Author Topic: Anatomy of an RV Furnace  (Read 39726 times)

Offline DaveVA78Chieftain

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Anatomy of an RV Furnace
« on: March 23, 2009, 02:14 PM »
Introduction
Over the years there has been 2 basic types of furnaces:
  • Pilot Model RV Furnace
  • Direct Spark Ignition (DSI) Model RV Furnace
RV Furnace  - Overview

Furnaces installed in recreational vehicles are classified as Forced Draft Furnaces. A forced draft furnace utilizes a sealed combustion chamber that is vented to the outside atmosphere. Intake air for combustion is taken from outdoors and is isolated from the exhaust. A motor is used to drive an impeller wheel to draw intake air into the combustion chamber and force the exhaust gases through the furnace chamber to the outside atmosphere. A second impeller wheel (driven by the same motor yet totally isolated from the combustion air) is used to circulate room air across the furnace chamber where it is heated. The blower then forces the hot air into the living area either through a duct system or through a front grille on the furnace cabinet.
WARNING
While the information here is for general reference, using it is at your own risk.
If you do not feel comfortable with working on Propane Gas appliances,
then enlist the services of a qualified technician.
CAUTION
While the simplified drawings posted here help in describing how a furnace works,
always refer to the actual unit drawing when evaluating or working on a RV Furnace.
Components (i.e. extra Limit Switches, etc.) may be installed that are not represented here.
Typically, the elctrical schematic for the furnace can be found glued to the inside of the Furnace.
Pilot Model RV Furnace  - Overview

The Pilot Model RV Furnace was the first style of furnace used.  They started replacing them with the Direct Spark Ignition (DSI) Model RV Furnace in the mid to late 70's.  These furnaces use a pilot light port off of the gas valve.  No electronic control board is installed.
As shown in the drawings below, there are various implementations of a Pilot Model RV Furnance, however they all share the same basic components.
Description, Operation, and Troubleshooting of the Pilot Model RV Furnace is provided in the 2nd Post.

Note:
Some Model RV furnaces may also have a 115VAC input.  They have a built in 12VDC power supply which is used as the +12VDC furnace supply source when the RV is connected to a 115VAC source.  The information here only covers the 12VDC portion of the furnace, not the 115VAC or 12VDC Power Supply portion.
WARNING
Purging of all Propane Gas from the Combustion Chamber for 5 minutes prior to lighting/re-lighting the Pilot Light is extremly important.
This prevents a build of gas that could result in an explosion.


  • Fuse or Circuit Breaker - Protects against high Current Draw
  • System Switch - Allows the user to secure the furnace to prevent it from operating.
  • Glow Coil switch - Only on a few models.  Allows the user to electrically light the pilot light without a match.  Thermostat will not operate when in Pilot light position.
  • Fan Switch
       
    Normally open.  This switch jumpers the Fan relay contacts.  When the furnace heats up this switch closes.  When the Thermostat is satisfied, the gas is shut off however this switch allows the fan to continue to operate in order to cool the furnace down.  On some furnaces, this switch is replaced by a time delay relay.
  • Thermostat - Controls the gas and fan circuits.
  • Fan Relay
       
    Allows the smaller current capability of the thermostat to control the higher current requirements of the Fan.
  • Sail Switch
       
    Used to detect proper fan operation. The fan has to achieve 75% of capacity before the micro switch will engage.  Low DC voltage, blocked air flow, or a defective fan motor can keep the fan from achieving full speed.
  • Limit Switch
       
    May be more than one in a furnace.  Normally closed.  Opens if the furnace overheats and shuts down the gas supply.  Fan will continue to operate because the Thermostat is still ON.
  • Gas Valve
       
    Electrically controlled gas valve.  Provides gas to pilot light.  A feed back thermocouple (small electic signal) at pilot light is used to indicate to the gas valve that the pilot is lit.  Gas valve will not work if pilot light is not lit.
Direct Spark Ignition (DSI) Model RV Furnace - Overview



