QuestionDarren,
sorry it tok so long, the model number is NT 24sp.
I hope this helps.
Dave
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Followup To
Question -
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Followup To
Question -
'94 Winnebago Chief, the fan kicks on but no heat comes from furnace. This happens on AC or DC power and the propane is nearly full.
Thanks in advance, Dave
Answer -
Dave,
The first thing to check would be the circuit board. You should be able to call and find a dealer with a fenwal board tester and have it checked. If thats not it if you will get me the model and serial numbers I can give you some more ideas on where to go next.
Darren
Darren,
I pulled the entire furnace out and it looks like you can not remove the board, it's inside the metal enclosure. But I did put a Meter on the leads going into the furnace, the side of the furnce shows +12 -12 and thermistat +12 I do not get the -12vdc. Do you know if that voltage is constant ?
Thanks for the help.
Dave
Answer -
Dave,
I still need the model #.
Darren
AnswerSEQUENCE OF NORMAL OPERATION
The electronic ignition system is made up of three main parts: the module board, the
electrode assembly, and the electrode wire. The module board is the brain of the electronic
ignition system and it has several functions.
1. When the blower reaches approximately 75% of the normal rpm and sufficient airflow is
established, the sail switch engages and completes a 12-volt circuit through the limit
switch to the module board. (NOTE: Low voltage power supply will not provide
sufficient motor rpm to engage the sail switch.)
2. After a 12-18 second delay, 12-volt current will pass through the module board to the
solenoid valve. The current to the valve opens it and allows gas to the main burner,
simultaneously, the module board sends high voltage through the electrode wire to the
electrode assembly. The voltage seeks a ground between electrode and ground probe
and a spark occurs. The spark then ignites the main burner.
3. The module board also performs the lockout function in cases where the spark fails to
light the burner. When lockout occurs, the spark stops, the voltage from the module
board to the gas valve is discontinued, and the valve closes. The unit will remain in
lockout and the blower will continue to run until the thermostat is turned off. Turning
the thermostat off disengages the lockout function of the module board. After the
blower has stopped, the ignition sequence can be started again.
OPERATION INFORMATION
Figure 6 illustrates a typical electrical circuit for direct ignition models. The room air blower
must be operating at approximately 75% of the normal rpm at 12-volts DC before ignition
can occur. This is unlike most residential furnaces that have burner operation without the
blower running.
The operation of a forced draft combustion furnace is controlled through a 12-volt
thermostat (24-volt thermostat in the case of a park model furnace). When the thermostat
is set above room ambient, the contacts close completing a circuit through the coil of the
time delay relay. The contacts on the relay then close, completing a circuit through the
windings of the motor and the motor comes on.
As the motor reaches approximately 75% of its normal rpm, the airflow generated by the
room air blower pushes against the paddle on the sail switch (sometimes referred to as a
microswitch or an air prover switch) sailing it in and closing the contacts. A circuit is then
completed through a high temperature limit switch to the module board on a direct ignition
furnace.
On a direct ignition furnace, as current passes through the ignition module to the valve, a
high voltage spark is produced at the electrode assembly located over the burner.
Simultaneously, the valve opens, gas flows into the burner and is ignited by the spark.
All DYNATRAIL direct ignition furnaces must have a flame sensing circuit through the
control module that senses the presence of main burner flame. If proper flame sensing is
not obtained or if the burner fails to light, the furnace must go into 100% lockout. When
lockout occurs, the valve closes and will remain closed until the thermostat circuit is
broken. Re-ignition will not occur automatically. If it could, it would not be 100% lockout.
While 100% lockout is a safety factor, it can prove to be frustrating for the RV owner who
has experienced lockout problems. Not only can a lockout problem be difficult at times to
diagnose, it will result in continuous operation of the furnace blower (that is, until the
thermostat circuit is broken) and can drain the battery if the owner is not in the
recreational vehicle.
The solution for this complaint is not to try and eliminate the lockout function of the
furnace, but rather to find the cause for lockout (which will be covered later), and correct it.
