Gas/Propane Basics Archives | Indoor Comfort Marketing

Part 1 from the May/Jun 2024 Indoor Comfort

Part 2 from the Jul/Aug 2024 Indoor Comfort

Part 3 from the Sep/Oct 2024 Indoor Comfort

As we continue to look at modern systems found on equipment and the controls associated with them, we start to see a shift toward Universal Replacement Controls. This is a first step toward simplifying technician truck stock requirements, where one control will potentially replace several previously used on Forced Warm Air Systems.

Today’s modern electronic equipment incorporates the principles of Electronic Flame Detection. There are different applications determined by the equipment manufacturers’ design applications. Each of these systems has its own distinct set of advantages and problems. The ST9120U-1003 is the forerunner to the ST9120U-1011.

Honeywell is not alone in this trend of moving toward Universal Replacement Controls. White Rodgers has also taken this direction with their controls.

As Industry moves toward the modulation condensing class of equipment (often abbreviated to mod-cons), this is the last step in replacement for some of the older systems still out it in the field today. Those systems do not require complete replacement; often just a control replacement will suffice.

ST9120U Universal Electronic Fan Timers
The first step toward universal replacement Electronic Fan Timers was the ST9120U-1003 (there is also a newer version, the ST9120U-1011). Carrying this product means a reduction in truck stock and, once the technician gets accustomed to using it, a reduction in replacement time. The ST9120U Universal Electronic Fan Timers integrate controls of all combustion blower and circulating fan operations in a gas warm air appliance. This control is the central wiring point for most of the electrical components in the furnace. The basic purposes of the ST9120U are to monitor the thermostat for heat, cool and fan demands, run the induced draft blower motor and run a circulating fan (up to two speeds) as required. The electronic fan timers also monitor limit switch strings and energize separate ignition control systems through SPST pressure switches.

The ST9101 to ST9120U wiring conversion is illustrated in Table 1. The ST9120U can replace any ST9101 or ST9120 listed in Table 2, which also gives the switch settings for the unit being replaced to ensure compatibility and lists the particular original equipment manufacturer (OEM). The electronic fan timers feature a field-adjustable heat “Fan-On” delay, a field-adjustable heat “Fan-Off” delay, a field-adjustable cool “Fan-On” delay and a fixed cool “Fan-Off” delay.

Electronic air cleaner (EAC) and humidifier (HUM) convenience terminal connections are provided as an option. Continuous low speed indoor air circulation is also provided.

See Figure 2 to adjust timings. Figure 3 shows the wiring adapter to be used to connect the ST9120U to the existing harness on the board being replaced (there will be further instructions on this). P1 is the nine-pin connector, which will have the connector off the board being replaced, plugged into it.

Figure 4 shows the harness connected to the ST9120U board; to save time, the technician can pre-wire this. The portion of the harness at the top of Figure 4 is what connects together with the harness being removed from the old board.

ST9120 to ST9120U Wiring Conversion Instructions
1. Turn off power to appliance. Carefully remove each wire and connect directly to the corresponding location on the new ST9120U control board. Be careful to directly connect to the new terminal with the same-labeled identity or label each wire prior to removing from the original board. If original thermostat connection was a quick connect, cut off quick connect, strip wire and insert into terminal block.

2. Identify the model number of the board being replaced and set the dip switches based on Table 2.

ST9101 to ST9120U Wiring Conversion Instructions
1. Turn off power to appliance and carefully remove each wire and connect directly to the new ST9120U control board before removing the existing control. Use Table 1 to determine the proper ST9120U terminal for each corresponding ST9101 connection. Be careful to directly connect to the new terminal or label each wire prior to removal from original board.

2. Unplug the nine-pin connector and plug existing wiring harness directly into the nine-pin connector found on the ST9101 wiring adapter.

3. Plug the six-pin connector found on the ST9101 wiring adapter directly to the open six-pin connector on the new ST9120U control board.

4. Take the two remaining white wires on the ST9101 wiring adapter and connect to “Neutral” quick connect terminals three and four on the ST9120U.

5. Take the black wire on the ST9101 wiring adapter and connect to the S1 quick connect terminal on the ST9120U.

6. Take the blue wire on the ST9101 wiring adapter and connect to the D1 quick connect terminal on the ST9120U

7. Identify the model number of the board being replaced and set the dip switches based on Table 2.

Table 3 gives the wire adapter connections and the wire adapter schematic. This can be compared with Figure 5, the wiring diagram showing the connections from the six-pin connector and the nine-pin connector along with the color of the wires.