At first glance, a DSI Furnace looks very simular to a Pilot Light style furnace. The DSI Model furnace uses a high voltage electric spark to light the burner rather than a pilot light.  Operational control of a DSI furnace is provided via a electronic control board (inserted between the Limit Switch and the Gas Valve).  Additionally, on start up, the DSI control board delays 15 to 20 seconds before turning on the gas valve.  This purges any residual gas in the combustion chamber prior to ignition.  On start-up, if the burner does not light, older DSI control modules turn the gas valve off, discontinue ignition, but leave the blower running.  Newer versions retry the ignition cycle 3 times and then shut everything down.  These cycles are called lockout mode.  Lockout mode is reset by turning the Thermostat off for 30 seconds and turning it back on.  Older DSI control boards can be upgraded to the newer style control boards.  While the ignition method is different, a DSI furnace shares many of the same components of a Pilot model RV furnance.
Description, Operation, and Troubleshooting of the DSI Model RV Furnace is provided in the 3rd Post.

Note: DSI boards are also used in newer electronic controlled RV reefers and RV Water Heaters.

  • Fuse or Circuit Breaker - Protects against high Current Draw
  • System Switch - Allows the user to secure the furnace to prevent it from operating.
  • Fan Switch
       
    Normally open.  This switch jumpers the Fan relay contacts.  When the furnace heats up this switch closes.  When the Thermostat is satisfied, the gas is shut off however this switch allows the fan to continue to operate in order to cool the furnace down.  On some furnaces, this switch is replaced by a time delay relay.
  • Thermostat - Controls the gas and fan circuits.
  • Fan Relay
       
    Allows the smaller current capability of the thermostat to control the higher current requirements of the Fan.
  • Sail Switch
       
    Used to detect proper fan operation. The fan has to achieve 75% of capacity before the micro switch will engage.  Low DC voltage, blocked air flow, or a defective fan motor can keep the fan from achieving full speed.
  • Limit Switch
       
    May be more than one in a furnace.  Normally closed.  Opens if the furnace overheats and shuts down the gas supply.  Fan will continue to operate because the Thermostat is still ON.
  • Control board
       
    Controls the gas valve, produces the high voltage for the ignitor and monitors the ignition/burner process.
  • Ignitor/flame Monitor
       
    Provides the ignition spark. Heat from the flame produces a small current in the electrode which is monitored by the control board.  Some furnaces use a seperate electrode as the monitor; some use the actual spark electrode as a monitor.
  • Gas Valve
       
    Electrically operated gas valve under control of the DSI control board.

Offline DaveVA78Chieftain

  • 14 year member
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  • Chassis: Dodge M400
  • Engine: 440-3
Re: Anatomy of an RV Furnace
« Reply #1 on: March 23, 2009, 02:17 PM »
Pilot Model RV Furnace

Operation

Electrically, the Furnace basically consists of a Fan Circuit and a Gas Circuit.  Both circuits are controlled by the Thermostat.

WARNING
While the information here is for general reference, using it is at your own risk.
If you do not feel comfortable with working on Propane Gas appliances,
then enlist the services of a qualified technician.

WARNING
Purging of all Propane Gas from the Pilot Model RV Furnace Combustion Chamber for 5 minutes prior to lighting/re-lighting the Pilot Light is extremly important.
This prevents a build of gas that could result in an explosion.

[
WARNING
Over time the combustion chamber rusts through allowing Gas fumes to enter the interior of the RV.
You will need to replace the furnace if this occurs.

CAUTION
While the simplified drawings posted here help in describing how a furnace works,
always refer to the actual unit drawing when evaluating or working on a RV Furnace.
Components (i.e. extra Limit Switches, etc.) may be installed that are not represented here.
Typically, the elctrical schematic for the furnace can be found glued to the inside of the Furnace.





Referring to the drawing above:

