Gas Furnace Controls

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

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.

1. How is the pilot gas ignited?

2. What is/are the safety device(s) that are used?

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.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.Look also in your textbook for this switch.

Pilot flame goes intwo directions. To left to heat the bimetal switch and to the right to lightthe main burners.

Bimetal switch

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:1. Manually light pilot.

2. 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?

1. Power

2. Control

3. Safety

What are the first three things you do when you

arrive on the job?

1. ASK the customer what they observed.

2. Check that the system switch on the

thermostat is properly set.

3. Set the thermostat to call for MAXIMUM heat.

Continue onto the next power point

presentation in this series. Look for

the title: Page A4

THE END