gas furnace controls

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%).’

ON

PILOT

OFF

Knob to turnTip of knob indicates position

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.

Flame makes contact on 3/8 – 1/2 inch of the tip –

HOT JUNCTION

No flame on base – COLD JUNCTION

Connection to a BASO switchor 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. It

produced 750 mv under no load conditions.

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

Push thatdirection

Mechanicallinkage movesbutton to close

switch

Plunger moves pin up into solenoid

Thermocoupleconnection

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

Main manual shut-off valve

Pilot Valve

Main Valve

Regulator

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.

Gas Flow

Connection for tube going to pilot assembly

Example of a CGC valve.

Mechanism/button used to manually push-in the pilot

solenoid plunger.

Example of a CGC valve.

Pilot solenoid insidehousing at this location.

Thermocouple connectionto pilot solenoid.

Pilot Soleniod

electromagnetShaft 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.

Thermocouple connectionto pilot solenoid.

No separate mechanism used to

manually push-in the pilot solenoid plunger.

Here it is part of the valve knob assembly.

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

The End