Adjusting the Offset

Alan (California Radiant) Forbes

I've always been fearful of adjusting the bias/offset on a negative pressure gas valve. HTP used to tell me not to touch it on Munchkin boilers as it would kill the gas valve if you adjusted it too much.

@GroundUp has said: "If you adjusted the low fire screw far enough to raise the high fire reading by .7%, you've created bigger problems."

And just yesterday, I spoke to a techie that said to not be afraid of making adjustments.

So, what's the big deal? What does the adjustment screw actually do and how can it be damaged irreparably? 

mattmia2

I suspect it isn't that it is damaged but that it is so far out of adjustment that it is very difficult to get it back in the right range without a test rig of some sort. BTW the manufacturers of that valves have much more in depth documentation than what HTP gives us.

GroundUp

I don't know that I'd agree with it "killing the gas valve", but as mattmia2 and I both alluded above, if you adjust it so far that it's making a .7% difference while in high fire, it's so far out of whack that it's probably not going to run well at low fire anymore (if it even lights at all). Some manufacturers only want you touching one screw which works well for that particular piece of equipment, but the Noble in question when I made the above statement is not one of those machines.

The 2.5MM throttle screw on those units is to be adjusted at and for high fire only. The 4MM bias/offset screw is to be adjusted at and for low fire only. If you were to adjust the bias far enough to make a significant difference at high fire, the low(er) fire is going to be affected more than you would have seen at high fire which causes the mixture to be so poor that they often won't light at all. I made this mistake myself when I switched from HTP/Laars to Lochinvar a few years back (since they are a one screw unit and I was too dumb to read the manual for the Noble). There is nothing to be afraid of in regards to adjustment of the bias, it just needs to be done with care. Once you get it so far out of whack that it's not lighting anymore, it's nearly impossible to find that spot again as 1/8 turn can often move the CO2 several % at low fire.

hot_rod

One time I had to mess with that adjustment on a Munchkin. I got tech support on the phone to talk me through it. Some tech support personal can FaceTime now. That can be a plus for a troubleshooter on the job.

HVACNUT

I had a Weil McLain Rep on a site once and he told me if low fire is off and you break the seal and adjust it, do it in eighths on the turns. Then when its good, recheck High fire. It's easy to think the turns aren't making a difference because the analyzer needs time to analyze. And before you know it, you over or under shot by a ton. Nice and easy. Keep a load on the boiler and don't rush it.

GGross

I took apart a similar gas valve from triangle tube a few years ago to see what the adjustment screw was all about. There was a little air pillow/diaphragm back there the screw would push against. It made sense after that why you would need to wait between adjustments because that little air pillow would need to settle in to the changes from the screw pushing against it. I don't know much more than that and I can't find the photos I took of it back then. Seems like you could possibly damage the sensitive air pillow thing back there if you screwed it down too far but I never tried that

The throttle screw on it just turned a cat's eye shaped plastic disc which would turn indefinitely, no possibility of breaking that one. The older models where the throttle screw was on the venturi it would screw all the way in or out, couldn't break that one either. I'm sure many are familiar with both of those throttle screws.