Figure 6 is a breakdown of the ST9120U; on the left hand side is the twinning connections “Z1” and “Z2”. There is also a five-amp fuse located next to six-pin connector CN1. Relay K1 is for the Circulating Fan, K2 is for Heat/Cool and K3 is for the Combustion Air Blower.

Setting Adjustable Heat Fan DIP Switches
Set the heat “Fan-On” delay and cool “Fan-On”delay DIP switches as shown in Figure 2. The “Heat-On”delay time switch #4 (30 or 60 seconds) starts when the main gas valve is energized at the start of a thermostat call for heat; the “Heat-Off”delay time switches #3 and #4 (60, 100, 140 and 180 seconds) starts when the main gas valve is de-energized at the end of a thermostat call for heat. The cool “Fan-On”delay time switch #1 (four or 30 seconds) begins when the compressor is energized at the start of a thermostat call for cool.

Table 4 is the operating sequence for the ST9120U. ICM

Timmie M. McElwain is President of Gas Appliance Service, which provides training for those servicing gas combustion equipment. He is a certified instructor and test proctor for the Propane Gas Association in their CETP program.

Part 1 from the May/Jun 2024 Indoor Comfort

Part 2 from the Jul/Aug 2024 Indoor Comfort

Different equipment manufacturers may have different ways of applying these controls—from intermittent pilot application to direct spark ignition and including hot surface. It is important to understand how the controls operate and not jump to conclusions without going through the operating procedures. As we continue our series on Advanced Controls for gas heating, we are going to address two-stage heating.

As many of you know, we have closed the Gas Training Institute. The eight manuals that we have used in our one-week course are still being printed and are available for purchase, and I am available for classroom training.

Honeywell ST9162 Two-Stage Electronic Fan Timers
The ST9162 Two-Stage Electronic Fan Timers integrate control of all the circulating fan operations in a two-stage gas warm air furnace. These controls are the central wiring point for most of the electrical components of the appliance. The ST9162 monitors the thermostat for heat, cool and fan demands and controls a multi-speed circulating fan as required. Communication with the SmartValve System Control initiates appliance lightoff.

The ST9162 connects to either a single-stage or two-stage thermostat. For both thermostats, it provides sequencing of two heat stages in a two-stage gas warm air appliance.

The ST9162A features a field-adjustable heat fan-on delay and a field-adjustable heat fan-off delay. Cooling fan-on and -off delay is fixed. Specific timings vary. See appliance label or instructions for timings that are available for a specific appliance.

A single-stage thermostat mode with a fixed, 12-minute, second stage delay is field-selectable. Electronic air cleaner (EAC) and humidifier (HUM) convenience terminal connections can be provided as an option. Continuous low speed indoor air circulation is also available as an option. Thermostat connections are screw terminals.

Figure 2 shows the dipswitch location and also the Y2 connection, which is for humidification.

The ST9162 can be set up for a two-stage or a single-stage thermostat as shown by the settings on dip switch 4 in Figure 3 (seen in the lower left hand corner).

In addition, Figure 3 shows the different settings for dip switches. Switch 3 is for heat fan-on delay. Switch 1 and switch 2 are for the heat fan- off delay of 60, 100, 140 or 180 seconds.
Figure 4 shows the location of connections and switches on the circuit board. Figure 5 is the diagram for the SmartValve two-stage valve with the ST9162. ICM

Part 1 from the May/Jun 2024 Indoor Comfort

As we continue looking at some of the more advanced electronic controls used on gas heating, and particularly the more advanced high efficiency equipment, it is important to understand how the controls are applied. The majority of new equipment today involves some form of electronics, as well as the principal of electronic flame detection and the use of flame rectification as a safety and flame proving system.

Different equipment manufacturers may have different ways of applying these controls—from intermittent pilot application to direct spark ignition and including hot surface. It is important to understand how the controls operate and not jump to conclusions without going through the operating procedures. This article continues our series on Advanced Controls for Gas Heating.

On a personal note, I want to take this opportunity to let everyone know that after 30 years in business, we have closed the Gas Training Institute. The eight manuals used in our one-week course are still being printed and are available for purchase.

