Negative gas valve

MikeJ

Please see the photo, wondering will gas come out of a negative gas valve if unhooked from the inducer. When you apply power to the valve? Have never done this check before, I have spark, have gas to valve but not lighting.
Unhooked the valve from inducer, apply power and no gas came out. Bad valve?blocked vent pipe? or ??
https://www.screencast.com/t/ClTssCgcDc

HVACNUT

The inducer must be connected. With a manometer on the outlet side, it should pull a good negative on prepurge, and move more towards 0 on ignition and run.

Tim McElwain

Do you have gas pressure on the inlet side of the valve?

MikeJ

Yes, have 6" on inlet side

MikeJ

@HVACNUT no gas at all is going to come out of valve, must be blocked discharge vent pipe. You think a little gas would come out, thank you

ratio

No gas should come out. The negative means that the inducer sucks the gas out. No inducer suckage, no gas.

EBEBRATT-Ed

Looks like a Trane negative pressure burner on a RTU??

If you have gas at the inlet of the gas valve yes gas should come out.

High pressure goes to low pressure always.

You will get more gas when under a negative pressure on the outlet but should get gas if the valve is good

MikeJ

hopefully find out tomorrow, have a new gas valve and have a plan to fine out if flue is block. i'm sure there is ice in the flue but don't know if its completely block. Just got back from Menards with heat gun and other supplies. Talking -20 Sunday night

rbeck

If the vent was blocked I would think the pressure switch would shut you off.

DZoro

Gas wont come out without the negative draw on the valve. Sounds like venting issue. There is a damper/flapper in the air inlet by the gas valve make sure it swings freely. New install? or older install?

EBEBRATT-Ed

6" wc at inlet, outlet disconnected how can gas not come out?

ratio

Outlet pressure is above -.2 in wc, gas valve regulator tries to throttle down, shouldn't try to pass gas until outlet drops below -.2 in wc. I suppose there may be some leak through, but I would think that's just what leaks through.

NB: I have not actually tried this, and may be completely wrong. But it's what I'd expect. Remember that it's just like if the manifold pressure rose above (say) 3.5 in wc, the valve would try to stop it down.

EBEBRATT-Ed

I agree more gas will flow if the outlet is in a negative. But I would think some gas will flow enough to troubleshoot the valve

MikeJ

Found out had a block flue pipe, and gas will not flow through a neg gas valve when it is unhooked. Had the valve unhooked, from the inducer, gas peruser on the inlet, no gas came out when power was apply.
After unblocking the flue pipe of ice, Gas valve worked beautifully,
HVACNUT it worked just the the way you described, after unblocking it.

Thanks to all for the input.

psb75

Occam's Razor: check the for the simplest, more obvious problem FIRST. That is, don't go further up the "troubleshooting chain" to more complex problems...before eliminating the simpler problems first.
Yours would be an example of the latter.

Tim McElwain

There can be no gas on a negative pressure valve unless a clear path of suction from the inducer and a clear path through the vent to outdoors. Usually the pressure switch will hold it off.

I often tell techs if a sidewall vented appliance is not working try running it with out the flue attached if it will run then the flue and air intake is the problem.

NY_Rob

What was blocking the flue BTW?

Edit... I see it was ice.

How do you plan on preventing that from occurring again?

MikeJ

psb75 sounds easy now, after the fact. Vent on the roof could not climb on to roof. Had spark, had inducer on, had power at valve would not light. Could be block vent or bad valve, question was how to tell if valve was good or bad.

Insulated the vent pipe in the attic to hopefully prevent it from happening again.

JUGHNE

So with exhaust plugged, the inlet pressure switch must have closed in order to get spark and power to gas valve.
How did the inlet pressure switch close if discharge plugged......perhaps by compressing the air in the discharge??

So the only safety method to keep gas flow off if you have an exhaust blockage is lack of negative pressure on the outlet of the fan?
What if you have a partial blockage which allows some flow but not enough for proper burning?

ratio

I think that with the negative gas valve, at least with the Tranes that I'm familiar with, the burner system is balanced to be in tolerance; that is, we suck enough gas to burn with the air we gots. I know for a fact that they modulate via a 3Φ inducer motor—more suckage = higher firing rate. Might not be the cleanest burn below low fire though.

Jamie Hall

ratio said:

I think that with the negative gas valve, at least with the Tranes that I'm familiar with, the burner system is balanced to be in tolerance; that is, we suck enough gas to burn with the air we gots. I know for a fact that they modulate via a 3Φ inducer motor—more suckage = higher firing rate. Might not be the cleanest burn below low fire though.

Sounds suspiciously to me like one of them thar old-fashioned carburetors...

ratio

Now you're getting all analogue on us.

MikeJ

There is no inducer proving switch, there is a drainage blockage switch. It was a KHN85 Lochinvar boiler.
Seems like with the exhaust pipe block I should of been getting a Fan Speed lock out.
Never did get that, the inducer fan was runing, gas valve would energize, the inducer was not able to pull gas out of the gas valve with the blockage.
This is out of the service manual for Lochinvar
https://www.screencast.com/t/GLMSOeSf

DZoro

The boiler systems with the negative gas valve have different operating characteristics. No air flow, No gas. Safety is built into basically the negative pressure gas valve. Fan was doing its thing at the correct rpm, but it didn't know it wasn't moving the air. In this case only the gas valve was aware of this.