The motor circuit contains a time delay relay (see wiring diagram). The purpose of a time
delay relay is to provide a means of the blower operation on initial startup and after the
thermostat is satisfied. When the thermostat contacts open, the circuit through the coil of
the time delay relay is broken and the contacts open after approximately 3-5 minutes,
depending on the furnace model.
MAINTENANCE
Preventative maintenance is essential if an RV owner is to have reliable, safe operation of
his furnace. By far, the two most important areas to watch closely in order to assure safe,
reliable operation, are the venting and the main burner.
An obstruction in the vent or main burner will reduce the intake of combustion air that
results in incomplete combustion. Whenever incomplete combustion occurs, the
by-products are carbon monoxide (CO) and soot. If the furnace outside exhaust vent shows
black soot may be forming, the furnace should not be operated until the problem is
corrected. Possible causes are air restriction combustion air, low gas pressure, dirty
burner, slow motor, low voltage, etc.
If operation of the furnace continues under these conditions, it could result in serious
injury to the occupants of the RV or even death.
Cleaning of the main burner and an inspection of the venting system should be done at
least once a year, preferably just before the beginning of the heating season. Some RV
owners and service personnel have the false assumption that if a furnace has not been
used, it will not require cleaning. NOT SO! A furnace that has not been used for some time
could be more in need of cleaning than a furnace that has been used extensively.
Dust and lint should be removed from the room air blower wheel and sail switch. A buildup
of dust and lint on the blower wheel can cause the motor to drag and not generate enough
airflow to engage the sail switch. Dust accumulation on the sail switch will restrict the
travel of the actuator arm to where the airflow across the paddle will not sail it in and
complete the valve circuit.
A yearly inspection should be made of all gaskets on the furnace. If any gaskets show signs
of leakage or deterioration, they must be replaced to assure proper pressurization of the
sealed combustion chamber.
Motors used on RV furnaces are permanently lubricated and do not require oiling.
CAUTIONS
1. Never operate the furnace with the electrode wire disconnected or with the electrode
assembly removed from the furnace.
2. Never use a battery charger to checkout an electronic ignition furnace -use a 12-volt
battery.
3. Never use a screwdriver or any part of the electrode assembly while the furnace is in
operation.
4. Be certain that the spark from the electrode never reaches the flame sensor portion of
the electrode assembly. (See Figure 2 for correct gapping).
5. Be sure the electrode assembly screws are snug at all times, especially after the
electrode has been removed and reinstalled.
6. If the module board is found to be defective, it must be replaced - it is not field
repairable. Any attempts to repair the board may alter the board and cause it to
operate in an unsatisfactory manner.
7. Ensure that the gap between electrode and ground is always 1/8". The gap between the
ground and the flame sensor should be approximately twice the gap between electrode
and ground to ensure no sparking to sensor. Sparking to sensor will damage the
module board (see Figure 7).
FURNACE REMOVAL
To replace parts or service the latter generation furnaces, the M and S series, it is
necessary to follow these steps:
1. Disconnect gas and power supply.
2. Remove vent cap assembly (4 sheetmetal screws and one 3" exhaust baffle screw, see
Figure 2).
3. Remove the cabinet front.
4. Remove the shipping screw securing the chamber shield to the cabinet.
5. Pull chamber outward completely to gain access to all controls and parts for adjustment
or replacement.
NOTE: Our service technicians are available to assist you in making repairs or parts
replacements from 8:00 a.m. to 5:00 p.m. EST, Monday through Friday (except holidays),
by calling (615) 775-2131.
SERVICE HINTS, DIAGNOSIS, AND CORRECTIVE MEASURES
FOR THE IGNITION SYSTEMS OF
SUBURBAN 12-VOLT DC ELECTRONIC IGNITION GAS FURNACES
Furnaces with Direct Spark Ignition:
Each step in this operation must be completed in the listed order before the next function
will occur. To properly diagnose a malfunction and correct it, it must be determined at what
step the operation of the furnace failed.
