Chicago Heating and Furnace Components Part 2, Furnace Repair Tips

This is going to be the second part in a two-part article that will describe the main components of a furnace and some tips to resolve problems that can be associated with the furnace.

The following is for informational use only and is not intended as a “How to Guide”.  Gremillion Mechanical always recommends that if there is a problem with any complex appliance in your home to call a professional service technician trained in the repair of that equipment.  Remember to always “stay in your comfort zone” when trying to perform any task related to your furnace and if you feel unsafe, call for professional help!

In this article I will be speaking about furnaces that are made prior to draft induced furnaces. The type of furnace problems that will be associated with furnaces that have natural (gravity) draft flue rather than newer furnaces with induced draft are similar but definitely different and some of the components are the same on both.  Yet this type of natural (gravity) draft system is no less complicated.

The main components of a natural draft furnace are:

  • gas valve
  • thermocouple
  • pilot light assembly (standing pilot, spark igniter)
  • belt drive blower (fan)
  • blower motor
  • fan limit
  • electrical relay
  • burners
  • heat exchanger
  • low voltage transformer
  • draft diverter box

Before you start, and if you are comfortable in doing so, remove the upper door from the furnace.  You should now be able to see inside the compartment that has the gas valve, gas burners and burner compartment of the heat exchanger, gas manifold, pilot assembly, gas valve safeties and maybe some other components.  If at any time you decide that what you are doing is too much for your ability level, stop and call for a qualified professional service technician.

Follow these steps if your furnace doesn’t run at all:

