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  1. #1
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    Double Hosed Pressure Switch Gas Furnace 90+

    Hi All.
    I have a American Standard 90+ AUX120C960C1, made in 1999, up flow gas furnace with both outside air intake and exhaust, and blower fan at the bottom of unit. The pressure switch has 2 air hoses connected to it. One goes from the back of the switch to the bottom of the burner box area, and the hose on the front of the switch tees to the gas valve- furnace side of valve body, and the other side of the tee goes to the burner box, right under the venturis- the chamber where I can see the flame and watch the ignition process. My switch stopped working right. I found by either blowing or sucking on the various tubes I could get it to kick on again. Eventually I discovered by leaving the front tee'd tube slightly loose on the pressure switch, the furnace runs fine.

    My question is: how does this double hosed pressure switch work? It seems to monitor the gas flow, and dump the measured gas into the ignition side of the burners. It also measures the suction side of the burners, closer to the exhaust area. As a safety issue, this design doesn't make sense to me, because if the hose is accidentally disconnected, it would vent gas into the house. Never mind that I partially disconnected it. It still has only one micro switch, the supply wire on one terminal, the other safety switches/sensors on the other leg, and a ground wire attached to the mounting bracket, which is connected to all furnace metal in the end.

    I do not get a abnormal blinking code on control board? Should I just replace the valve? Model is Tri Delta FS6002A-1669. I cannot find one for a reasonable price, unless someone can guide me to one or a suitable substitution.

    As an aside, my second furnace which is identical accept for using room air for combustion, only has one hose connected to the end of the flame path as far as I can judge. Thanks for any answers, Malcolm

  2. #2
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    Quote Originally Posted by Malcolm View Post
    Hi All.
    I have a American Standard 90+ AUX120C960C1, made in 1999, up flow gas furnace with both outside air intake and exhaust, and blower fan at the bottom of unit. The pressure switch has 2 air hoses connected to it. One goes from the back of the switch to the bottom of the burner box area, and the hose on the front of the switch tees to the gas valve- furnace side of valve body, and the other side of the tee goes to the burner box, right under the venturis- the chamber where I can see the flame and watch the ignition process. My switch stopped working right. I found by either blowing or sucking on the various tubes I could get it to kick on again. Eventually I discovered by leaving the front tee'd tube slightly loose on the pressure switch, the furnace runs fine.

    My question is: how does this double hosed pressure switch work? It seems to monitor the gas flow, and dump the measured gas into the ignition side of the burners. It also measures the suction side of the burners, closer to the exhaust area. As a safety issue, this design doesn't make sense to me, because if the hose is accidentally disconnected, it would vent gas into the house. Never mind that I partially disconnected it. It still has only one micro switch, the supply wire on one terminal, the other safety switches/sensors on the other leg, and a ground wire attached to the mounting bracket, which is connected to all furnace metal in the end.

    I do not get a abnormal blinking code on control board? Should I just replace the valve? Model is Tri Delta FS6002A-1669. I cannot find one for a reasonable price, unless someone can guide me to one or a suitable substitution.

    As an aside, my second furnace which is identical accept for using room air for combustion, only has one hose connected to the end of the flame path as far as I can judge. Thanks for any answers, Malcolm
    The hose going to the gas valve is letting the vent gas go to the burner
    The gas valve does not vent enough gas to worry about.Each time the valve opens it has one little squirt and that it. What the other two hoses are doing,
    is measuring the difference in pressure across the heat exchanger..
    So this can mean blockage in the heat exchanger, crack in hose, weak pressure switch, or plugged drain, drain not draining fast enough, blockage in one of the stacks. Later Paul

  3. #3
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    I almost understand..
    Quote Originally Posted by paul52446m View Post
    The hose going to the gas valve is letting the vent gas go to the burner
    What is "vent gas"? Is it just getting rid of the tiny squirt of gas?
    So 2 hoses required to measure pressure drop across exchanger, opposed to single tube just measuring a fixed pressure drop? But the tube going to the gas valve is tee'd to the pressure switch and the airspace near the burner box. Where is the vent gas coming from? Does gas or air flow through the pressure valve, or is it sealed so no flow between the two sides? I have read up a little on the gas valve connection, seems to have something to do with making sure switches work right independant of how long 2 combustion air pipes are, but since they are tee'd I can't see it working for that. Thanks for quick reply.

  4. #4
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    Quote Originally Posted by Malcolm View Post
    I almost understand..

    What is "vent gas"? Is it just getting rid of the tiny squirt of gas?
    So 2 hoses required to measure pressure drop across exchanger, opposed to single tube just measuring a fixed pressure drop? But the tube going to the gas valve is tee'd to the pressure switch and the airspace near the burner box. Where is the vent gas coming from? Does gas or air flow through the pressure valve, or is it sealed so no flow between the two sides? I have read up a little on the gas valve connection, seems to have something to do with making sure switches work right independant of how long 2 combustion air pipes are, but since they are tee'd I can't see it working for that. Thanks for quick reply.
    Ok the gas valve has a die-a-frame in it and when the gas valve opens it has to relieve the pressure on the top of the die-a-frame,so when it does this it is just a real small squirt. The inducer fan is pulling a vacuum on the hose that covers the burner box so that small amount of gas is pulled into the burner box. The other hose also has a vacuum on it but one is moue than the other so that makes the switch. later Paul

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