1. When the temperature drops to a set temperature, the wall thermostat contacts close.
2. The fan relay coil is energized in the thermostat relay, completing the circuit to the
blower motor. (Some models equipped with a time delay relay have a 5-25 second time
delay after the thermostat contacts close).
3. The motor starts and, after reaching 75% of its normal rpm, the room air blower wheel
or blade activates the sail switch (microswitch), sending current to the temperature
limit switch (a normally closed switch), and on to the module board.
4. When energized, the module board has a built-in 15-20 second delay, allowing the
combustion air to purge the combustion chamber.
5. The module board supplies a high-voltage spark through high-tension wire to the
electrode assembly, at the same time sending voltage to the gas valve, opening the
valve.
6. Burner ignites.
7. The sensor probe (left probe) signals the presence of flame and the spark stops. (If
flame is not established within 7 seconds, the system closes the gas valve and goes into
lockout).
8. The fan switch (normally open) closes on temperature rise. This does not apply to
models with the time-delay relay. These models will keep the motor running for 3 to 5
minutes by the delay of the relay.
9. The gas valve closes when either the limit switch or thermostat contacts open.
10. The blower motor goes off when the fan switch or time delay relay opens.
Do not use a battery charger to power or test the furnace, as they sometimes provide more
than the 14.5 volts DC that will damage the control module board.
All testing and repair should be done by qualified personnel only. Do not use a screwdriver
or touch any part of the electrode assembly while the furnace is running.
Do not operate the furnace with the high-tension lead wire disconnected or the electrode
assembly removed from the burner access plate.
Do not perform any high-pot tests on this furnace!
A. Thermostat Is Calling For Heat, but The Blower Does Not Run:
1. Check for 12 volts DC at the furnace connection. Correct power source if "0" volts or
below 12 volts DC.
2. Check all wiring for correct polarity (refer to wiring diagram), loose connections and
possible shorts.
3. Check the wall thermostat and wiring for continuity. Be sure thermostat points are
contacting.
4. With thermostat points closed, check for circuit completion across terminals 2 and 4
of the thermostat relay. If no continuity, and wiring to the relay is okay, replace the
relay.
5. If there is continuity across terminals 2 and 4, and the wiring to the motor is okay,
replace the motor.
6. On time delay relay models, allow 3 to 10 seconds for the circuit to be completed to
the motor, as the coil in the time delay relay must heat up to close the contacts.
B. Blower Is Noisy:
1. Check for lint accumulation or debris on the blower wheels or possible damage to the
wheel.
2. Check for excessive voltage and correct, if present.
3. Check polarity. If reversed, the motor will whine and run backward. Air delivery will
be too low to close the microswitch.
4. Wheel may be hitting the housing. Remove the combustion air housing and adjust
the wheel with a T-handle 1/8" Allen wrench. On the room air wheel side, a 90* angle
1/811 Allen wrench will be needed to remove or adjust the wheel.
5. At times, insects such as mud daubers and wasps will build nests inside the
combustion air housing, coming in from the fresh air intake. Remove any foreign
materials and check the combustion air wheel for warpage.
6. If the bushings of the motor are worn and noise persists, replace the motor. Check
both wheels for balance and, if not proper, replace the wheel or wheels.
C. Blower Runs but Burner Does Not Light:
1. Make sure the manual shut-off valve is in the "ON" position.
2. Possible air in the gas lines. Reset the thermostat and cycle the furnace several times
to bleed the system.
3. Check the gas supply. Be certain 11" WC pressure is present while the furnace is
operating. If not, adjust the pressure regulator.
4. Check the wiring for proper polarity.
5. Check for proper clearances down the sides and across the top for return air to feed
the room air blower. Each model furnace has a minimum requirement of return air
outlined in the installation manual. With improper return air, the air volume may not
actuate the sail switch that sends voltage to the module board. All models must have
I" clearance along the sides and across the top except the NT-34, -42 and -45 models,
which require 2" on the sides and top.
