Transcript Gas Furnace Controls - hvacr

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.
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:
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