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Heat does not come on. Furnace light blinks 3 times, indicating faulty pressure switch. I jumped the pressure switch, which caused the ignition filament to light up. However, it quickly shut off again. No gas comes out so the blower never starts up. Have checked the lines, they appear to be clear. Furnace is only about 6 years old. Any idea what is causing this and how to fix it?

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  • mbt110 Sep 17, 2010

    Thanks for the feedback; is there a test I can run to test if the exhaust fan circuit is the problem?

  • mbt110 Sep 17, 2010

    Heat does not come on. Furnace light blinks 3 times, indicating faulty pressure switch. It does not appear that the exhaust flue motor turns on when the furnace turns on. I jumped the pressure switches, which caused the ignition filament to light up and the gas flames to ignite (normal operation). All pressure hose lines appear to be clear and there does not appear to be a drainage backup. Furnace is only about 6 years old. Am I right in thinking that the problem is either a bad exhaust flue motor or the connection to the motor? How do I isolate one or the other?

  • mbt110 Sep 17, 2010

    Thanks. I have made sure all connections (including that probe) are clean and free of debris. I actually got the furnace to fire up after jumping both pressure switches (I was previously only jumping the first one). The exhaust flue fan/motor does not seem to be coming on when it should - I think that may mean it is faulty. Or, that the connection to that is bad. Do you think that's right?

  • mbt110 Sep 19, 2010

    Thanks, much. Some additional information - the fan doesn't come on at all, regardless of jumpering sequence. Also, when I jump the pressure switches correctly (in the order you mention), the system fires up normally.


    These things lead me to believe that the flue fan motor (or possibly the connection to it) is bad. Can you confirm if this sounds right?

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Hi,
When jumpering the pressure switch ...you need to let the blower start and then apply the jumper... the brain needs to see the proper sequence or it will not allow the gas valve to open...
and when it shuts down the jumper must come off of the switch and be reattached at the proper time for start up to happen again... this is done for safety reasons...

heatman101

Posted on Sep 18, 2010

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Check the inside of the pilot. There is a probe inside. This probe needs to remain clean. It provides a certain temperature that is needed to keep the pilot lit. Make sure that probe is clean.

Posted on Sep 17, 2010

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  • Master
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I believe your problem might be your exhaust fan circuit . A exhaust flue fan turns on when the heat demand circuit is closed. The fan usually will be running for about ten seconds before enough pressure is built up in the pressure diaphragm assembly . It is connected to the exhaust fan by a small vacuum hose . Sometimes debris will accumulate in the hose itself preventing the pressure build up in the diaphragm switch assembly . It is important that the circuit operates correctly as to not allow carbon monoxide to Backdraft into your home .

Posted on Sep 17, 2010

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  • Cool Blue Services
    Cool Blue Services Sep 17, 2010

    try disconnecting the hose between the pressures switch and the fan assembly . Blow through the hose to make sure it is free and clear . Then check the small connection on the motor to be sure the hole is clear . You can use a paper clip or a piece of wire to clean it out . I'm assuming that the exhaust motor is functioning correctly . If it is not , replace the exhaust flue motor.

  • Cool Blue Services
    Cool Blue Services Sep 17, 2010

    regarding testing the circuit itself , it only has two parts . The exhaust flue motor / blower assembly and the pressure switch assembly which is nothing more than a diaphragm and a switch . Of course , there is the little vacuum hose that connects between the blower case and the diaphragm switch which is also called the pressures switch . When the heater first comes on , the exhaust blower motor should come on first. Once enough pressure is built up , the diaphragm presses against the switch closing the pressure switched circuit . The fan runs continuously as long as the heater is running . Do not confuse the exhaust fan with your air Handler blower which circulates the air through your home . Either the pressure circuit works or it doesn't . If the exhaust fan works , it can only be a plug vacuum hose or a bad diaphragm pressure switch .

  • mbt110 Sep 17, 2010

    Ok, checked all the lines. They seem clear, no apparent leaks in the hoses or anything either. I don't hear a fan turning on when I turn on the furnace, either. I believe I was improperly jumping the pressure switch (there are two on my model). I was previously only jumping the first one, which I believe may be why the ignition turned off. I tried jumping both, in the proper sequence, and the system fired up just fine. Am I right in thinking this points more firmly to the exhaust flue motor being the problem?