6. Check the ducting of the furnace to determine proper airflow away from the furnace.
See the installation manual for correct duct requirements. If the ducts have severe
bends or kinks in the hose, the airflow may be restricted enough to cause the
microswitch to bounce back from the volume of air hitting the cabinet front. This again
will not allow voltage to the module board. Many times, a furnace will operate
satisfactorily on the bench or without the cabi.net door on the unit. Check for
microswitch energizing when this happens. Correct ducting for proper airflow and/or
replace the microswitch (could be stiff).
7. If the furnace will ignite with the cabinet front off, but will not with the cabinet front
secured, and the ducting is free, check the gaskets around the burner access door. If
the burner access is not properly sealed, air may be deflected from the cabinet front
into the burner area, thus disturbing air and gas mixture for proper ignition.
8. Check for 12 volts DC on both sides of the limit switch. If there is voltage on one side of
the switch, but not on the other, replace the switch. If voltage is present on both sides
of the switch, proceed to Step 9.
9. Check for 12 volts DC at the power terminals on the module board (refer to the wiring
diagram - red wire positive and yellow wire negative). If no voltage, check wiring from
the limit switch. Correct wiring.
10. If sparking is audible approximately 15 seconds after the blower starts, but no ignition,
check the high-tension wire for continuity, grounding and secure connections. Repair if
necessary.
11. Make sure electrode is not grounding to burner, the gap between the spark probe and
the ground probe is 1/8" (see Figure 7), and the ceramic insulator is not cracked,
broken or sooted. Adjust, clean or replace as necessary.
12. If sparking is not audible, check the module board and observe if the neon bulb on the
circuit board is flashing during the trial for ignition. Remove the white plastic wire
connector from the circuit board. Clean the board terminals on back with a soft rubber
eraser. Check the little pin terminals in the plastic connector for good contact. Reinstall
connector. If the tube does not flash, replace the module board.
13. Check for voltage at the gas valve during the trial for ignition. If no voltage and
connections are okay, replace the board.
14. If the valve does not open with 12 volts DC present, replace the valve.
15. If the electrode sparks and the valve opens but the burner will not light, check the gas
supply for 11" WC pressure to the main burner office. If 11" is present to the valve but
not to the orifice, replace the valve. Check vent and air intake installation. Check the
main burner for correct relationship to the electrode assembly (burner should be 3/16"
from the spark probe and sawports or charge ports directly under the spark gap).
Check main burner for soot build-up in ports (see Figure 8). Clean cast-iron burners by
passing a hacksaw blade through the sawports, being careful not to enlarge the
openings. Wire brush stainless tube burners to remove build-up.
16. Check the combustion air wheel for proper rotation. Insufficient air will not sustain the
flame if the wheel is installed backward.
D. Burner Ignites But Shuts Off:
1. Check to be certain that flame sensor is over slots in the main burner and that the
main burner flame is burning against the tip of the flame sensor - adjust by bending
the sensor probe. NOTE: Sensor probe should be in the inner blue cone of the
burner flame, approximately 1/4" to 5/16" above the burner.
2. If the burner still goes off and into lockout, check the wire connections at the flame
sensor and the module board.
3. If wire connections are okay, check for continuity through the flame sensor wire.
4. If the continuity of the flame sensor wire is okay, check with microamp meter in
series with the flame sensor and flame sensor wire to be certain the flame sensor is
generating at least seven microamps within seven seconds after the burner is ignited.
Connect the meter as follows: (+) to sensor wire, (-) to sensor probe. Adjust the
position of the sensor probe, check air adjustment and check for carbon deposits on
the sensor probe if the reading is less than seven microamps.
5. When the flame sensor circuit is generating at least seven microamps, but the burner
still goes off and into lockout, replace the module board.