  • +12VDC power (+10.5VDC to +13.5VDC) is applied to the circuit breaker or fuse if so equipped.
  • The +12VDC is then feed through the Unit Power ON/OFF switch and distributed to the Thermostat, Fan Switch and the input side contact of the Fan Relay.
    The Fan Switch is a thermo switch and is normally open.  The Fan Switch closes when the furnace heats up.
  • When the Thermostat requests heat (contacts close), +12VDC is feed to the Fan Relay (Fan Circuit) and the Sail Switch (Gas Circuit).
  • The Fan Relay energizies and applies power to the Fan Motor.  On some models, a starting capacitor is installed that assists the Fan Motor start.  DC Voltage for the fan returns to the battery via the +12VDC return path.
  • The sail switch is normally open.  When the fan reaches to 75% of it's rated capacity, the Sail Switch closes routing +12VDC voltage to the Thermal Limit Switch.
  • The Thermo Limit switch is normally closed and passes the +12VDC power on to the Gas Valve.  The Limit switch will open shutting power to the Gas Valve if the furnace internal temperature goes to high.
  • The Gas Valve is a electricly operated solinoid valve, provides gas to the main burner, provides gas to the Pilot Light, and senses the status of the Pilot Light.  Sensing of the Pilot Light is performed electrically via a voltage induced in the Thermocouple by the Pilot Light.  If the Gas Valve does not sense the Pilot Light signal it will not provide gas to the main burner.
  • When the Thermostat senses the desired temperature in the room has been achieved, it opens the contacts shutting off voltage to the Fan Relay and Gas Valve circuits.  The Fan Switch will keep providing +12VDC supply voltage to the Fan Motor until the temperature inside the Furnace is reduced at which point the switch opens securing power to the Fan Motor.
  • Glow Coil Models - When the Pilot Light switch is depressed, the Thermostat and fan circuits are disabled.
  • DuoTherm Models - Even though the Thermostat is located on the return leg side of the circuit, it essentially controls both the Fan and Gas valve circuits the same way.

CAUTION
Just as the motor on a vacuum cleaner will increase speed if you place your hand over the intake hose,
the RV Furnace fan motor will increase speed if the inside air intake ports are blocked by camping supplies, linen, cookie sheets, etc.
This can result in a increase in combustion air flow that causes the furnace to run hotter.
The overheating can cause damage to the furnace unit.  Do not block the furnace air intake and exhaust ports.


Notes:
1.  If the Thermostat applies +12VDC voltage to the Fan Relay and the Gas circuit does not operate (i.e Pilot Light is not lit), the Fan Motor will keep running until either the Thermostat is turned OFF or the input supply voltage is drained.
2.  If the propane supply runs out and the Thermostat is on, the Fan Motor will keep running until either the Thermostat is turned OFF or the input supply voltage drained.
3.  If either the Combustion side Air Intake/Exhaust path (i.e. birds/wasp nest) or the Room side Air Intake/Exhaust path (i.e. camping gear) is blocked, the fan will typically not get up to 75% capacity.  This will prevent the Sail Switch from closing and allowing voltage to get to the gas valve.
4.  Overtime the combustion chamber rusts through.  You will need to replace the furnace if this occurs.
5.  If the propane system in the RV has been purged, then it is best to purge most of the air out of the system using the burners on the stove prior to attempting to light the furnace.  Even so, it my still take multiple attempts to start the furnace in order to purge the air from the furnace gas lines.
6.  When turning the Propane system on at the tank, turn the gas on very slowly.  There is a safety valve in the propane tank that is used to drastically reduce the flow from the tank to the regulator if it senses a large volumn of propane is discharged from the tank.  This is safety feature where if the RV is in an accident and the propane lines are ruptured, the gas flow is greatly reduced.  If the safety valve is triggered, to reset it, you have to turn the tank supply OFF for 60 seconds and then very slowly open the tank supply valve.
7.  Even though Propane suppliers strive not to get moisture in their delivery systems, a small amount of moisture does get in.  On very cold days, this moisture can then freeze in your regulator stopping the flow of gas into the RV.  You have to warm up (no flames!; warm towel, etc.) the regulator to get it working again.


Troubleshooting

BURNER FAILS TO IGNITE AND BLOWER FAILS TO RUN
   
Condition
Solution
1. No electrical power to the furnace;
current overload protector device defective or,     
tripped (circuit breaker)
Reconnect or replace power source
Reset circuit breaker
Check amp draw from motor according to furnace's specification.
2. Thermostat defective
Replace thermostat
3. Thermostat wires broken
Replace wire or wires
Relocate thermostat
4. Blower motor defective
Replace motor
5. Wire off Blower motor
Reconnect wire
6. Blower relay defective
Replace relay
7. Wire off relay
Reconnect wire
8. Improper ground
Clean and secure grounds