Honeywell ST9160A-C Electronic Fan Timers
The ST9160A-C Electronic Fan Timers integrate control of all circulating fan operations in a gas warm air furnace. This control is the central wiring point for most of the electrical components in the furnace. The ST9160 monitors the thermostat for heat, cool and fan demands, and controls a two-speed circulating fan as required. Communication with the SmartValve System Control initiates appliance light OFF. The ST9160 features a fixed or field-adjustable heat fan ON delay and a fixed or field-adjustable heat fan OFF delay, depending on model. Cooling fan ON and OFF delay is fixed; specific timings vary. See the label or instructions for timings that are available for a specific appliance. Refer to Figure 2 for model specifications.

Electronic air cleaner (EAC) and humidifier (HUM) convenience terminal connections can be provided as an option, as can continuous low speed indoor air circulation. Thermostat connections are screw terminals or quick-connects.

The ST9160A-C Electronic Fan Timers are used exclusively with Generation III SmartValves SV9510/SV9520 and SV9540/SV9640.

Figure 2 covers the settings of the dip switches on the various models. The ST9160A is a fixed setting all the way. The ST9160B has an adjustable OFF delay, giving four different settings. The ST9160C has two adjustable heat ON delays—30 and 60 seconds, set on switch three. The four adjustable OFF settings are 60, 100, 140 and 180 seconds.

Figure 3 shows the component parts of the ST9160 EFT. Of particular interest is the twinning connection shown on the left hand side just below the R12 resistor. There is also a separate connection for the DATA signal from the SmartValve.

The diagram in Figure 4 shows the ST9160 with the SV9540 or SV9640 SmartValves, which use the Q3450 24 volt igniter. The Figure 5 illustration shows the ST9160 with the SV9510 or SV9610 series or the SV9520 or SV9620, which used 120-volt igniters.

Figure 6 shows the connections from the ST9160 to the SmartValve. The red dotted line is the connection that is different for the SV9540 and SV9640 series. ICM

The majority of new equipment today involves some form of electronics, as well as the principal of electronic flame detection and the use of flame rectification as a safety and flame proving system. This is the case with both Forced Warm Air systems and Forced Hot Water systems.

There are, however, different ways it is applied—from intermittent pilot application to direct spark ignition and including hot surface ignition. Each of these systems has its own distinct advantages as well as some disadvantages. It can be easy to jump to conclusions and just keep changing parts until perhaps you fix the problem, but that can be time-consuming and expensive.

The controls that are used on equipment today are unique to a particular facet of operation. Understanding their operation and troubleshooting them is covered in this article on the Honeywell ST 9141A control, which is used on Gas Forced Warm Air systems.

I invite you to visit our Facebook page, Timmie’s Tips on Gas. I look forward to seeing you there.

Honeywell ST 9141A control
For use on gas forced warm air systems

The ST9141A-1002 Electronic Fan Timer integrates control of all combustion blower and circulating fan operations in a gas warm air furnace. This control is the central wiring point for most of the electrical components in the furnace. The basic purposes of the ST9141A-1002 are to monitor the thermostat for heat, cool and fan demands, and run the induced draft blower motor up to a two-speed circulating fan as required. The ST9141A-1002 monitors an SPDT pressure switch, burner limit and primary limit.

A Honeywell SV9500 Hot Surface Pilot Ignition System Control manages the burner, which is energized through the pressure switch. A light-emitting diode (LED) indicates system status.

In addition, this model includes electronic air cleaner and humidifier convenience terminal connections.

Note: Use this control to replace an identical ST9141A-1002 Electronic Fan Timer only. This control is not for use in general applications.

The difference between the ST9120 series and the ST9141 is that the LED status indicator was added to the left of the two dipswitches.

ST9141 is similar to ST9120:

• Heat fan

Ø ON delay 30 seconds
Ø OFF delay adjustable (60, 100, 140, 180 seconds)

• Cool fan

Ø ON delay six seconds
Ø OFF delay 60 seconds

Light emitting diode

• Pulsing dim/bright—normal operation
• Off with control powered—defective control
• No pulsing dim/bright when powered—defective control
• Flashing

Ø One flash: burner limit open
Ø Two flashes: primary limit open
Ø Three flashes: pressure switch did not recycle during off cycle
Ø Four flashes: control in lockout due to overheating (limit open for more than 150 seconds on each of three consecutive cycles)

Figure 2 shows the two dipswitches for Fan Off time setting. The Fan On time is fixed at 30 seconds. Figure 3 expands on the LED status light diagnostics. This along with the operating sequence and the diagrams to follow will make troubleshooting much easier.

