The mod motor turns the jackshaft, and arms, rods and an adjustable cam carry that motion to the fuel valve and the air damper so their ratio follows a fixed curve. It is simple and robust, but linkages slip, wear and drift with temperature, so excess air has to be generous. Parallel positioning replaces it with separate servos on fuel and air driven by an electronic curve.
Set screw on the jackshaft arm slipped, air damper's lagging the gas valve and it's smoking at mid-fire.
Mark every linkage arm and set screw with paint after tuning, so a slipped arm is visible on the next walk-through.
Also heard as
- single-point positioning
- jackshaft linkage
Related terms
Mod motor
The modulating motor that drives a burner's air damper and fuel valve through their range on command.
Excess air
Combustion air supplied beyond the exact amount needed to burn all the fuel, expressed as a percentage.
O2 trim
A control that measures flue gas oxygen and continuously adjusts the air-fuel ratio to hold it at target.
Modulation
Varying a burner's firing rate continuously between low fire and high fire to match the load.