BLOWER RUNS BUT FURNACE FAILS TO IGNITE
   
Condition
Solution
1. Low voltage (less than +10.5VDC)
Correct Power Supply
2. 12 volt polarity reversed
Correct polarity
3. Furnace grounding wires not secure   
Clean and secure grounds established
4. Gas valve defective
Replace valve or valve coil, depending on problem encountered
5. Gas pressure incorrect
Set pressure to a minimum of 11"W.C. with all appliances running (Replace regulator if not obtainable)
6. Limit switch defective or wire off
Reconnect wire or replace Limit Switch
7. Sail switch defective or wire off
Reconnect wire or replace Sail Switch
8. Burner orifice defective
Clean main burner orifice or replace.
9. Combustion air wheel loose
Reposition and tighten
10. Burner head adjustment
Reposition and tighten
Adjust burner according to furnace specifications.
11. Exhaust blocked
Clean exhaust
12. Air intake restricted
Clean air intake
13. Broken or loose wire
Replace or tighten
14. Motor running slow
Check voltage first  (greater than +10.5VDC)
If 12 VDC while running, replace motor

BLOWER SHUTS OFF AT SAME TIME BURNER SHUTS OFF
   
Condition
Solution
1. Faulty Fan Switch or Fan Relay   
Replace Fan Switch or Fan Relay
2. Thermostat wired wrong
Correct wiring

BURNER CYCLING ON AND OFF - BLOWER RUNS CONTINUOUSLY WITH THERMOSTAT "ON"
   
Condition
Solution
1. Restricted return air supply
Make sure return air meets furnace's minimum requirements.
2. Restricted discharge duct system   
a. Make sure ducting meets furnace's minimum requirements
b. Make sure there is no excess ducting or unnecessary bends
c. Make sure any closeable registers are fully open and unrestricted
3. Defective limit switch
Replace limit switch.
4. Furnace over fired
Set gas pressure to a minimum of 11"W.C. with all appliances running. Replace regulator if not obtainable
Also, check main burner orifice, it must comply with furnace's specifications.

BLOWER VIBRATES OR IS NOISY
   
Condition
Solution
1. Damaged blower wheel
Replace blower wheel.
2. Motor shaft bent
Replace Fan Motor.
3. Loose motor
Tighten motor mounting bracket.
4. Restricted discharge duct system   
a. Make sure ducting meets furnace's minimum requirements
b. Make sure there is no excess ducting or unnecessary bends
c. Make sure any closeable registers are fully open and unrestricted


PILOT FAILS TO IGNITE OR STAY LIT
   
Condition
Solution
1. Plugged pilot orifice
Clean or replace orifice.
2. Electrode out of adjustment (Glow coil or Piezo sparker equipped furnace)   
Adjust electrode to furnace's specifications.
3. Pilot tube defective
Replace tube.
4. Piezo sparker defective
Replace sparker.
5. Defective thermocouple
Replace thermocouple.
6. Defective valve
Replace valve.
7. Vent kit not sealed
Seal vent where it meets the furnace.
8. MICA window missing
Replace MICA window.
9. Combustion air hose plugged or collapsed
Replace combustion hose.
10. Burner plate gasket or pilot not sealed
Replace gaskets.
11. Water in propane
Add alcohol to the propane bottles.

LAZY FLAME (Yellow)
   
Condition
Solution
1. High gas pressure
Set pressure to a minimum of 11"W.C. with all appliances running.
Replace regulator if not obtainable
2. Burner out of adjustment
Adjust burner according to furnace specification
3. Combustion wheel installed backwards   
Reposition wheel and tighten.
4. Air leakeage at gaskets
Replace gaskets.
5. Low voltage  (less than +10.5VDC)
Correct power supply.

FAN RUNS CONTINOUSLY WITH THERMOSTAT "OFF"
   
Condition
Solution
1. Defective thermostat
Replace thermostat.
2. Shorted thermostat leads   
Replace/repair wire or wires.
3. Defective Fan Switch
Replace Fan Switch.
4. Defective Relay
Replace Relay.

INSUFFICIENT HEAT
   
Condition
Solution
1. Defective thermostat   
Replace thermostat.
2. Furnace under fired
1. Set gas pressure to 11"W.C. with all appliances running.
Also, check main burner orifice, it must comply with furnace's specifications
2. Check ducting and return air according to furnace's specifications.