Figures 4, 5 and 6 show both the internal and wiring connections for the ST9141. Figure 6 shows an expanded view of the nine pin CN1 connector. That connector is also identified in Figure 4 and Figure 5. ICM

Part 1, from the Nov/Dec 2023 Indoor Comfort
Part 2, from the Jan/Feb 2024 Indoor Comfort

We continue our discussion of modern heating systems and the advanced electronic controls associated with furnaces and boilers. It is always important to understand the basic fundamentals associated with these systems.

The majority of our equipment today uses electronic controls as well as the principal of electronic flame detection and flame rectification as a safety and flame-proving system. This is the case with both Forced Warm Air Systems and Forced Hot Water Systems.

There are, however, different ways it is applied—from intermittent pilot application to direct spark ignition and including hot surface ignition. Each of these systems has its own distinct advantages and problems. Next in this series of resolving burner issues related to these systems, we offer corrections and diagnostics. It can be easy to jump to conclusions and change parts until you hopefully solve the problem. That is, however, time consuming and costly.

In this series, Advanced Controls for Gas Heating, we will continue to break down and examine individual controls, working toward Universal Replacement Controls—one control that replaces many others, making servicing these systems much easier—as well as simplifying truck stock.

I invite you to visit our Facebook page, Timmie’s Tips on Gas, I look forward to seeing you there.

Electronic Furnace & Boiler Controls
The Honeywell ST9120A-H/ST9150B Electronic Fan Timers integrate control of all combustion blower and circulating fan operations in a gas warm air system. This control is the central wiring point for most of the electrical components in the furnace. The basic purposes of the ST9120/ST9150 are to monitor the thermostat for heat, cool and fan demands; run the induced draft blower motor; and run a circulating fan (up to two speeds), as required. The electronic fan timers also monitor limit switch strings and energize separate ignition control systems through Single Pole Single Throw (SPST) pressure switches. The electronic fan timers feature either a fixed or a field-adjustable heat fan-on delay, a fixed or field-adjustable heat fan-off delay and a fixed or field adjustable cool fan-on delay, depending on the model.

Cooling fan on/off delay, extended combustion air blower postpurge, and an enclosure are available on selected models as noted in Figure 2. Electronic air cleaner (EAC) and humidifier (HUM) convenience terminal connections can be provided as an option. Continuous low speed indoor air circulation is also available as an option.

Figure 3 shows the various functions of this Electronic Fan Timer, which was the next step after the ST9101 series of EFTs with more control functions.

Combustion air blower

• ON with call for heat

• OFF with end of call for heat + post-purge timing (typically five seconds)

• ON with limit open

System (indoor) fan

• Heat

■ ON delay (30 or 60 seconds) starts when main valve is energized

■ OFF delay (adjustable: 60, 100, 140, 180 seconds) starts when call for heat ends

■ ON two seconds after a manual call for fan operation (thermostat sub-base switched to ON).

• Cool

■ ON delay six seconds after call for cooling starts

■ OFF delay on some models

• Continuous circulation fan

■ ON all the time when the fan is not running at the heating or cooling speed

• EAC is ON with heat or cool fan (hot wire with continuous fan)

• Humidifier is on when combustion air blower is running.

The electronic air cleaner and humidifier terminals require the knockouts to be broken away in order to connect. This is illustrated in Figure 4.

Function depends on the number of dip switches on the different versions of the ST9120. The units with only two dip switches (Figure 5) allow for heat fan-off delay only (times 60, 90, 120 and 150). The fan on time would be fixed at either 30 or 60 seconds. The units with three dip switches (Figure 6) allow heat on delay using Switch #3 set to 30 or 60 seconds, Switches #1 and #2 (times 60, 100, 140 and 180). The four dip switch units (Figure 7) allow Switch #1 to be set for four or 30 seconds cool on delay, Switch #4 heat on delay of 30 or 60 seconds and Switches #2 and #3 allow heat off delay of (times 60, 100, 140 and 180).

