The problem:No heat.

The apprentice: One year on the job.

The technician: Highly qualified and experienced.

The diagnosis and repair sequence:

The technician has completed discussing the heating problem with the customer.

His conversation came up with the following facts:

  • The heat was out a month ago.
  • The apprentice diagnosed the problem as the gas furnace igniter and replaced it.
  • The heat was fine up until today, when it went out again.
  • The technician, being the mentor of the apprentice, asks a series of questions to make sure the apprentice properly diagnosed the original problem and that the repair was made properly.

    The apprentice explains that when he arrived on the occasion of the first call, the thermostat was calling for heat, and the home furnace was being fed power. Knowing a furnace ignition control may have a lockout due to non-ignition, the apprentice informed the technician that he cycled off the call for heat, waited five minutes, and then cycled on the heat. He observed that the igniter didn’t glow. Follow along with the conversation and test your own knowledge. Would you have known what to do in this situation?


    “Did you check the igniter voltage? What should the proper igniter voltage be?”


    “I checked the voltage. The proper voltage should be…”

    a) 24 Volts AC.

    b) 240 Volts AC.

    c) 120 Volts AC.

    d) 120 Volts DC.


    c)120 Volts AC.

    “That was a good first step,” advised the mentor. “Making sure the igniter is fed the proper voltage confirms that the ignition module is providing the right voltage and the problem usually isn’t the controller.”


    “When you physically checked the igniter, was it intact? And, by the way, what is the igniter called and what material is it made from?”

    Apprentice: “The Hot Surface Igniter (HSI) was cracked. It is made of…”

    a)Silicon carbide.

    b) Silicon carbon.

    c) Poly butane.

    d) Poly carbon.


    a)Silicon carbide.

    “Good,” said the mentor. “It’s important to know what the HSI is made of.”


    “What is the resistance of the HSI and why does it need that resistance?”


    “The HSI has…”

    a) High resistance.

    b) Low resistance.

    c) About the same resistance as the thermostat heat anticipator.

    d) No resistance.


    a)High resistance.


    “How hot is the HSI when it is glowing?”


    “HSI temperatures range from…”

    a) 800ºF to 1,000ºF

    b) 100ºF to 500ºF

    c) 500ºF to 800ºF

    d) 1,100ºF to 3,000ºF


    d1,100 degrees Farenheit to 3,000 degrees Farenheit.

    “Great again,” replied the technician. “You know it needs to be that hot so the gas/air mixture ignites.”

    Looking at the igniter, the team sees that it is broken again. They have a replacement with them and start the repair. The technician asks the apprentice to do the installation to see if he is doing anything wrong. He stops him just before he installs the new part. “What did I do wrong?” said the apprentice.


    “How should you handle the HSI?”


    “I’m not sure. I guess…”

    a) It doesn’t matter how it is handled.

    b) It must be handled carefully.

    c) It can be handled roughly.

    d) It can be handled just like a thermocouple.


    b)It must be handled carefully.

    “You’re right,” said the technician. “If you don’t handle it carefully or you get the grease or oils from your hands on the HSI, it can break or fail prematurely. Let’s make sure we have on gloves when we replace it. We have to be careful installing it. We’ll also check to make sure it is not short-cycling or getting a higher-than-normal voltage that could reduce the operational life of the part.”

    “I think I know what I did wrong last time,” said the apprentice. “I dropped the HSI when I did the first replacement. It must have been damaged, and I couldn’t see it. I’ll be more careful when I have to do this again. Thanks.”

    Publication date: 04/23/2001