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Gas Furnace Controls
Part 2
Gas furnace controls – part 1 reviewed Group I of the four
groups of controls systems. Part 2 will review Group II.
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 II.
II. Manual Ignition with Standing Pilot and
Thermocouple
Manual Ignition
Manual Ignition means the pilot gas
must be lit, generally with a match.
Standing Pilot
Standing Pilot means the pilot is
always lit and available to light the
main burners.
Thermocouple
The thermocouple is the safety device
which proves the pilot is lit and
available to light the main burners.
Here is an example of a thermocouple.
This group will have either:
A.
Main burner shut-off Only or
B.
Complete shut-off (100%)
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
up with raw (unburned) gas.
Complete Shut-off (100%)
Complete shut-off means that if the pilot
should go out for any reason, the gas valve
will not allow any gas to flow either to the
main burners or to the pilot burner, thus
100%.
Here is an example of a
‘Complete Shut-off (100%).’
This application is identified because
there are three positions on this valve:
OFF, PILOT and ON.
Here is an example of a
‘Complete Shut-off (100%).’
This ‘knob’ requires one to manually turn it.
Here is another example of a
‘Complete Shut-off (100%).’
PILOT
ON
Tip of knob indicates position
OFF
Knob to turn
Before we go any further with the
valves, we need to back up and
consider the thermocouple.
Thermocouple
The thermocouple is a device which converts
‘heat’ to electricity. The voltage is so small that it
is referred to as mv(millivolt). Thermocouple use
on furnaces is a thing of the past, but there are
many furnaces still operating that use the
thermocouple as the ‘flame proving device.’
Thermocouple
Even though the thermocouple is not being used
on today’s furnaces, the concept of changing heat
to electricity is sometimes applied when we use
our temperature testers that use probes.
Thermocouple
The thermocouple as it is applied to furnaces to
detect the pilot flame is composed of two different
metals.
Refer to your textbook for more
information on the metals used.
Thermocouple
The thermocouple, when heated on one end by a
flame, will have a ‘hot junction.’ The other end of
the thermocouple, which is not heated by a flame,
is the ‘cold junction.’ When the ‘hot junction’ is hot
and the ‘cold junction’ is cold, the result will be the
generation of a DC voltage.
Thermocouple
The DC voltage is then used by either a Baso
switch or a solenoid valve to allow gas to flow to
the pilot when voltage is present or close gas off
to the pilot if there is no voltage present, thereby
‘proving’ that there is a flame or no flame.
Thermocouple
Refer to your textbook for the value of the DC
voltages as well as the two conditions the
thermocouple will be producing voltage: load or
no load conditions.
Thermocouple
The thermocouple shown in the next slide is
published as producing a voltage of 25 – 30 mv
under no load conditions.
No flame on base –
COLD JUNCTION
Flame makes contact on
3/8 – 1/2 inch of the tip –
HOT JUNCTION
Connection to a BASO switch
or gas valve.
Thermocouple
The next slide will show a thermocouple that is
NOT part of this control system, but was used
previous to this control system. It is known as a
Powerpile, because it was 25 thermocouples in
series.
It was also known as a ‘matchbook’
because it looks like a book of matches.
produced 750 mv under no load conditions.
It
A ribbon of flame makes contact
with the thermocouple(s) along
the top so this becomes the HOT
JUNCTION.
Pilot gas enters here.
We will now move onto the two devices that will
use the thermocouple millivolts.
1. Baso Switch
2. CGC Valve
Baso Switch
A baso switch is a relay. It has three parts, just
like any relay.
It has a coil, contacts and
mechanical linkage.
Example of Baso Switch
Cover Removed
Another example of a Baso Switch
Cover Removed
Another example of a Baso Switch
Front
Baso Switch
A baso switch coil is designed to become
energized when millivolts is applied to it. Since
the millivolts are so low, the relay mechanical
linkage will not automatically work, so we have to
move the lever to close the contacts and they will
remain closed so long as millivolts are applied to
the coil.
Lever
Plunger moves pin
up into solenoid
Push that
direction
Thermocouple
connection
Mechanical
linkage moves
button to close
switch
CGC Valve
CGC is short for ‘combination gas control’ valve.
But what is it a combination of? Many years ago
there were three devices on a gas furnace piping
system:
pilot valve, main valve and regulator.
The CGC valve combines all three into one
housing.
The Original
Pilot
Valve
Main manual
shut-off valve
Regulator
Main
Valve
Gas flow, as shown here, was from left to right.
The ‘Pilot Valve’ was before the main valve, so if
pilot went out, pilot gas continued to flow.
The ‘Pilot Valve’ was known as the
‘B’ valve, for obvious reasons.
These two valves are made of brass. They are tapered and
are considered to be self sealing, although there is a gas
valve grease that can be used if they do not seal.
CGC Valve
The CGC valve design uses the millivolts from
the thermocouple to control a ‘solenoid’ valve
within the valve body. It controls the ‘pilot portion’
of the valve.
CGC Valve
When millivolts are present, the pilot solenoid is
open and gas flows through the pilot port to the
pilot assembly and to the main valve port. When
millivolts are not present, the pilot solenoid
closes its port and does not allow any gas to flow
to either the pilot assembly or main valve, thus the
name: complete shut-off, also known as 100%.
Example of a CGC valve.
Connection for tube going to
pilot assembly
Gas Flow
Example of a CGC valve.
Mechanism/button used to
manually push-in the pilot
solenoid plunger.
Example of a CGC valve.
Thermocouple connection
to pilot solenoid.
Pilot solenoid inside
housing at this location.
Pilot Soleniod
electromagnet
Shaft which is pulled
to the electromagnet
when energized.
Rubber flapper which
opens and closes the
pilot port in the CGC.
Another example of a CGC valve.
No separate
mechanism used to
manually push-in the
pilot solenoid plunger.
Here it is part of the
valve knob assembly.
Thermocouple connection
to pilot solenoid.
Sequence of Operation for this Group.
1. Turn gas CGC knob to the PILOT position.
2. DEPRESS either the knob or button so that you
manually open the pilot solenoid valve port. Gas flow
to pilot assembly.
3. Manually light the pilot and hold know for 60 seconds
for thermocouple tip to get hot – hot junction.
Sequence of Operation for this Group.
4. Release knob or button and pilot will stay lit because
the millivolts are present to energize the pilot solenoid
and keep the port open.
5. Turn CGC gas knob to ON position and the port to the
main burners is now open, so that when the
thermostat calls for heat, the main valve will be
energized and will open and allow gas to flow to the
main burners.
This concludes the information relating to ‘Group
II.’
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 3

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