For a complete system diagram see Figure 8.

S8600 Module
Figure 9 shows the CN1 6 pin connector terminal numbers and what they are attached to. ICM

Part 1, from the Nov/Dec 2023 Indoor Comfort

As the gas industry has progressed, so have the requirements for electronic controls.

There are many setups that use rather sophisticated electronic controls to accomplish many of the functions required on systems today. Almost everything in modern-day homes uses some kind of microprocessor-type control.

The mandates for efficiency have pretty much driven the industry into the electronic age. We could probably still do some of the things we do with electromechanical controls, but it would be with great difficulty. These functions are much easier with microprocessor-type controls.

We will be discussing electronic controls used on furnaces and boilers. On the furnace side, we have electronic fan timers (EFTs) and integrated controls. On the boiler side, we have multi-zone panels and, specifically, integrated boiler controls.

Electronic Fan Timers
Electronic fan timers have been around a while. The ST9101 EFT is used on some standing pilot furnaces and the ST9103 is used on oil furnaces. The ST9103 EFT integrates control of burner and circulating fan operation in an oil furnace. The ST9101A; ST9120A-C, G; ST9141A integrates control of the combustion blower and circulating fan operation in gas warm air furnaces. The use of twinning on warm air systems today is also addressed by the twinning features on these controls. It is important to realize that these controls are configured for application to specific appliance models. They are intended for direct replacement of Original Equipment Manufacturer (OEM)-installed controls only as noted in the order table. Do not attempt to install these controls except as direct replacements for the specific Honeywell models noted in the order table.

We will also subsequently address the use of Universal Replacement Controls ST9120U-1003 and ST9120U-1011.

Figure 1 shows the ST9101A controls and the particular OEM they were used by. Later, as we address universal replacement controls, one can see that many of these are replaceable by a universal application.

The ST9101A EFT integrates control of all combustion blower and circulating fan operations for a gas warm air appliance. The basic purposes of the ST9101A are to monitor the thermostat and run a combustion blower with a two-speed circulating fan. The ST9101A monitors the thermostat for heat, cool and fan demands. The ST9101A also monitors a limit switch string, energizing the circulating fan when any of the limits open.

The basic ST9101 EFT is used principally on Rheem standing pilot furnaces with induced draft blowers.

• Fan control delays

Heat ON: 20 seconds

Heat OFF: 90, 120, 150, 180 seconds (field adjustable)

Cool ON/OFF: no delays (switches cooling fan ON and OFF with a call for cooling)

Fan switching rated for 12 amps/120 volts

HEAT fan energized when limit opens

• Induced draft blower

ON and OFF with the thermostat call for heat 2 amps/120 volts

Setting Fan-Off Delay

Usually the fan-ON time is fixed at either 30 or 60 seconds to meet the needs of a particular furnace, but the fan-OFF timing is adjustable to meet the needs of a particular application. This control from Honeywell is a set of screws that must be adjusted, as shown in Figure 2. The screw heads make contact between two wires to create an input to the electronic circuits.

Some models use a different EFT, such as a Robertshaw Model RBC-1 shown in Figure 3, as the adjustment for blower off delay using the jumper on the control board to the pins desired.

The operating sequence for the ST9101A is shown in Figure 4.

In the diagrams that follow, we will go through the sequence. In this step-by-step process, the pattern from various pins on the 9-pin connector labeled CN23 through the printed circuit board soldered connections will be followed. The schematic diagram shows those as dotted lines. The diagram fails to show the three relays used, so we will point them out as we trace the sequence. Figure 5 shows the various connections including the three relays labeled K1, K2 and K3. Figure 6 shows the CN23 pin numbers.

The schematic shown in the following drawings are a good example of a typical circuit used with EFTs.

The initial circuit is the 120 VAC circuit from L through the interlock applying 120 VAC to S1 to N2 the transformer primary. This puts 24 VAC on the secondary side of the transformer, which is an external transformer. What actually happens is, out of the interlock, 120 VAC goes to S-3/CN23 pin 6 to N4/CN23 pin 2. From pin 6, 120 VAC goes through the printed circuit (dotted lines) to S1 then an external wire to the transformer then to N2 and back through the printed circuit board wiring to CN23 pin 2/N4 back to N (neutral). This is shown in Figure 7.

