The answer to 'why is my boiler freezing up' is that a boiler can freeze up due to a lack of water circulation, low water pressure, or extremely cold outdoor temperatures. This can cause the water inside the boiler to freeze, preventing the system from operating properly.
Some common symptoms of a frozen boiler include no heat, strange noises coming from the system, and visible ice buildup on the boiler or pipes. To resolve a frozen boiler, you may need to increase the water pressure, insulate exposed pipes, or in some cases, thaw the frozen components. However, attempting to fix a frozen boiler on your own can be dangerous, so it's generally best to call a professional HVAC technician for assistance.