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Welcome to Just Answer, my name is ***** ***** I will do my best to help you with your issue. If my initial response doesn’t answer your question then let me know and we can continue our conversation.
Based on your initial description it sounds like you have a bad thermostat on the Weil tank. We could do some troubleshotting if you like.
Do you have a dedicated pump for the tank or does a zone valve control boiler water flow?
It's one or the other. The answer can't be no
Ok, does your temp dial on the tank have numbers or just warm, hot, very hot etc
Ok, where is it set now and do you have a thermometer that you can use to check the water temp?
You'd just put the thermometer under running hot (only) water but if it's set for very hot and the water isn't scalding then you likely have a bad thermostat. One more test to confirm it's not something else. Trace where the thermostat wires connect to either the separate pump relay (for the tank's pump) or the boiler controller and jump those terminals. You can also connect the 2 wires together at some point if that's easier. When you do does the pump run? It can be difficult to tell so you can put the handle of a screwdriver to your ear and the tip on the pump. You'll hear the motor spin if it's running. Does the piping from the boiler to the tank get hot? If it does then replace the thermostat. If not you have a circulation problem.
If the piping does get hot then replace the thermostat
You didn't mention a mixer (3 pipes connected to it) until now. If the hot line into the mixer is very hot and the mixed water out to the house is only 100 then the mixer is the problem and it needs to be rebuilt.
All mixer/tempering valves can be rebuilt. Sometimes (with aggressive water) the valve mechanism can fail internally and the entire valve must be replaced.
The knob should turn (easily) but that won't solve this problem even if you could turn it. The fact that it's seized up is another indicator of the need to repair or replace it.
I have been called away. If you have more questions I'll respond when I get back (should be less tahn 1 hour). If not I'd appreciate a favorable rating. The thread won't go away and I'll answer related follow up questions even after you rate me favorably