Offline DaveVA78Chieftain

  • 14 year member
  • *
  • Posts: 3506
  • Member since: 2003
  • I own: a Motor Home
  • Year: 1978
  • Make: Winnebago
  • Model: Chieftain
  • Chassis: Dodge M400
  • Engine: 440-3
Re: Anatomy of an RV Furnace
« Reply #2 on: March 23, 2009, 02:18 PM »
Direct Spark Ignition (DSI) Model RV Furnace

Operation

Like a Pilot style furnace, electrically, the Furnace basically consists of a Fan Circuit and a Gas Circuit.
Both circuits are controlled by the Thermostat however the following features are added by a DSI Control Board that is inserted between the Limit Switch and Gas Valve.

  • The Pilot light is replaced by a high voltage ignitor electrode.
  • The Burner Flame is monitored by the DSI Control Board and shuts the Gas Valve OFF if the Burnner does not light or extinguishes (lockout mode).
  • For safety, the Combustion Chamber is purged of residual gas for 15 to 20 seconds prior to burner ignition.
  • Most DSI Control Boards retry the ignition sequence 3 times before going into Lockout Mode.  To exit Lockout Mode, turn the Thermostat OFF for 60 seconds then back ON.
  • Newer DSI Control Boards will also secure the fan if the burner does not light or extinguishes.
  • If not provided by the stock DSI furnace, the 3 try and fan securing features can be added by replacing the stock DSI Control Board with a Dinosaur FAN50 Control Board.  It does require some minor rewiring of the Fan circuit.

WARNING
While the information here is for general reference, using it is at your own risk.
If you do not feel comfortable with working on Propane Gas appliances,
then enlist the services of a qualified technician.

WARNING
Over time the combustion chamber rusts through allowing Gas fumes to enter the interior of the RV.
You will need to replace the furnace if this occurs.

CAUTION
While the simplified drawings posted here help in describing how a furnace works,
always refer to the actual unit drawing when evaluating or working on a RV Furnace.
Components (i.e. extra Limit Switches, etc.) may be installed that are not represented here.
Typically, the electrical schematic for the furnace can be found glued to the inside of the Furnace.




Referring to the drawing above:

  • +12VDC power (+10.5VDC to +13.5VDC) is applied to the circuit breaker or fuse if so equipped.
  • The +12VDC is then feed through the Unit Power ON/OFF switch and distributed to the Thermostat, Fan Switch and the input side contact of the Fan Relay.
    The Fan Switch is a thermo switch and is normally open.  The Fan Switch closes when the furnace heats up.
  • When the Thermostat requests heat (contacts close), +12VDC is feed to the Fan Relay (Fan Circuit) and the Sail Switch (Gas Circuit).
  • The Fan Relay energizies and applies power to the Fan Motor.  On newer DSI models, the +12VDC voltage is first routed through the DSI Control Board.  This allows the control board to shut down the fan motor (and gas valve) if it detects a low supply voltage, no ignition, or loss of burner flame (prevents the fan motor from running the house battery down).  On some models, a starting capacitor is installed that assists the Fan Motor start.  DC Voltage for the fan returns to the battery via the +12VDC return path.
  • The sail switch is normally open.  When the fan reaches to 75% of it's rated capacity, the Sail Switch closes routing +12VDC voltage to the Thermal Limit Switch.
  • The Thermo Limit switch is normally closed and passes the +12VDC power on to the Gas Valve.  The Limit switch will open shutting power to the DSI Control Board if the furnace internal temperature goes to high.  This in turn will cause the Gas Valve to be secured.
  • The signal from the Limit switch is applied to pin 1 of the control board.  In order to purge residual gas from the Combustion Chamber, the control board will wait 15 to 20 seconds (fan is running during this period) before attempting the ignition sequence.  After the purging delay, it provides high voltage to the ignitor electrode and sends +12VDC to the gas valve via pin 4.
    The Control board monitors the burner flame status via pin 5 (Some furnaces monitor the flame via the ignitor lead rather than pin 5).  If the Control Board does not senese the flame within approximatly 10 seconds, it will stops the ignition cycle and secure the +12VDC signal to the Gas Valve (lockout mode).  Newer DSI Control Boards retry the ignition sequence 3 times before going into Lockout mode.  Some DSI furnaces also secure the Fan Motor if the Furnace goes into Lockout Mode.
  • The Gas Valve is a electricly operated solinoid valve that provides gas to the main burner.
  • When the Thermostat senses the desired temperature in the room has been achieved, it opens the contacts shutting off voltage to the Fan Relay and Gas Valve circuits.  The Fan Switch will keep providing +12VDC supply voltage to the Fan Motor until the temperature inside the Furnace is reduced at which point the switch opens securing power to the Fan Motor.