When there is a call for heat, as shown in Figure 8, from the thermostat “R” to “W” the circuit is from the transformer to “X” though the printed circuit to CN23 pin 5. Then to the auxiliary limit, blocked vent switch, rollout switch and main limit to CN23 pin 3 to the pressure switch. The pressure switch is in the normally open (NO) position and will not close until relay K3 is energized. When K3 is energized, it will cause the Combustion Air Blower to come on (120 VAC) DI CN23 pin 9 to N3 CN23 pin 1, this will pull in the pressure switch.

Figure 9 shows the pressure switch (now closed); the gas valve is energized H to C CN23 pin 7 back to “C” (common) on the transformer.

Figure 9

Figure 10 shows that there is also 24 volts fed to W1 CN23 pin 8, which starts the fan on delay timer to bring on the fan by energizing relay K1. This is turn causes switch K1 to close putting 120 VAC through relay switch K2 (set by putting the thermostat Heat/Cool switch on Heat).

Figure 10

The system is now up and running. Figure 11 shows when the thermostat is satisfied “R” to “W” breaks de-energizing relay K3; this will shut down the Combustion Air Blower and the pressure switch will open. This will cause the gas valve to shut down and the fan-off delay will start as pre-set on the A & B screws for time.

Figure 11

When the fan-off delay time is complete, the system will be totally shut down and waiting for the next call for heat.

Understanding this entire sequence can make the job of troubleshooting these systems much simpler. ICM

As we look at modern systems and what specific problems they present, it’s important to understand the basic fundamentals associated with these systems.

Most of our modern heating equipment in some way or another involves electronics; along with the use of electronics is the use of flame rectification as a safety and flame-proving system. It doesn’t matter if it is a forced warm air furnace or a forced hot water boiler; the same basic system is used to perform safe ignition, and then consistent operation, throughout the entire call for heat.

There are, however, different ways it is applied—from intermittent pilot application to direct spark ignition and including hot surface ignition. Each system has its own distinct advantages and problems. Next in this series of resolving burner issues related to these systems, we offer corrections and diagnostics. It can be easy to jump to conclusions and change parts until you hopefully solve the problem. That is, however, time-consuming and costly.

I invite you to visit our Facebook page, Timmie’s Tips On Gas, I look forward to seeing you there.

We are starting a new series titled Advanced Controls for Gas Heating. In future columns, we will break down and examine individual controls, leading up to universal replacement controls—one control that replaces many others, making servicing these systems much easier, as well as simplifying truck stock.

Electronic Furnace & Boiler Controls
I have observed the gas industry since I began working in the 1960s until the present day and we have come a long way. Gas company technicians back then were using test lights instead of meters and there were no electronics to speak of. My background was in electronics in the Navy, and I owned my own multimeter and was quite capable of using it correctly to diagnose and troubleshoot the rather simple gas system circuits.

The slow addition of electronics into gas systems has surely taken off today. There are currently numerous systems, using rather sophisticated electronic controls, to operate many of the required functions. Almost everything in homes today uses some kind of microprocessor control. The mandates for efficiency have driven the plumbing, heating and HVAC industry into the electronic age. While we could probably still do some of the things we do with electromechanical controls, it would be with great difficulty. These functions are much easier with microprocessor-type controls.

Let’s discuss electronic controls used on furnaces and boilers. On the furnace side, we have Electronic Fan Timers and Integrated Furnace Controls. On the boiler side, we have multi-zone panels and (to be covered in a future column) Integrated Boiler Controls.

The traditional fan-limit control for warm air furnace:

• Has both functions combined into one device (L4064)
• Is difficult to apply in horizontal and counterflow furnaces, and especially in multipoise furnaces.

Ø Location for the fan control won’t work for the limit and vice versa
Ø Two separate controls cost too much

• Even though it perhaps has made controlling duct temperatures easier, due to its ability to be adjusted for ON–OFF temperature, it was just not adaptable to modern furnaces.
• Time assist fan-limit control (L4064T, E)

Ø Fan turns on after 90 seconds whether the sensor feels heat or not
Ø Limit control works as usual

Time-only fan control:
• The solution is to abandon temperature altogether as the basis for switching the fan and go to time only
• Bimetal timers have been available for a long time to do this job.

