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90% furnaces

drhvac Member Posts: 190
usually have a hose going from the burner box then teed, one side going to the pressure switch, then the other one going in to the manifold side of the gas valve. Could anyone tell me what exactly this hose is doing? I know it has something to do with gas valve regulator, but I don't understand it. It is a low pressure, much lower than the lowest pressure the gas valve is sending to the manifold. Thanks


  • unclejohn
    unclejohn Member Posts: 1,833
    Tim Will Know

    But my guess is it's just a vent line and not sensing pressure. Carrier is using the same valve on there 80% & 90% single stage and the instructions say on the 80% that the tapping for the hose does not need to be sealed.
  • JStar
    JStar Member Posts: 2,752

    I've always been told that it equalizes the pressure between the gas valve and the sealed combustion box. In an 80% furnace, there is no sealed combustion box, so the gas valve and burners are naturally equalized.
  • Ironman
    Ironman Member Posts: 7,376
    That's It

    Joe's got it. The gas regulator is maintaining pressure relative to the atmospheric pressure at the burner. Since the burner is in a slightly negative atmosphere from the induced draft, the regulator needs to "sense" pressure at that point.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Tim McElwain
    Tim McElwain Member Posts: 4,621
    Very good "grasshoppers" Regulators on appliances

    always have to have the vent (small hole in the regulator) no gas coming out at the same atmospheric pressure as the burners them selves. Soon atmospheric burners (air from with in the room) the vent is open (may have a vent limiter).

    On sealed chambers you will almost always find a hose going from the built in regulator on the gas valve to the combustion chamber so they are at the same atmospheric pressure. The vent in theory is so the regulator can "breathe".
  • drhvac
    drhvac Member Posts: 190

    to help me understand this better, maybe by answering this question would help me. What would happen if that hose wasn't there? I'm assuming it would affect the manifold pressure a little?
  • SWEI
    SWEI Member Posts: 7,356
    It would make

    the gas flow match the ambient air pressure instead of the CAZ pressure.