Notes:
1.  On older DSI Furnaces (DSI Control Board is not connected to fan circuit), if the Thermostat applies +12VDC voltage to the Fan Relay and the Gas circuit does not operate (i.e Pilot Light is not lit), the Fan Motor will keep running until either the Thermostat is turned OFF or the input supply voltage is drained.
2.  On older DSI Furnaces (DSI Control Board is not connected to fan circuit), if the propane supply runs out and the Thermostat is on, the Fan Motor will keep running until either the Thermostat is turned OFF or the input supply voltage drained.
3.  If either the Combustion side Air Intake/Exhaust path (i.e. birds/wasp nest) or the Room side Air Intake/Exhaust path (i.e. camping gear) is blocked, the fan will typically not get up to 75% capacity.  This will prevent the Sail Switch from closing and allowing voltage to get to the DSI Control Board.
4.  If the propane system in the RV has been purged, then it is best to purge most of the air out of the system using the burners on the stove prior to attempting to light the furnace.  Even so, it my still take multiple attempts to start the furnace in order to purge the air from the furnace gas lines.
5.  When turning the Propane system on at the tank, turn the gas on very slowly.  There is a safety valve in the propane tank that is used to drastically reduce the flow from the tank to the regulator if it senses a large volumn of propane is discharged from the tank.  This is safety feature where if the RV is in an accident and the propane lines are ruptured, the gas flow is greatly reduced.  If the safety valve is triggered, to reset it, you have to turn the tank supply OFF for 60 seconds and then very slowly open the tank supply valve.
6.  Even though Propane suppliers strive not to get moisture in their delivery systems, a small amount of moisture does get in.  On very cold days, this moisture can then freeze in your regulator stopping the flow of gas into the RV.  You have to warm up (no flames!; warm towel, etc.) the regulator to get it working again.
7.  DSI Control Board life span is typically only 2 to 3 years.


Troubleshooting

BURNER FAILS TO IGNITE AND BLOWER FAILS TO RUN
   
Condition
Solution
1. No electrical power to the furnace;
current overload protector device defective or,     
tripped (circuit breaker)
Reconnect or replace power source
Reset circuit breaker
Check amp draw from motor according to furnace's specification.
2. Thermostat defective
Replace thermostat
3. Thermostat wires broken
Replace wire or wires
Relocate thermostat
4. Blower motor defective
Replace motor
5. Wire off Blower motor
Reconnect wire
6. Blower relay defective
Replace relay
7. Wire off relay
Reconnect wire
8. Improper ground
Clean and secure grounds

BLOWER RUNS BUT FURNACE FAILS TO IGNITE
   
Condition
Solution
1. Mis-adjusted (1/8" Clearance) or defective ignitor/monitor electrode(s)   
Adjust or replace ignitor/monitor electrode(s).
2.  Ignitor/monitor electrode(s) wire off or broken
Replace/repair wiring.
3. Low voltage (less than +10.5VDC)
Correct Power Supply
4. 12 volt polarity reversed
CAUTION
Reversed polarity will most likely damage the DSI Control Board.

Correct polarity
5. Furnace grounding wires not secure   
Clean and secure grounds established
6. DSI Control Board wires off or broken.
Repair or replace wires.
7. Gas valve defective
Replace valve or valve coil, depending on problem encountered
8. Gas pressure incorrect
Set pressure to a minimum of 11"W.C. with all appliances running (Replace regulator if not obtainable)
9. Limit switch defective or wire off
Reconnect wire or replace Limit Switch
10. Sail switch defective or wire off
Reconnect wire or replace Sail Switch
11. Burner orifice defective
Clean main burner orifice or replace.
12. Combustion air wheel loose
Reposition and tighten
13. Burner head adjustment
Reposition and tighten
Adjust burner according to furnace specifications.
14. Exhaust blocked
Clean exhaust
15. Air intake restricted
Clean air intake
16. Broken or loose wire
Replace or tighten
17. Motor running slow
Check voltage first
If 12 VDC while running, replace motor
18. DSI Control Board defective.
Replace Control Board
19. Water in propane
Add alcohol to the propane bottles.

