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Old 06-14-2016, 08:36 AM   #1
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Furnace Problem

Iím having a problem with my Suburban furnace (sf-20) model year 2005. When you set the thermostat to have it come on, after the fan runs to clear out the ducts the burner ignites but stays light for only a few minutes then the burner goes out but the fan runs for several minutes then the burner goes on again but as before only for a few minutes. This cycle repeats all night long, the fan never shuts off as the temperature never reaches the thermostat setting and we end up with dead batteries. As a note, I replaced the limit switch but it made no difference.

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Old 06-14-2016, 08:57 AM   #2
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From your description it may be going off on the high limit switch. Are you sure the fan you hear is the actual furnace fan and not the inducer fan? Is there actual air blowing thru the vents?

The sequence of events should be 1. stat calls for heat. 2. inducer fan comes on to clear flu. 3. spark ig comes on gas valve open burner ignites. 4. Temp comes up fan switch closes furnace fan comes on.

I would check for any loose wires and then check the fan switch and limit switch for proper operation

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Old 06-14-2016, 11:50 AM   #3
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RV furnaces only have one motor for the warm air supply and forced draft.

Here is the sequence of events.

Time Line Description - Sequence of Events
Thermostat Calls for Heat

The wall thermostat controls the operation of the furnace by reacting to room
temperature, this allows current to flow through the On/Off switch to the module board.
The module board constantly checks for a minimum 9.5 volts. If there is not 9.5 volts,
the module board will go into a stand by mode until adequate power is supplied. It will
then resume normal operation.

Upon a call from the thermostat, the module board thermostat circuit will go active. The sail switch circuit is verified as being open. The blower output is energized. Blower
motor starts.

15 Seconds Purge Cycle

The module board will then verify that the sail switch circuit is closed and motor is up
to speed. If this circuit remains open for 30 seconds after the blower motor starts, the
module board will go into lock out and shut down the blower motor.
The module board checks that the gas valve relay contacts (which are located on the
module board) are open before the ignition sequence starts.
The board has a pre-purge timing circuit of (approximately 15 seconds). This allows the
chamber to purge.

7 Seconds Ignition Cycle and Flame Sense

The module board will energize the gas valve and enable the high voltage spark output
to the electrode for 7 seconds of ignition time.
The module board will then check for flame sense to verify successful lighting of the
main burner flame. Sparking will then be terminated and the gas valve and blower
outputs will remain energized.
If ignition is successful the module board will monitor the flame sense, sail switch and
limit switch circuits, and the thermostat inputs during the heating period.
The flame is sensed through the spark wire and electrode. Therefore, it is essential that
the electrode is properly positioned in the burner flame.

2nd and 3rd Ignition Cycles if Required

3 Try Ignition Board

If the flame is not sensed after seven (7) seconds, a second 15 second purge cycle will
begin followed by a second Trial-For-Ignition sequence. After three (3) Trial-For-Ignition attempts with no ignition of the main burner, the module board will de-energize the gas valve immediately and blower will run for 3 minutes and then shutdown in lockout.

Heating Cycle

If during the heating cycle, the limit switch circuit opens and remains open for 5
minutes, the module board will go into lock out and shut down the blower motor. If this
occurs, the thermostat will need to be reset for the furnace to operate.

90 Second Shut Down

When the thermostat has reached its set point and the demand for heat ends, the gas
valve will be de-energized and the flame will go out. The post purge period of 90
seconds begins. When it times out, the blower motor output is removed, and the blower stops
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Old 06-15-2016, 12:18 PM   #4
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Thank you for everyone’s help. I have determined that the high limit switch is kicking off the burner. I wired the limit switch together as a test and the burner was not shutting down. The limit switch is stamped 150F so I tested it in hot water and when the temperature got to 150 you could hear it click and the continuity was broken. It would seem that the fan is not clearing the plenum enough so now I will check the duct pipes and look for a blockage. The fan seems to run strong.
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Old 06-15-2016, 12:26 PM   #5
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the high limit switch can be a pain mostly because of where they get mounted to the heat ex-changer and then how it was ducted in . i have seen where the H-limit was put of top of the heat exchanger and then the furnace was ducted into the floor causing the h-limt to trip all the time cause it was not getting good air flow
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Old 06-15-2016, 12:29 PM   #6
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something else to consider is the flame sensor will sometimes get carboned up and not sense flame like it should a cleaning very lighly with a lite abrasive emery or even paper will clean it up

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