  • mbt110 Sep 17, 2010

    Ok, checked all the lines. They seem clear, no apparent leaks in the hoses or anything either. I don't hear a fan turning on when I turn on the furnace, either. I believe I was improperly jumping the pressure switch (there are two on my model). I was previously only jumping the first one, which I believe may be why the ignition turned off. I tried jumping both, in the proper sequence, and the system fired up just fine. Am I right in thinking this points more firmly to the exhaust flue motor being the problem?

  • mbt110 Sep 17, 2010

    Ok, checked all the lines. They seem clear, no apparent leaks in the hoses or anything either. I don't hear a fan turning on when I turn on the furnace, either. I believe I was improperly jumping the pressure switch (there are two on my model). I was previously only jumping the first one, which I believe may be why the ignition turned off. I tried jumping both, in the proper sequence, and the system fired up just fine. Am I right in thinking this points more firmly to the exhaust flue motor being the problem?

  • mbt110 Sep 17, 2010

    Ok, checked all the lines. They seem clear, no apparent leaks in the hoses or anything either. I don't hear a fan turning on when I turn on the furnace, either. I believe I was improperly jumping the pressure switch (there are two on my model). I was previously only jumping the first one, which I believe may be why the ignition turned off. I tried jumping both, in the proper sequence, and the system fired up just fine. Am I right in thinking this points more firmly to the exhaust flue motor being the problem?

  • Cool Blue Services
    Cool Blue Services Sep 17, 2010

    CHECK THE PRESSURE HOSE BETWEEN THE FLUE EXHAUST BLOWER AND PRESSURE DIAPHRAGM ASSEMBLY. MAKE SURE IT IS CLEAR- YOU CAN BLOW THROUGH IT...CHECK THE BLOWER HOSE NIPPLE TO BE SURE IT IS CLEAR. USE A PAPER CLIP TO CLEAN THE HOLE OUT. NOW, HOOK THE HOSE BACK TO THE PRESSURE SWITCH (UNIT OFF AND NO POWER TO UNIT) BLOW INTO THE END OF THE HOSE AND CHECK THE 2 TERMINALS WITH A OHM METER SET ON RX1 OR LOWEST SCALE...IT SHOULD READ "0 OHMS (CLOSED CIRCUIT) WHEN YOU BLOW INTO IT--- AND INFINITE (OPEN CIRCUIT) WHEN YOU TAKE YOUR MOUTH AWAY. WHEN ALL IS HOOKED UP AND RIGHT, THE BLOWER COMES ON FIRST, THEN AFTER A FEW SECONDS, WHEN THE PRESSURE IN THE BLOWER BUILDS UP, THE SWITCH CLOSES AND THE IGNITER FIRES UP. THEN THE GAS VALVE OPENS AND GAS IGNITES. WHEN THE SET HEAT IS REACHED IN THE BURNER BOX, THE AIR HANDLER BLOWER COMES ON TO HEAT THE HOME. SOME UNITS HAVE A THERMOCOUPLE NEXT TO THE IGNITER. IF THE UNIT FAILS TO STAY LIT, BUT EVERYTHING ELSE CHECKS OUT, REPLACE THE THERMOCOUPLE.

  • Cool Blue Services
    Cool Blue Services Sep 18, 2010

    Good evening to you . I believe we have isolated the problem . Yes , your exhaust fan motor is probably faulty . However , as you seem to be knowledgeable in the AC electrical areas , I will walk you through one more test before we condemn the motor. The motor is either 110 or 220 VAC. I'm sure you can read the label on the motor to determine the voltage . Regardless , using your DMM or voltage meter, test the leads the supply the voltage to the motor for voltage when the unit first comes on (heat demand as energized by the thermostat). It is important to take his reading when the unit energizes as the failsafe features will cause it to shut down within seconds . If you read the proper voltage then yes replace the exhaust motor assembly. To purchase a new motor , you'll need the model number off of the air Handler / furnace to be sure and get the right part . Sometimes you can use the motor tag number itself , but I would recommend using the air handler model number. Good luck & make sure you disconnect the power to the unit before replacing the motor .

  • Cool Blue Services
    Cool Blue Services Sep 20, 2010

    I posted a comment yesterday but I don't believe it went through . Any ways , you can test for voltage at the connector for the exhaust motor . There should be an inline connector you can test for voltage at when you first turn on the furnace . There should be line voltage to that motor soon as there is a demand for heat from the thermostat via the circuit board controller . If you have line voltage to that motor , and the motor is not operating , replace the motor . Use the model number of of the furnace air Handler unit to be sure you get the correct replacement motor .

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