Ø S876 is still in the Honeywell product line and is a good solution to fan switching problems
Ø There are still Carrier/Bryant/Day Night Payne furnaces that use a bimetal control

Electronic fan control:
• The ability to design and build reliable electronic fan controls made a substantial change in the appearance of warm air furnaces:

Ø Electronic, and later digital, controls resulted in very precise fan-on and fan-off timing. We could have exactly the right timing for each furnace design. On delays and off delays could be specified exactly.
Ø Application became much easier because the issue of locating the fan sensor was completely gone.
Ø By switching the fan at exactly the right time, there are some small, but measurable, improvements in efficiency.
Ø Because of the increased efficiencies, the protection of the heat exchanger due to thermal stress was also a factor in precise timing, in particular the fan-on times.

Electronic Fan Timers
Electronic fan timers have been around for a while. The ST9101 is used on some standing pilot furnaces and the ST9103 is used on oil furnaces. The ST9103 integrates control of burner and circulating fan operation in an oil furnace. The ST9101A; ST9120A-C, G; and ST9141A integrate control of the combustion blower and circulating fan operation in warm air gas furnaces.

The use of twinning on warm air systems today is also addressed by twinning features on these controls. It is important to realize that these controls are configured for application to specific appliance models. They are intended for direct replacement of OEM installed controls only; as noted in Figure 3 below (do not attempt to apply these controls except as direct replacement for specific Honeywell models noted).

Later in this series, we will address the use of universal replacement controls on both the ST9120U-1003 and the newest version, the ST9120U-1011. As the systems grow older, the development of universal replacement controls makes the service techs’ job a lot easier.

Figure 3 shows the controls and the particular original equipment manufacturer (OEM) that used them. As we address universal replacement controls in future columns, you will see that many of these are replaceable by a universal application. ICM

Part 14, from the Sep/Oct 2023 Indoor Comfort.

Parts 1 & 2, from the Jul/Aug and Sep/Oct 2021 Indoor Comfort can be found here.

Part 3, from the Nov/Dec 2021 Indoor Comfort can be found here.

Part 4, from the Jan/Feb 2022 Indoor Comfort can be found here.

Part 5, from the Mar/Apr 2022 Indoor Comfort can be found here.

Part 6, from the May/Jun 2022 Indoor Comfort can be found here.

Part 7, from the Jul/Aug 2022 Indoor Comfort can be found here.

Part 8, from the Sep/Oct 2022 Indoor Comfort can be found here.

Part 9, from the Nov/Dec 2022 Indoor Comfort can be found here.

Part 10, from the Jan/Feb 2023 Indoor Comfort can be found here.

Part 11, from the Mar/Apr 2023 Indoor Comfort can be found here.

Part 12, from the May/Jun 2023 Indoor Comfort can be found here.

Part 13, from the Jul/Aug 2023 Indoor Comfort can be found here.

As we look at modern systems and what specific problems they present, it’s important to understand the basic fundamentals associated with these systems.

We will start with the basics and go into operation, typical problems, diagnosis, troubleshooting procedures and a final solution to a particular problem. It can be easy to jump to conclusions and change parts until you hopefully solve the problem. That is, however, time-consuming and costly.

I invite you to visit our Facebook page, Timmie’s Tips On Gas, I look forward to seeing you there.

We are presently doing a series on Honeywell Smart Valve and will continue to cover Smart Valve Generation III, with step-by-step troubleshooting of these controls.

Gas Valve/Induced Draft Fan/Pressure Switches
The Gas Valve is connected to the combustion chamber through a series of tubing connections, which include the Induced Draft Fan, the Low Fire Pressure Switch and the High Fire Pressure Switch.

The gas valve has a knurled fitting at what would be the normal regulator vent to atmosphere. The knurled fitting is for the tubing to fit over; the valve is then connected to the combustion chamber so that the valve senses, not room pressure, but combustion chamber pressure. The two pressure switches are set up to be differential switches, which must sense a certain differential pressure in order for them to close. The differential pressure for the Low Fire Switch is .58″ W.C. and the High Fire Switch is 1.38″ W.C.

