Offset on a Gas Valve

Peregrine

What is an Offset on a SIT 848 Sigma negative regulation gas valve. Please see attached manual. It is on a Lochinvar Noble Combi 150 NKCN.

I can adjust to manufacturer spec targets of O2 5.4% and CO2 of 8.9%. And within ranges of O2 5.1%-5.7% and CO2 8.7%-9.1% at low fire. No Problem.

I found this attached chart but very little anywhere I researched.

But I don’t understand Offset.

Please help.

Have a great Easter.

Offset Chart.pdf

848-SIGMA-GB gas valve.pdf

mattmia2

the offset changes the ratio between the high and low fore mixture. if the low and high fire numbers are within spec there is no need to touch it. if there were a need to touch it, it needs to be adjusted on very small fractions of a turn or it can be set so far off that it is extremely difficult to get it to the right setting. it is like the main vs idle circuit on a carburetor feeding fuel to an engine.

Peregrine

thanks you