Why won’t my furnace heat?

Why won’t my furnace heat?

If your furnace isn’t heating at all, start here:

  • Go to the control cabinet and turn the control power and the element power off.
  • Then turn first the control power and then the element power back on.
    Contactor
    Contactor  
    Control and element power switches on control cabinet
    Control and element power  switches

    You should hear a “clunk”. That’s the contactor (image right) allowing power to the system.

     

  • No clunk? The contactor is part of the over temperature protection system, which also includes a limit controller. If the limit controller isn’t functioning properly, or if the limit thermocouple which is the third part of this circuit is broken or not properly connected, the contactor can’t engage. To check, look at the limit controller display. Is it giving you a temperature reading, or is there an “error” message? Different limit controllers display different error messages; just be aware that if it’s not displaying a temperature it’s not functioning correctly. (Note however that if the furnace has been in an over temperature condition and the limit controller has done it’s job  and interrupted power to the elements, then you will see “exceed” or similar on the display. But that’s another problem.)
  • If you see an error message on the limit controller, don’t give up on it just yet. Check the connections to the limit thermocouple, both at the controller and at the thermocouple where it attaches to the thermocouple block on the outside of the furnace. If you don’t detect any problems, then proceed with the following.

The following troubleshooting instructions apply whether your furnace is not heating at all or lagging behind the setpoint.  You need to determine whether or not you have one or more broken elements. This is the most likely and most common cause of such problems.

If your furnace system is designed for single phase power, you might have, for example, just one series of six elements. If your furnace control power and element power are on and you are running a program, you should get a reading on your ammeter indicating that power is going to the elements. If you see all zeroes on the meter, chances are one or more elements in that single series are broken. If one goes out, the circuit is interrupted and no heating occurs.

You might instead have elements in series parallel; that is, two series of elements. In this case if an element goes out in one series, that series will be interrupted but the remaining series – with no broken elements – can and will continue to heat. In this case, the ammeter might have a lower-than-expected reading. For example, you might be accustomed to seeing 20 amps indicated at this particular segment of the program you are running, but this time the reading is ten amps. That could be because one of the two series of elements is out.

If your furnace system is designed to run on three phase power, you will have three series of elements. If one or more elements in one series is out, one of your ammeters might read, for example, 25 amps, while the others read 12 and 13 amps, respectively. Readings may vary because three phase power is somewhat complicated. What’s significant is that they aren’t all reading the same, which they would be if all the elements were intact.

Now it’s time to check for broken elements. While the titles are a bit misleading, the instructions for doing so can be found in our video tutorials entitled, “3-6mm Element Removal Tips” and “6-12mm Element Removal Tips”.

 

 

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