Oil fired steam boiler continues to run, despite hitting the 180 degree high limit of aquastat

randerson039

This is a pretty simple Weil Mclain, oil fired, single pipe, low pressure steam system. No control valves, 1 zone of heat governed by 1 thermostat, 1 zone for an indirect fired hot water heater governed by an aquastat and associated circulator pump. The boiler has a Honeywell L6006A aquastat for boiler temp control. It is summer, so the thermostat is off and boiler is just supposed to make domestic hot water. Recently though, and intermittently, the boiler will begin to fire and the radiators will begin to heat up. This happens despite there being no call for hot water or heat. I have not tested the temp of the boiler water when this happens, but I'm pretty sure it is higher than the 180 degree hi point set on the B-control aquastat (diff is set to 20 degrees). I suspected the aquastat was faulty so I replaced it as well as the probe well (and used the thermal grease). The problem remained. Replaced the oil burner control yesterday and tested the system. The aquastat appears to be functioning properly, shutting down when hitting the hi point temp and restarting when the high point temp is raised, shutting down again when the high point is lowered. I disconnected the thermostat for the heat zone last night to eliminate that as a cause, and the system worked properly all night and most of today making hot water and or maintaining the boiler core temp, until my wife texted me this afternoon to tell me the boiler is running hard and the radiators are heating up. She shut the system down. What could cause this? Is it possible I have had 2 faulty aquastats? Is there something in the Carlin burner that could cause this? I've replaced everything else.

HVACNUT

Check the aquastat on the indirect. Os the domestic hot water too hot? If no then it's not the indirect aquastat. 
Is there a relay controlling the indirect and heat thermostat? Either or both? 
On the Carlin primary, what is TT connected to? Is TT jumped?
You've replaced most of the operating controls so I would lean towards an intermittent low voltage short. Start with the TT wires and work back from there.

EBEBRATT-Ed

Or muck around the aqustat well. Do you have enough water in the boiler to cover the aqustat well??

randerson039

So the aquastat on the indirect hot water heater runs a circulator pump and has a transformer and relay to run all that. That setup appears to be working correctly and is not tied into the oil burner circuit at all. It just turns on the pump when the water in the tank gets below 140. The TT terminals on the Carlin burner are normally wired to the boiler control aquastat and to the Ecobee thermostat, which has a transformer and a relay as well. Right now, the thermostat wire is not connected to the TT terminals. I also disconnected the boiler control aquastat from the TT terminals to see if somehow an intermittent short in the oil burner wiring was causing the problem. As would be expected, the boiler is just a 400 lb paperweight with nothing attached to the TT terminals. Im going to replace the wire for the boiler control aquastat tonight to eliminate a possible short there. So if it’s not that, a faulty aquastat or the aquastat probe is somehow not able to consistently read the water temp in the boiler? Could mud in the boiler cause an intermittent cold spot where the aquastat probe is? The water level is set to what it’s always been, half way up the sight glass. When I replaced the aquastat well, a ton of water came out, despite dropping the water level to below the sight glass, so I don’t think water level is the issue. I do appreciate the input.