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 volt
glow coil
Thermocouple
Standing
Pilot
Assembly
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 Switch
24v to 2.5v stepdown
transformer for glow coil
glow coil
connection
LARGE coil that produces
very strong magnetic field
to pull in relay.
thermocouple
connection
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

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