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Tech Talk => Info Articles => Tech Talk => Topic started by: DaveVA78Chieftain on March 23, 2009, 02:14 PM

Title: Anatomy of an RV Furnace
Post by: DaveVA78Chieftain on March 23, 2009, 02:14 PM
Introduction
Over the years there has been 2 basic types of furnaces:
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.
(http://i286.photobucket.com/albums/ll120/BaileyDave/RVFurnace/Small-ColemanPilotModelRVFurnace.gif) (http://i286.photobucket.com/albums/ll120/BaileyDave/RVFurnace/Small-ColemanPilotModelRVFurnacew-g.gif) (http://i286.photobucket.com/albums/ll120/BaileyDave/RVFurnace/Small-DuoThermPilotModelRVFurnace.gif)

Direct Spark Ignition (DSI) Model RV Furnace - Overview

(http://i286.photobucket.com/albums/ll120/BaileyDave/RVFurnace/Small-ColemanDSIModelRVFurnace.gif)(http://i286.photobucket.com/albums/ll120/BaileyDave/RVFurnace/Small-ColemanDSIModelRVFurnacewithF.jpg)

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.

Title: Re: Anatomy of an RV Furnace
Post by: DaveVA78Chieftain 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.


(http://i286.photobucket.com/albums/ll120/BaileyDave/RVFurnace/ColemanPilotModelRVFurnace.gif)(http://i286.photobucket.com/albums/ll120/BaileyDave/RVFurnace/ColemanPilotModelRVFurnacew-glow.gif)
(http://i286.photobucket.com/albums/ll120/BaileyDave/RVFurnace/DuoThermPilotModelRVFurnace.gif)

Referring to the drawing above:


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.

Title: Re: Anatomy of an RV Furnace
Post by: DaveVA78Chieftain 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.


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.


(http://i286.photobucket.com/albums/ll120/BaileyDave/RVFurnace/ColemanDSIModelRVFurnace.gif)(http://i286.photobucket.com/albums/ll120/BaileyDave/RVFurnace/ColemanDSIModelRVFurnacewithFan50.jpg)

Referring to the drawing above:




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.

Title: Re: Anatomy of an RV Furnace
Post by: DaveVA78Chieftain on March 23, 2009, 07:44 PM
Typical Internal Ducting Hookup
(http://i286.photobucket.com/albums/ll120/BaileyDave/RVFurnace/FurnaceWithDuctingSmall.gif)

Typical Mechanical View
(http://i286.photobucket.com/albums/ll120/BaileyDave/RVFurnace/FurnaceBlowUp.gif)

Pilot Furnace - Electrical Side
(http://i286.photobucket.com/albums/ll120/BaileyDave/RVFurnace/PilotElectSide.gif)

DSI Furnace - Electrical Side
(http://i286.photobucket.com/albums/ll120/BaileyDave/RVFurnace/DSIElectSide.gif)

Pilot or DSI Furnace - Chamber Side
(http://i286.photobucket.com/albums/ll120/BaileyDave/RVFurnace/PilotChamberSide.gif)

Outside Vent
(http://i286.photobucket.com/albums/ll120/BaileyDave/RVFurnace/OutsideVent.gif)

Title: Re: Anatomy of an RV Furnace
Post by: JDxeper on March 24, 2009, 03:06 PM
Good Job Dave :)ThmbUp
Title: Re: Anatomy of an RV Furnace
Post by: thomas on April 30, 2019, 02:52 PM
HI DAVE<  I was just looking through different posting on thr forum,and came across yours on the haeters!! WOW !! realy good job!!  IM so glad I found this forum!  You guys are awsome!!