RV Furnace 101

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“Winter is coming” is an always true statement. You need some heat in your RV so make sure the furnace (propane) is functional and in case there is trouble you need to know how it works to be able to troubleshoot. Better find out before winter is there!

This article is geared towards an RV propane furnace (Suburban) that most RV’s have.

INDEX:
How it works
PDF (Includes all the parts with pictures)
Quick troubleshooting tips from RVtravel.com
Various furnace manuals
Troubleshooting and solutions
Video: Overview of furnace parts
Video: taking apart a furnace
Video: reassembling a furnace

Let’s find out what happens when you hit the furnace thermostat and crank it up. We have to make sure there is enough propane to start and run the furnace and there are 12 volts available for the furnace control board to even start the furnace.

TIP: When you did not use the propane for a while just use the stove a little so any air in the gas lines will be replaced by propane.

HOW IT WORKS:

How it heats the air;
Most RV propane furnaces are designed on the concept of sealed combustion with two blower wheels.
1. Room air wheel-
This pulls air from the inside of the RV and blows it over the outside of the heating chamber to wipe the heat from the chamber and force it back into the duct system to distribute it throughout the RV.
2. Combustion air wheel-
Pulls the air from the outside of the RV and blows it into the chamber into the burner to mix the gas for combustion and then blows the combustion air out the exhaust.
3. The combustion air and room air channels are separated from each other.
4. All these furnaces work the same just different shapes and sizes. 

What happens when I use the thermostat;

The thermostat will signal the furnace control board that we need some heat. First, the blower motor will start to purge any gas out of the furnace that was left from previous use, it also activates the “sail switch” that enables power to another switch called the “thermo switch“. When both switches are connected (they are in series) the 12-volt power will reach the control board and opens the gas valve and activate the igniter. It takes about 15 seconds before the igniter activates.

Now let’s back up a little and explain what this all means;

The “sail switch” will switch to ON when there is initial airflow right after you hit the thermostat. In case the airflow is obstructed for example by a clogged exhaust the switch will not work and the furnace will not function. The sail switch moves by the flow of the air so a slow running blower, because of a low voltage, will also NOT engage the sail switch to ON!
The “thermo switch” (also called a high limit temperature control) is ON when the temperature does not exceed a set limit to prevent overheating. This switch is installed after the sail switch. Both switches have to be ON for the furnace to work.
The igniter is a Direct Spark Ignition board system (DSI board) This board has a timing circuit that will purge the chamber of any gas that was sitting in the chamber from the last use, next it will ignite the gas. After about 8 seconds the DSI checks for the presence of a flame. In case there is no flame it will try 2 more times (15 seconds interval) and goes to lock-out when there is still not a flame detected. (That is why you should use the stove to purge air out of the gas line) In case of a lock-out consult the manual for the action to take. Most furnaces are being reset by switching off the thermostat followed by waiting time.

When the thermostat has reached the set temperature it will remove power from the system, the blower will remain ON for some time to vent the system.

Download this PDF (click the picture) for a more elaborate explanation including troubleshooting tips, also watch the videos below for an explanation and see a disassembling and reassembling of an RV furnace.

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Suburban furnace information PDF

RVtravel has a nice quick troubleshooting guide;

Steve Savage submitted this article to RVtravel.com when he was a Master Certified RV Technician with Mobility RV Service.
When winter arrives, most service calls I get are for RV furnace problems. So let’s review the do’s and don’ts.
First of all, not knowing what you are doing with a gas-fired appliance can be dangerous, no matter how easy it looks on YouTube. Knowledge of multi-meter use is essential. So is understanding how the furnace sequence of operation goes.

Direct Spark Ignition (DSI) Model RV Furnace (Coleman 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 hire 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 thermal 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 energizes 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 too 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 sense the flame within approximately 10 seconds, it will stop 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 an electrically operated solenoid 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 may 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 volume of propane is discharged from the tank.  This is a 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

This picture shows a furnace burner fan wheel that has been used by a bird to build a nest. After cleaning it did run fine again.

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 the 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 the main burner orifice or replace it.
12. Combustion air wheel lose 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 the 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 the 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 the main burner orifice, it must comply with the furnace’s specifications.

BLOWER VIBRATES OR IS NOISY

Condition Solution
1. Damaged blower wheel Replace the blower wheel.
2. Motor shaft bent Replace Fan Motor.
3. Loose motor Tighten the 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 backward Reposition wheel and tighten.
4. Air leakage at gaskets Replace gaskets.
5. Low voltage (less than +10.5VDC) Correct power supply.

FAN RUNS CONTINUOUSLY 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 underfired 1. Set gas pressure to 11″W.C. with all appliances running.
Also, check the main burner orifice, it must comply with the furnace’s specifications
2. Check to duct and return air according to furnace’s specifications.

Overview, Parts and naming video


These videos show an RV tech dismantling a Suburban RV Furnace while he points out all the parts with the proper name. In part 2 he reassembled it again.

Part 2

 

 

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