High low aquastat wiring - L4081A

walnut

I have a hydronic system running on a slantfin cast iron 80k btu boiler.

The boiler has a L4081A high low aquastat.

The circulator is being controlled by thermostats and zone valves.

The high / low has two wires going into it. One feeds the B terminal of the high limit, one feeds the B terminal of the low limit. There is a jumper between the two R terminals.

The controller doesn't appear to be working properly but I can't understand how it would with this wiring configuration. I need to trace the wires back but generally my assumption is that these safety switches work by interrupting power to the gas valve and I can't see how this controller could work that way. If the temp is higher than the high point I can understand how it would interrupt. If temp is lower than the high point it would flow across the jumper and stop at the low limit as the temp is higher than the low limit.

The last plumber I had work on the system left the heat shield off the burner assembly and the heat killed the gas valve and some of the wiring to the high/low limit. The gas valve was replaced and the burnt up wires were redone but maybe the tech who did that work messed it up or maybe it was never setup properly.

The triadic which is on the side but higher in the heat exchanger well than that high/low showed 108C / 30 psi. The relief has been dripping and is very hot. The expansion tank had failed but has been replaced. The relief was replaced first. Shutting down the boiler had the triadic reading lower temperatures and pressures ( relief still dripping ).

Watching the operation of the boiler with the set points at 170F and 140F when a zone was calling for heat I watched the temp climb from 130F to 200F over 20 minutes, the boiler only stopped firing when the zone stopped calling for heat.

I thought maybe the high/low was bad but now I am thinking it just isn't wired correctly. When would you ever jump the burner terminals on a high low aquastat ? Looking at the manual and figure 7 - that seems like my setup but assuming the power comes into the high passes through jumps to the low, at 150F it would break and has the gas valve off.

https://digitalassets.resideo.com/damroot/Original/10015/60-2105.pdf

I've always thought it seemed to run hotter than it should. The relief and secondary aquastat probably prevent this thing from going super nova.

System was installed in 1994 so there have been more than a few people touching it over the years.