gas furnace controls

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.


The sequence of operation of pilotless ignition

systems will either:

1. 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

2. light the main burners, prove they are lit and

keep operating.


There are two systems this power point

presentation will review. They are:

1. Hot Surface Ignition (HSI)

2. Direct Spark Ignition (DSI)


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.

1. Flame conductivity

2. 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 mainburners make contact with both flame probes.

Flame Conductivity

Control Module

In this situation, the flame will ‘conduct’ the ACfrom 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 lookslike in the rectification circuit. A lot of flow in one direction, but very little flow in the opposite direction.

Now onto

Hot Surface Ignition

(HSI)

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 components

found on this HSI system.

1. Main valve/Redundant valve

2. Flame Sensor Probe

3. Control Module

4. Hot Surface Ignitor

Main valve/Redundant valve

The main valve and redundant valve are actually 2

valves in one housing that are piped in series.

They both must be open for gas to flow to the

burners, but if either one does NOT open or

closes during operation, no more gas flows to the

main burners.


One valve would do the job, but because there are

two, it is considered ‘redundant.’ This feature is

for safety considerations.


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.


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 SensorProbe

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 ignitor 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

SiliconeCarbide

SiliconeNitride

Several Hot Surface Ignitor Configurations

Silicone Carbidewith 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.

aµa

µa meter inserted here

Now onto

Direct Spark Ignition

(DSI)

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

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.

MV PV


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.



1. Thermostat calls for heat.

2. 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.



3. Pilot gas flows to the pilot assembly and is lit

by the spark.

4. The pilot flame will then allow current to flow

across the spark gap from the high tension

cable through the GND wire.



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 or

2. 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 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: Fan and Limit

The End

gas furnace controls

Documents

flame probes

flame characteristics

manual ignition

size flame sensors

automatic ignition

flame conductivitythe

better flame sensing

pilot burner