  1. First, you need to check to make sure you have power (electricity) to the unit.  A very easy way to check this is by setting your thermostat to call for the furnace fan to run (turn the fan on/auto switch on the thermostat to on) then go to your furnace and listen for the fan running (some older furnaces are not wired properly for the thermostat “fan on” function so this may not work as a test).  If the fan does not turn on, check to see if you circuit breaker (should be marked as furnace) in your circuit panel is tripped.  If the circuit breaker is tripped, reset the breaker and see if the fan comes on.  There may also be a user switch on the side of your furnace and it looks like a light switch, make sure this switch is on.  If you do all this and still no power to the furnace, contact a heating and air conditioning professional to diagnose the problem.
  2. You also need to test the control transformer located inside of the furnace.  Before you test this component to see if it has line voltage (120 volts) to it and control voltage (24 volts) out of it, turn the user switch off.  Find the component in the furnace and get the wires to the transformer ready to be tested, then turn the user switch back on.  If you have both line voltage and control voltage to and from the transformer, follow the next step.
  3. Now you know if you have line voltage to the unit and low voltage to control components, you need to make sure you are getting control voltage to the thermostat and back to the furnace.  If you know how to separate the outer portion of the thermostat from the backplate, test the wire on the “R” terminal to see if you have 24 volts.  If you do, then turn the power off and take the wire that is on the “W” terminal and install it on the “R” terminal along with the wire that is already on the “R” terminal (you should have two wires on the “R” terminal and no wire on the “W” terminal).  Then turn the power to the furnace back on and if the furnace runs you have a bad thermostat.  Turn your furnace power back off and replace the thermostat.  If your furnace does not run, continue to the next step after you replace the wire to the “W” terminal.
  4. If you know you have power to the furnace but it still doesn’t work, the next thing that needs to be checked to see why your furnace is not operating is the pilot light.  If you removed the upper door of your furnace, you can now see if the pilot light is on.  Look at the gas valve and follow the metal tubing that goes from the gas valve to the pilot assembly.  Once you locate the pilot assembly, look to see if you can see a small flame.  Not seeing this flame indicates that your pilot light has gone out and so your furnace will not run.  This could be caused by several problems but may be able to be relit.  Before you start, turn the thermostat to the off position so if you fix any problems with the furnace, it will not start to operate before you are ready.  To relight a standing pilot, you have to have something that is long enough to reach where the pilot assembly actually is.  Once you have something to ignite the pilot again, turn the knob on to of the gas valve to the pilot position.  Now, carefully place whatever you are using to light the pilot next to the pilot assembly while it is lit, then press the knob on top of the gas valve down.  This action will send natural gas from the gas valve to the pilot assembly.  If the pilot lights, then continue to hold the knob on the gas valve down for about 30 seconds.  This allows the thermocouple to heat up and keep the pilot lit.  The thermocouple is the safety device that will not allow the gas valve to open if the gas sent to the the burners if the pilot lite out. The thermocouple requires no external power but produces very small amounts of power itself allowing it to function even without power to the furnace itself.  After you perform this task, and if the pilot stayed lit, turn the knob on top of the gas valve to on then try to turn your furnace to heating mode and see if it works.  If the furnace turns on, you have fixed the issue at hand, but ask yourself “why did the pilot light go out”?  If the pilot light will not stay lit, you may need to replace the thermocouple that controls the gas valve pilot or the gas supply to your furnace has been interrupted.
  5. Now that you have made sure your furnace has power to it, the has a pilot light is lit and you have the heat set for 85 degrees but the gas burners are not working, you may have a bad gas valve.  The gas valve is allowed to operate only after the thermocouple (gas valve safety device) proves that there is a pilot light to ignite the gas once the gas valve is opened.  You have now gotten to the point where you need to make a decision about how comfortable you are, and if you have a good electrical  multimeter, in testing for power inside the furnace (testing the electrical components is daunting for a lot of people, and if this is you, call for trained professionals to fix your equipment.  If you decide to continue, remember to be very careful while testing any electrical circuit, it can cause you great bodily harm if you accidently touch a live wire.  Now you need to understand two terms, control voltage & line voltage.  The line voltage is the power from the circuit panel to the furnace and is around 120 volts ac.  The control voltage is the power that comes out of the secondary winding side of the transformer and is around 24 volts ac.  The control voltage is used to turn off and on line voltage components along with some components to run on low voltage.  That being said, while testing this aspect of the furnace be safe and turn the user switch on the side of the furnace off till you get ready to test the circuit you are looking at.  Check for low voltage going to the gas valve, if you find that there is low voltage to the gas valve and you can’t smell any gas or see even a little flame, the valve is bad.  Finding no low voltage to the gas valve means that some other furnace component is not working.  Again you need to choose whether you are comfortable changing the gas valve yourself or calling for service.
  6. Now lets assume that the furnace has power (both control and line), the pilot light is lit and the gas valve is working but you still have no blower running.  Once the gas valve has started and the burners are lit, another control takes over.  Some natural (gravity) draft furnaces use a control that is called a “combination fan limit“.  This controls three different heating sequences of your furnace, fan on – gas off – fan off.  This control has a bi-metal probe that sits in the air stream between two cells of the heat exchanger to sense the temperature coming off the heat exchanger and airstream.  There is a metal disk inside and this will turn clockwise as the heat exchanger heats up and causes the closing of the appropriate circuit.  When the temperature reaches the temperature that the control is set for (usually around 120 – 130 degrees), it allows the fan to turn on.  After that the control turns the gas valve off if the sensed temperature gets too high (usually around 180-200 degrees), and then will turn it back on if the sensed temperature reaches the “fan on” setting.  Lastly, this control turns off the fan at a preset temperature after the gas valve turns off and the thermostat is satisfied.  This device controls both line voltage and control voltage components in the furnace and great care should be taken while testing this safety.  This component is located in the compartment that has the gas valve in it, above the gas valve and should look like a shinny metal rectangle box.  This is the cover and it needs to be removed.  Again, turn the user switch off before testing any component.  Remove the cover and you should see a round metal disk with numbers on it.  On the bottom of this metal disk there is a slot with three metal tabs.  The first tab shuts the fan off when the thermostat is satisfied, the second turns the fan on after the heat exchanger is sufficiently hot, and the third is the gas valve off.  If your gas burners light but your blower doesn’t come on, see if the disk turns past the second (fan on) tab after the burners have been lit for a minute or two.  If it does and you still have no fan running, test the line voltage side of the control for power into and out of the control.  If you find power in & out, you will have to check the motor (more on that later) and blower drive assembly.  If you have power into but not out of the control, and the disk moves past the second tab, you have a bad combination fan limit.  This control also controls the power to the gas valve and can cause the valve to not work.  If you have tested the power to the gas valve and got no voltage, this would be the control that sends control power to the valve.
  7. Lets say that now we have the burners burning and the combination fan limit seems to be working, but we still have no air coming out of the vents.  Again, turn the user switch off and now remove the lower furnace door.  You now can see the blower housing, the fan wheel, the blower motor and the belt & drive sheaves.   First make sure your belt is over both sheaves (pulleys) and tight enough to not slip.  Carefully see if you can turn the blower wheel, if not, you either have a frozen blower shaft (metal rod that runs through the blower wheel and has a sheave / pulley on one end), or frozen motor.  You can remove the drive belt and see if the fan and motor turn without a lot of effort and thus determine if you have a frozen blower shaft or frozen motor shaft.  If you can turn the blower with the drive assembly in tact continue the testing.  Again, call for heat and turn the user switch back on.  When the combination fan limit tells the fan to turn on, and nothing happens inside the blower compartment, and you have line voltage to the motor, you have a bad blower motor.  If you hear the motor try to turn and then shuts off (overload protection built into the motor does this), and the blower turned in the previous step, you have a bad blower motor or if there is a capacitor on the motor this could be bad.  Checking of the capacitor requires a meter that is made for this purpose.  You might be able to bring the capacitor to where ever you get parts for your furnace and have them test it.  If it proves to be good, change the blower motor with one that has the same horse power, voltage and rpm, along with the appropriate capacitor for the new motor.
  8. The last thing you should check is your flue.  Improper installation of the flue, birds nests and other issues can cause your flue gases to not draft up and out of your home.  Some homes that would have this type of natural (gravity) draft furnace will use a chimney that will be along side of the fireplace chimney.  Sometimes these chimneys need to heat up some before the natural (gravity) draft effect begins.  This is ok but over the years, debris, birds or other animals may try to nest in there and cause real problems.  In this case, you need to call for a professional to clean your chimney.  To test to see if your flue is drafting, light a piece of paper near the draft diverter box (located right under where the flue pipe connects to the furnace and is open facing out from the furnace) then slowly move it closer to the opening of the diverter box and see if the flame moves toward the diverter box.  If it does, then your flue is drafting properly, if it blows back at you, call for a professional to clean the chimney.  Non-drafting flues can cause your pilot light to be blown out and cause your furnace to malfunction.

This is just a guide on things to check and is not intended as a repair manual.  Gremillion Mechanical always tries to help our clients save money by doing some simple things themselves.  We don’t want our clients to have a terrible time trying to sleep on the first cold nights of the year.  However, we do not recommend the “do-it-yourself” approach when the problem is serious.   I am a professional heating and air conditioning contractor in Chicago Illinois. This is an article for someone who has at least some experience with repairs to look through. If you feel that you can not handle the things that have been described please call a heating and air conditioning contractor to diagnose your system properly.

Chicago Heating Repair
3526 S. Paulina St. ChicagoIL60609 USA 
 • 773-234-4575

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