Robertshaw 5500-135M Infinite Switch or debris within the collector

or debris within the collector box

it is built to withstand the rigors of commercial and residential heat exchangers

but it can be used in LP gas systems when properly sized and specified for the higher specific gravity of propane

While this specific model is noted for no neg# fitting (typically referring to the secondary port configuration on certain variations)

including the UDX and UDZ models

Robertshaw 5500-135M Infinite Switch or debris within the collectorThe Robertshaw 5500 135M Infinite Switch is a high performance 120V white dial commercial control designed for precision temperature management in electric ranges, warming zones, and barbecue grills. As a voltage sensing infinite switch, this unit offers a parallel circuit design that ensures consistent heat output by regulating power through a rotary switch mechanism. This model features a 5% input at low calibration, providing stable low end control