Gas Furnace Controls

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Gas Furnace Controls

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Gas Furnace Controls

Part 4


Gas furnace controls – part 4 will review Group IV 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 IV.

IV. Pilotless Ignition Systems


Pilotless Ignition Systems

Pilotless Ignition Systems basically means that there is NO standing pilot.


Pilotless Ignition Systems

The sequence of operation of pilotless ignition systems will either:

light a pilot, prove the pilot is lit, open the main gas valve to the main burners and the pilot will light the gas and keep operating, or

light the main burners, prove they are lit and keep operating.


Pilotless Ignition Systems

There are two systems this power point presentation will review. They are:

Hot Surface Ignition (HSI)

Direct Spark Ignition (DSI)


Pilotless Ignition Systems

The two systems are represented in your Student Handout Packet. These two systems are found in the A9 section and the A10 section.


Before we take a look at these two pilotless ignition systems, we need to review two flame characteristics.

Flame conductivity

Flame rectification


Flame Conductivity

A physical property of a flame is that it will pass current through it. It will be a VERY SMALL amount of current.


Flame Conductivity

The way it works is that the control module sends out an AC voltage. The flame conducts current both ways with the same size flame sensors, so the control module will ‘see’ an AC input in response to the AC signal it sent out.


Flame Conductivity

Control Module

Flame Sensors


Flame Conductivity

Control Module

Gas from either the pilot burner or the main

burners make contact with both flame probes.


Flame Conductivity

Control Module

In this situation, the flame will ‘conduct’ the AC

from and back to the control module.


Flame Conductivity

Flame conductivity had some issues with false sensing and that is not good with gas equipment. So ‘flame conductivity’ is no longer used.


Flame Rectification

Flame rectification has replaced flame conductivity and is a better flame sensing system.


Flame Rectification

Control Module

Flame Sensors

Notice that there are still two sensors, but one is very large compared to the other one. The larger one is usually the metal housing of the furnace, so it is large compared to a pencil lead sized probe.


Flame Rectification

Control Module

Gas from either the pilot burner or the main burners makes contact with one probe. The other probe is the furnace housing, which is connected to the control module.

Furnace housing


Flame Rectification

Control Module

In this situation, the flame will ‘conduct’ AC from the control module, but due to the size difference of the probes, only a ‘rectified’ AC wave will return. So the rectified current will actually appear to be DC and the control module is designed to only sense this DC. So if it senses AC, it knows something is wrong and will shut down the furnace.


Flame Rectification

Control Module

This is what the sine wave looks

like in the rectification circuit. A lot of flow in one direction, but very little flow in the opposite direction.


Now onto

Hot Surface Ignition



Hot Surface Ignition (HSI)

A representation of this system is found in your Student Handout Packet, section A9.


Turn to page A9c to review the schematic,which is shown below.


A review of the following componentsfound on this HSI system.

Main valve/Redundant valve

Flame Sensor Probe

Control Module

Hot Surface



Main valve/Redundant valve

The main valve and redundant valve are actually 2 valves in one housing that are piped in series. They both


be open for gas to flow to the burners, but if either one does


open or closes during operation, no more gas flows to the main burners.


Main valve/Redundant valve

One valve would do the job, but because there are two, it is considered ‘redundant.’ This feature is for safety considerations.


Main valve/Redundant valve

Look at the schematic and notice how the two valves are electrically connected. They are wired in parallel. This means that both valves will get voltage at the same time and they will open together.


Main valve/Redundant valve

To review, the main valve and redundant valves are PIPED in series, but WIRED in parallel.


Flame Sensor Probe

The ‘flame sensor probe’ may also be known as the ‘flame probe’ or ‘flame rod.’ It is a piece of stainless steel that one end is inserted into the gas flame, and the other end is connected to the control module.


Flame Sensor Probe

Flame Sensor



Flame Sensor Probe

The ‘flame sensor probe’ function is to sense the flame. More on how it does this later.


Control Module

The ‘Control Module’ is the brains behind the operation. It starts the sequence of operation and will provide safety shutdown if it detects anything is wrong either during start-up or during the running time of the equipment. We will go over its sequence of operation later.


