needs help from a capacitor to help get it started.  If your fan doesn’t start running the capacitor can be checked (be sure to ground out the capacitor before working with it as it can hold a charge even when power has been disconnected to the furnace).  Capacitors can be tested, however the instructions can be fairly complicated.  This is a link to the Wikipedia page which has detailed instructions on how to test capacitors:

http://en.wikipedia.org/wiki/Equivalent_series_resistance

Once the fan starts up it will blow hot air through the heat exchanger, into the duct work, and out the heat registers.  If the fan doesn’t start up, or there is not enough flow the heat exchanger can heat up to a dangerous temperature.  To keep this from occurring a limit control is in place.  Limit controls can come in a few different styles however there are two basic types:

Mechanical Limit Control

Snap Disc Limit Control

 

Again, testing either of these is a very descriptive process and should be done by a professional or learned with hands on training.

If you have flame and your fan kicks on but everything shuts down within a couple seconds or up to a couple minutes of starting, without the room reaching the desired temperature, your limit control could be bad or there could be a blockage in the air flow, first check for a blockage of air flow at either the furnace filter or the A-coil.  Filters should be changed at least every 3 months, a dirty filter can block airflow into the heat exchanger.

An A-coil is used in split-style air conditioning systems.  This means there is a condensing unit outside the unit and an air exchanger inside the unit. The A-coil is the air exchanger and the fins can become clogged with dust, pet hair, and other debris.  These must be cleaned in order to facilitate proper air flow through the heat exchanger.  You can check for air flow at the heat registers also, although this can be deceiving unless you carry an air flow meter with you (I don’t).

 If the filter and a-coil are clean and allowing proper air flow the culprit is probably your limit control.   I usually change these out without testing them as they are a fairly cheap, disposable part.

If you fan doesn’t turn over or runs very slow you should check its operation.  Shut down the furnace and spin the blades by hand, if they seem hard to turn or don’t spin freely the bearings are bad and you will need to replace it.  When replacing a fan be sure to clean off the blades, which will be re-used, and the furnace cabinet while you have it out.

At this point there are no more steps for the furnace to go through, it should run normally and finish its cycle once the room reaches the set temperature.  If it is still shutting down early you may have a bad thermostat.  Again, you can jump the thermostat or you can replace it and see if it takes care of the problem.

The hardest thing to diagnose is the ignition control module.  If you look above the ignition control module plays a role in many of the steps.  They are also one of the most expensive parts of the furnace and it takes the most time to replace (although the fan motor is a close second).  This is usually the last part I replace when following the steps above. 

The biggest mistake technicians make when working on a furnace is rushing through the steps and getting frustrated.  If you are having problems with a furnace take a step back, calm down, and run the cycle again.  Be sure to follow each step and pay attention to what the furnace is telling you.  Having the right tool is also a big help, without the proper tools a 5 minute job can easily turn into 20 minutes of frustration.

Below is a list of tools that I find helpful when working on furnaces:

1)      6-1 screwdriver.  One of the most versatile tools be sure to get one which features ¼” and 5/16” nut drivers along with phillip and standard drivers.  Extra features which I have found handy include an LED light on the tip and an insulated handle.

2)      Multi-meter.  Testing parts saves the time and money for both the technician and the property.  Once you know how to properly use a multi-meter you won’t know what you did without it.  A few features I would look for are; a built-in noncontact voltage meter, CAP or uf testing, and a clamp on head (for air conditioning or refrigerators)

3)      Long set of nut drivers.  6” or longer is nice to reach back to the back of the furnaces

4)      Knipex Cobra pliers.  Some of the best pliers I have found, work flawlessly and are not as bulky as other models I have used.

5)      Headlamp or other light source.  Headlamps offer hands free work lighting but I have found a few other light sources which work just fine.

6)      Jumper wires.  I carry and insulated wire, about 8” long with alligator clips on both sides.  This allows me to jump over any troublesome circuit.  Again, please be sure you are trained properly before attempting this.

7)      Pipe wrench.  Although I can normally get by with my Knipex I usually try and carry a small pipe wrench with me for changing out gas valves.

8)      Small Drill/ Driver.  I really like the Rockwell RK2510K2 as it is small and powerful.  Look for something compact with a LED light on it.

9)      Digital Thermometer.