Gas Furnace Controls

Part 3

Gas furnace controls – part 3 will review Group III of the

four groups of controls systems. The next slide will show

all four groups as a review.

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 group III.

III. Automatic Ignition with Standing Pilot

Automatic Ignition

Automatic Ignition means the pilot gas must be lit,

not with a match, but by some automatic method.

Automatic Ignition

There are two methods that have been used:

A. Glow coil

B. Ignitor

Standing Pilot, Thermocouple

and

Main Burner Shut-off

These items were reviewed in Part 2, so if you

need to review them, go back to part 2.

We will now move onto ‘Glow Coil.’

The ‘glow coil’ is simply a piece of wire that when

energized will get very hot and ‘glow.’ This is like

a toaster used to toast bread. If we place this ‘hot’

coil close to gas, it will ignite. So the source of

ignition for the pilot gas is this ‘hot’ glow coil.

2.5 voltglow coil

StandingPilotAssembly

Thermocouple

Baso Switch

A baso switch is used with the glow coil ignition

system. The difference between the baso switch

used in Group II and this Group III, is that when

the coil is energized with the mv from the

thermocouple, the relay coil will automatically

move the mechanical linkage.

Baso Switch24v to 2.5v stepdowntransformer for glow coil

glow coilconnection

thermocoupleconnection

LARGE coil that producesvery strong magnetic fieldto pull in relay.

Baso Switch Circuit

The baso switch circuit is found on page A6a of

your Student Handout Packet.’ For a description

of the sequence of operation, please refer to the

power point presentation titled ‘A6a.’

We will now move onto ‘Ignitor.’

The ‘Ignitor’ is simply a gap between two pieces of

metal, much like a spark plug. When a very high

voltage is applied across the gap, a spark will be

created. If this spark is close to gas, as it would

be in a pilot gas assembly, the pilot gas will ignite.

Spark Gap

Ignitor Circuit

The ignitor circuit is found on page A5 of your

Student Handout Packet.’ For a description of the

sequence of operation, please refer to the power

point presentation titled ‘A5.’

This concludes the information relating to ‘Group

III.’ Refer to your textbook for additional

information.

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: Gas Furnace Controls – Part 4

The End