Hot Surface Ignitor

The hot surface


can be either silicone carbide or silicone nitride. When either device receives the voltage that it is rated at, it will get very hot and glow. This then becomes the high tech version of the older ‘glow coil’ described in a previous presentation.


Several Hot Surface Ignitor Configurations






Several Hot Surface Ignitor Configurations

Silicone Carbide

with a shieldfor protection


Now that we have gone through each of the components of an HSI system, we will go through the sequence of operation. This sequence is also available on page A9c of your Student Handout Packet.


HSI Sequence of Operation


Power is supplied to the furnace


Thermostat calls for heat,control module is energized


Control module starts the ignition sequence,silicone carbide ignitor is energized and gets ‘hot.’


After approximately 17 or 45 seconds, control module energizes terminals MV and MV, both gas valves are energized and gas flows to the burners.


Flame sensor senses flame and ‘rectifies’ the current. Proof of ignition has occurred.

Flame conducts current from sensor to ground of furnace housing



After proof of ignition has occurred,control module will de-energize HSI.


This completes the sequence of operation.


Look at the control module and the terminal identification for the valves. You see MV and MV.


MV and MV is abbreviated for Main Valve. What you don’t see is a PV for PILOT VALVE. This means that this control system does NOT have a separate pilot. The main burner gas is ignited and serves in place of the pilot gas. The control system proves ignition for the main burners.


Checking the flame circuits microamps (µa).


1. Remove the flame sensor wire from the control module.

Wire removed here


2. Insert your microamp meter, set for DC current, and read the µa. Check with manufacturer for µa value.



µa meter inserted here


Now onto

Direct Spark Ignition



Direct Spark Ignition (DSI)

A representation of this system is found in your Student Handout Packet, section A10.


Turn to page A10e to review the schematic,which is shown below.


You should observe the following about this circuit.


1. There is NO flame sensor identified. The ‘flame sensing loop’ is the ground (GND) wire plus the high voltage wire used for the spark.


2. When you want to check the flame rectification circuit, you would break the GND wire.


µa meter inserted here


3. Notice there is a ‘pilot burner.’ This means there is a pilot that is lit before the main burners.


4. Notice there are three terminals marked: MV, MV/PV, PV. This ignition system uses two valves in the same housing: pilot valve and main valve.


5. The MV/PV terminal would be ‘common’ to both valves as shown below.




Direct Spark Ignition (DSI)Sequence of operation

The sequence of operation and troubleshooting this circuit may be found in the Student Handout Packet A10 section. A brief version is found on the next several slides.


Direct Spark Ignition (DSI)Sequence of operation

Thermostat calls for heat.

The control module energizes the ignition transformer so that a high intensity spark is created at the pilot burner. The control module, at the same time, will energize terminals MV/PV to PV to open the pilot valve.


Direct Spark Ignition (DSI)Sequence of operation

Pilot gas flows to the pilot assembly and is lit by the spark.

The pilot flame will then allow current to flow across the spark gap from the high tension cable through the GND wire.


Direct Spark Ignition (DSI)Sequence of operation

5. The control module will sense ‘proof’ of pilot flame and will then energize terminals MV/PV to MV and allow the main valve to open to the main burners.


In conclusion, you can determine the sequence of operation of these two systems by looking at the schematic. If the schematic shows the control module with MV, MV/PV and PV, then there will be a pilot valve and a main valve. If the control module only has MV and MV, then there is no pilot valve.


The sequence of operation would be either:1. light pilot, prove pilot flame, then open main valve or2. light main burners, prove main burner flame.

In either sequence, there is NO standing pilot. Therefore it is a pilotless ignition system.


This concludes the information relating to ‘Group IV.’ Refer to your textbook for additional information.


Miscellaneous Information

(for tests)


When troubleshooting, what are the three circuitsa technician might have to troubleshoot?





What are the first three things you do when you arrive on the job?

ASK the customer what they observed.

Check that the system switch on the thermostat is properly set.

Set the thermostat to call for MAXIMUM heat.


Continue onto the next power point presentation in this series. Look for the title: Fan and Limit


The End





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