Looking at the diagram shown in Figure 1, when the initial W1 call for heat takes place, the SmartValve sends 120 VAC from P1-4 to C3-2 then from C3-1 on the Inducer In to Ind In on the EFT. Then 120 VAC is sent from Ind High on the EFT to the inducer, which comes up to high speed. This creates sufficient differential pressure to cause both Low Fire and High Fire pressure switches to close. The High Fire pressure switch will then pass DC voltage to the two-stage (black) solenoid, which will cause it to increase the gas pressure in order to reach High Fire setting. The complete tubing connection set-up is shown in Figure 1. The DC voltages expected are illustrated in Figure 3.

Figure 1 also shows the wiring diagram for this system. Prior to the main burner turning on, a three-second pre-purge will occur; then the igniter will light the pilot and the flame will be proved from the sensor. If the microamp signal is 1.5 microamps or greater, then the valve will be energized through the limit switch, rollout switch and low-pressure switch. The burner will turn on at High Fire setting and run until the delay is satisfied; the system will then shut down the DC signal to the solenoid and the system will go to Low Fire. The inducer will be running at low speed and the DATA signal from the SmartValve will be sent to the ST9162 EFT board and the fan [on delay] will start. In 30 seconds, the system fan will come on at low speed and the burner will be at Low Fire. If there is a subsequent “R” to “W2” High Fire call, then the DC signal will be sent to the two-stage solenoid due to the ST9162 sending a 120 VAC signal from the board to the inducer. Additionally, it will go up to high speed closing the high pressure switch. The system fan will then also go to high speed.

When the thermostat is satisfied, the power to the inducer will be shut down after a five-second post-purge. Timed from the gas valve de-energizing, the low heat fan speed de-energizes after the selected heat fan delay “off” time expires.

Figure 2 show various components and their location on the furnace. The condensing principle is accomplished by causing the water (vapor-laden) products of combustion to be drawn into the secondary heat exchanger by the inducer fan. This will, with the blower blowing across the heat exchanger, cause the vapor to condense; the latent heat will be drawn from it and then the air flow will continue to pass through the primary heat exchanger, picking up even more heat. The condensate is then dumped out of the condensate drain typically either into a neutralizer, a condensate pump or a combination of both. The electronically commutated motor (ECM) allows for variable speeds to control the cubic feet per minute (CFM) flow in order to cut down on the possibility of blowing cold air. The ECM motor is controlled by signals from the EFT board. Figure 2 further shows the location of the high and low-pressure switches, along with the two-speed inducer.

This system uses a DC blower called GE ECM motor. There will be a separate column on ECM motors, so we will be brief in our discussion here. This is not a regular 120 VAC motor that many may be familiar with, so special instructions are necessary in order to properly diagnose problems within this system.

The Modulating Furnace ECM Blower Motor
Motor Operation
The modulating furnace uses the GE ECM motor. The motor is a DC motor. Line voltage AC power is converted to DC by an inverter inside the motor electronics control. DC voltage powers the motor stator; therefore, it is a synchronous motor. Since it is a brushless motor, there are no brushes to wear out and it makes minimal noise.

The rotor uses a permanent magnet. This eliminates almost all rotor losses found on permanent split capacitor (PSC) induction motors. In fact, the magnetic pull can be felt when spinning the rotor by hand.

The motor has two inputs. One is 120-volt line power. This motor must be powered continuously with line voltage. It also has a 24-volt power input. Figure 4 shows how power flows to the ECM motor electronics, how that power flows to the blower motor and how the motor electronics send a signal back to the integrated furnace control board.

The airflow through the duct work is varied to meet the load demand. Airflow as low as 300 CFM can be achieved by the response of the brushless permanent magnet variable speed blower motor.

The ECM motor is wired from the 120 VAC source and the feed from the six-pin connector on the ST9162 EFT. Dual in-line package (DIP) switches must be set up at the time of installation. DIP switch settings will be covered in a future column.

One of the things to consider when diagnosing problems with the ECM motor is that, if the fan does not run on any thermostat call, then the fan is suspect. On the other hand, if the fan runs on all but one speed, then one would suspect the timer board. A quick test to see if the fan will run is to pull a wire off of one of the limits; this should cause the fan to turn “on” (see Figure 5).

The six-pin connector from the EFT board can be checked for proper voltage (Figure 6). The tests are done as AC tests, looking for around 14 VAC. The #3 pin is the Common Pin; Figure 7 shows the Common Pin relationships for various applications. ICM

Part 13, from the Jul/Aug 2023 Indoor Comfort.