BLOWER SHUTS OFF AT SAME TIME BURNER SHUTS OFF
   
Condition
Solution
1. Faulty Fan Switch or Fan Relay   
Replace Fan Switch or Fan Relay
2. Thermostat wired wrong
Correct wiring
3. Defective DSI Control Board
(If Fan Motor Control connection is provided)     
Replace DSI Control Board

BURNER CYCLING ON AND OFF - BLOWER RUNS CONTINUOUSLY WITH THERMOSTAT "ON"
   
Condition
Solution
1. Restricted return air supply
Make sure return air meets furnace's minimum requirements.
2. Restricted discharge duct system   
a. Make sure ducting meets furnace's minimum requirements
b. Make sure there is no excess ducting or unnecessary bends
c. Make sure any closeable registers are fully open and unrestricted
3. Defective limit switch
Replace limit switch.
4. Defective DSI Control Board
Replace DSI Control Board
5. Mis-adjusted or defective flame monitor electrode   
Adjust or replace flame monitor electrode.
6. Furnace over fired
Set gas pressure to a minimum of 11"W.C. with all appliances running. Replace regulator if not obtainable
Also, check main burner orifice, it must comply with furnace's specifications.

BLOWER VIBRATES OR IS NOISY
   
Condition
Solution
1. Damaged blower wheel
Replace blower wheel.
2. Motor shaft bent
Replace Fan Motor.
3. Loose motor
Tighten motor mounting bracket.
4. Restricted discharge duct system     
a. Make sure ducting meets furnace's minimum requirements
b. Make sure there is no excess ducting or unnecessary bends
c. Make sure any closeable registers are fully open and unrestricted


LAZY FLAME (Yellow)
   
Condition
Solution
1. High gas pressure
Set pressure to a minimum of 11"W.C. with all appliances running.
Replace regulator if not obtainable
2. Burner out of adjustment
Adjust burner according to furnace specification
3. Combustion wheel installed backwards   
Reposition wheel and tighten.
4. Air leakeage at gaskets
Replace gaskets.
5. Low voltage (less than +10.5VDC)
Correct power supply.

FAN RUNS CONTINOUSLY WITH THERMOSTAT "OFF"
   
Condition
Solution
1. Defective thermostat
Replace thermostat.
2. Shorted thermostat leads   
Replace/repair wire or wires.
3. Defective Fan Switch
Replace Fan Switch.
4. Defective Relay
Replace Relay.

INSUFFICIENT HEAT
   
Condition
Solution
1. Defective thermostat   
Replace thermostat.
2. Furnace under fired
1. Set gas pressure to 11"W.C. with all appliances running.
Also, check main burner orifice, it must comply with furnace's specifications
2. Check ducting and return air according to furnace's specifications.


Offline DaveVA78Chieftain

  • 14 year member
  • *
  • Posts: 3506
  • Member since: 2003
  • I own: a Motor Home
  • Year: 1978
  • Make: Winnebago
  • Model: Chieftain
  • Chassis: Dodge M400
  • Engine: 440-3
Re: Anatomy of an RV Furnace
« Reply #3 on: March 23, 2009, 07:44 PM »
Typical Internal Ducting Hookup

Typical Mechanical View

Pilot Furnace - Electrical Side

DSI Furnace - Electrical Side

Pilot or DSI Furnace - Chamber Side

Outside Vent


Offline JDxeper

  • 12 year member
  • *
  • Posts: 480
  • Tumble Bug
  • Member since: 2005
  • I own: a Motor Home
  • Year: 1968
  • Make: Winnebago
  • Model: F-17
  • Chassis: Ford
  • Engine: 300 cid 6
Re: Anatomy of an RV Furnace
« Reply #4 on: March 24, 2009, 03:06 PM »
Good Job Dave :)ThmbUp
Tumble Bug "Rollin in MO" (JD)

 

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