The technician has completed discussing the heating problem with the customer. His conversation came up with the following facts:
Question 1:From this information, what will the technician most likely check to determine the problem?
a) Electric power source
b) Fan high limit
c) Thermostat anticipator
d) Gas valve/thermocouple
Since the homeowner has advised the tech that the pilot is going out and that normally relighting the pilot would bring on the heat again, the tech would know that the system has power, the high limit is still completing its safety circuit, and the thermostat is operating within normal ranges. Relighting the pilot could be an indication of a number of problems.
Question 2.What is the technician’s next step in the repair sequence (especially since s/he is highly qualified and not a parts changer)?
a) Check for control voltage to the gas valve when the thermostat is calling.
b) Check for thermocouple voltage with the pilot lit.
c) Check for thermocouple voltage with the pilot off.
d) Check for control voltage to the gas valve when the thermostat is not calling.
e) Any sequence will work.
Depending on the technician, s/he might perform any one of the checks above. The particular sequence of investigation will be the technician’s uniquely and will provide him/her with the necessary information to determine the problem.
Question 3.During the checking of the thermocouple, the technician gets a reading of 30 millivolts using a standard millivoltmeter. Is this reading within the acceptable range?
From the above test, the technician determines that the thermocouple should be good and therefore the pilot holding coil in the gas valve is defective. The gas valve is replaced. When the technician attempts to restart the system pilot, it will not stay lit.
Question 4.What did the technician miss, if anything?
a) Nothing; the new valve is faulty.
b) Thermocouple checking should also have been done under a loaded condition.
c) The old thermocouple was a different length, producing a different mV range.
d) Any check of a thermocouple will produce inconclusive data.
Testing of an electric device such as a thermocouple may need to be done under load conditions. The thermocouple generates millivolts due to the dissimilar metals used in the device. The flame impingement causes a current to flow. When the technician checked the thermocouple, s/he did so without a loaded condition. Current was flowing as the meter measured. However, the resistance of the millivolt meter is not the same as the resistance of the gas valve pilot solenoid holding coil. The thermocouple may be able to generate a proper millivoltage range when unloaded, but will have an excessive voltage drop when checked under load and therefore will not be able to hold open the pilot assembly.
Question 5.What step should the technician take next?
a) Remove the new gas valve, reinstall the original gas valve, install a new thermocouple, and check system operation.
b) Leave in the new gas valve, install a new thermocouple, and check system operation.
c) Leave in the new gas valve, install a used thermocouple, and check system operation.
d)Remove the new gas valve, install a used gas valve, install a new thermocouple, and check system operation.
Answer:a.This technician, being highly qualified and not wanting to be a parts changer, would remove the new gas valve, reinstall the original gas valve, install a new thermocouple, and check system operation.
Publication date: 03/26/2001