E. Main Burner Will Not Shut Off:
1. Make sure the thermostat and contact points are open.
2. Check voltage at the valve terminals - should be "0" voltage.
3. Disconnect leads from the valve. If it does not shut off, replace the valve.
F. Continuous Sparking Of the Electrode:
1. Check the sensor wire and high-tension wire for good connections. Check for
separations in either wire - repair or replace. Check to be certain that the flame
sensor probe is over the slots in the main burner and that the main burner flame is
burning against the tip of the sensor probe. Adjust by bending the probe or
repositioning the burner. The burner flame should be a hard blue flame, lying down
on the burner.
G. Erratic Blower and/or Thermostat Operation:
1. Check the wiring to the furnace.
2. Check for shorts in wiring.
3. Disconnect leads to the gas valve. If the furnace runs properly with the leads
removed, replace the gas valve.
4. If the fan continues to run after the thermostat is satisfied and the burner goes off,
the fan switch or the time delay relay is defective and needs to be replace. If the fan
comes back on after a short period of time (within one minute) and cuts off and on,
replace the fan switch.
5. If the thermostat is not satisfied (points still made) and the fan cuts on and off,
replace the motor (thermal overload switch is defective.)
H. Unit Going Into Lockout Only Once In A While:
1. Thoroughly check the electrode and burner air adjustments.
2. Lockout can occur if the gas pressure fluctuates at the time that the thermostat calls
for heat. Pressure fluctuations can be caused by a malfunctioning gas bottle
regulator, and obstruction or a kink in the gas line, or moisture in the gas bottle
regulator or in the gas lines. It is difficult to check for these fluctuations that will not
noticeably affect any other appliance in the coach. However, isolating the furnace
from the coach gas system will determine if the gas system is responsible. This
isolation procedure can be done by connecting a separate upright bottle, regulator
and gas line directly to the furnace, eliminating the coach gas system. If the
occasional lockout still exists, then the furnace should be thoroughly tested to
determine the cause; however, if the furnace works properly on this separate system,
then the coach gas system should be checked.
3. Check the furnace return air and warm air discharge to be certain sufficient airflow
is present to engage the microswitch every time.
4. Check the microswitch to be sure it moves freely.
5. Remove the electrode and burner and clean them thoroughly.
6. When moisture in the gas system is suspected as being the problem, especially where
the horizontal type gas bottle is being used, the following steps should be taken to
prepare the gas system against further moisture problems.
Corrective Measures:
1. Disconnect gas bottle and drain it completely dry of all gas and all moisture.
2. Disconnect and blow out all gas lines completely dry.
3. Check pressure regulator on the gas bottle. Replace if necessary.
4. Add the drying agent. One half pint of methanol alcohol per 100 pound bottle
capacity is recommended.
Precautions:
1. Never fill the gas bottle over 80%.
2. Do not use the gas bottle completely dry to avoid using up the drying agent.
We have found the above procedures to be effective in solving most occasional lockout
problems; especially where the horizontal-type gas bottle is used. All of these steps must be
performed as described for the preparation of a contaminated gas system to be 100%
effective.
1. Repeated Module Board Failures:
1. Check to be certain that the electrode spark is not sparking against the flame sensor
portion of the electrode assembly.
2. Check to be sure the module board or high voltage wires are not shorted to the
chamber wrapper or other furnace parts.
3. Be sure the insulator covering the electrode wire connection on the coil of the module
board is in place and the insulator behind the module board is in place.
4. Check the high voltage - 14.5 volts DC maximum
High voltage could intermittently produce volt surges or AC spikes that could damage
the module board. Supply voltage to furnace should be checked if repeated module board
failures occur.
5. Be sure duct connections to the furnace are airtight. Seal duct collar connections to the
furnace cabinet with duct tape, if necessary, to prevent hot air leakage. No air leakage
should exist anywhere in the duct system, especially at connections on the furnace
cabinet.
6. The high voltage electrode wire should be routed away from any 12 volt wires (except at
grommet for sensor wire). No wires should pass over the module board.
7. Be sure the sensor wire terminal is tightly affixed to the sensor probe.
8. Be sure high voltage electrode wire is in good condition and properly positioned onto
pierce point electrode or spade terminal.
