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Gas valve w/safety draft inducer proof?

RickDelta
RickDelta Member Posts: 483
in Gas Heating

Hello HeatingHelp.com community! : )

One of my motel owner customers asked me if I could come take a look at his gas/hot air condensing furnace at his home. It was operating intermittently day and night (his normal service company said he needs to replace the entire system).

The problem was simply a leaf had got ingested into the fresh air intake, now sitting on top of the unit, blocking the air flow (outside screen was missing from intake pipe).

That was an easy fix! : )

In checking the vacuum hoses and vacuum diaphragm switch, I noticed that one vacuum hose branched over to the main gas control valve. I never saw this on a gas valve before!

Is this a new gas valve safety shut-off feature? ….. or has this been around for a long time?

(i'm not an HAVC guy!)

Comments

  • 109A_5
    109A_5 Member Posts: 1,587

    Hello RickDelta,

    I think it applies the burner chamber pressure or intake pressure to what was the atmospheric pressure side of the gas valve's pressure regulator diaphragm. Probably provides more accurate and/or stable gas pressure regulation to the burner. I suppose it could reduce the gas pressure if the intake pressure is incorrect.

    Love the easy fix ! kind of makes you wonder why it was not an easy fix for the other guy.

    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
    RickDelta
  • RickDelta
    RickDelta Member Posts: 483
    edited November 18

    @109A_5

    ….. yea! you could only see it with a mirror from the front.

    I didn't have a manometer , so I made one on site! Got some clear tubing he had laying around and made a "U" -tube manometer.

    Marked the water level during a working run time. When the unit would fail, it showed a significant drop in vacuum.

    I'm guessing that the vacuum line to this gas control valve is just a secondary main gas shut-off valve ….. should the exhaust fan fail or is blocked.

    Out of curiosity,

    A few days later, I called the manufacture of the gas valve, (Robertshaw) I think, ….. the tec said I have no information about that vacuum port! .. But did say they make that valve only for that HAVC company (Lennox).

    …..Interesting! : )

  • mattmia2
    mattmia2 Member Posts: 10,836

    Is it a modulating or 2 stage furnace?

  • RickDelta
    RickDelta Member Posts: 483
    edited November 18

    @mattmia2

    I'm sorry! ….. Its been over a year now ……. I don't remember : (

    It was just a single gas valve body.

  • mattmia2
    mattmia2 Member Posts: 10,836

    The pressure will change with the speed of the inducer, i was thinking maybe that is how it modulates.

  • RickDelta
    RickDelta Member Posts: 483
    edited November 18

    @mattmia2

    I hear ya!

    …… but this same vacuum line is driving the "proof" exhaust flow diaphragm switch.

    The vacuum port on this gas valve had a barbed hose fitting.

  • RickDelta
    RickDelta Member Posts: 483
    edited November 18

    Found this:

    Normally those three lines go to:

    1) burner box

    2) collector box which is what the exhaust fan/inducer attaches to

    3) VERY important, it goes to a port on the gas valve. Usually there is a barbed fitting on the side of the valve and the hose goes there. It is supposed to modulate the gas valve pressure if the intake pipe starts to plug with snow or debris. The vacuum in the burner box sucks on the gas valve diaphram and the pressure switch at the same time. Not all furnaces do it that way but if they have a closed burner box they should. If the burners are wide open when you remove the front door then there is no box to hook it up to.

    https://www.diychatroom.com/threads/vacuum-line-for-pressure-switch-pulling-atmosphere.366330/

    VacGasPort.png

    mattmia2

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