Gas Furnace Controls Part 1

If you were to look at all the controls used on gas furnaces over the years, the originator of this material believes they could be placed into four major groups. This can be especially useful when troubleshooting and trying to determine the control system you have on the furnace. Please refer to page A1 in your Student Handout Packet

The four groups are as follows: I. Manual Ignition with Standing Pilot and Pilot Heat Switch II. Manual Ignition with Standing Pilot and Thermocouple III. Automatic Ignition with Standing Pilot IV. Pilotless Ignition Systems

How is the pilot gas ignited? We will now look at the details of each of these groups and as we do, there will be two questions that will be answered regarding the control system to any gas furnace. How is the pilot gas ignited? What is/are the safety device(s) that are used?

Manual Ignition with Standing Pilot and Pilot Heat Switch – Group I Manual Ignition with Standing Pilot and Pilot Heat Switch – Main Burner Shut-off Only

Manual Ignition Manual Ignition means the pilot gas must be lit, generally with a match.

Here is an example of manual ignition Here is an example of manual ignition. This pilot requires someone to light it.

Standing Pilot Standing Pilot means the pilot is always lit and available to light the main burners.

Here is an example of a standing pilot.

Pilot Heat Switch Pilot Heat Switch is the safety device which proves the pilot is lit and available to light the main burners.

Here is an example of a pilot heat switch Here is an example of a pilot heat switch. Look also in your textbook for this switch. Bimetal switch Pilot flame goes in two directions. To left to heat the bimetal switch and to the right to light the main burners.

Turn to page A4 in the Student Handout Packet to see the use of this switch on the schematic.

The sequence of operation would be as follows: Manually light pilot.

When bimetal switch heats up, contacts will close.

3. When thermostat calls for heat, voltage will be able to get to the gas valve to energize it.

Main Burner Shut-off Only Main Burner Shut-off Only means that if the pilot should go out for any reason, the pilot gas will continue to flow, but the main burners will not be able to light. This will prevent the combustion chamber from filling with raw (unburned) gas.

Miscellaneous Information (for tests)

When troubleshooting, what are the three circuits a technician might have to troubleshoot? Power Control Safety

What are the first three things you do when you arrive on the job? ASK the customer what they observed. Check that the system switch on the thermostat is properly set. Set the thermostat to call for MAXIMUM heat.

Continue onto the next power point presentation in this series Continue onto the next power point presentation in this series. Look for the title: Page A4

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