When operating a hot water boiler, what could cause the safety valve to lift?

The lifting of a safety valve in a hot water boiler is typically triggered by excessive pressure within the system, which can occur due to a variety of reasons. When the expansion tank is filled with water, it is unable to absorb the additional water that expands when heated. This results in increased pressure in the boiler system, which may exceed the safe operating pressure. The safety valve is designed as a protective measure to relieve excess pressure, and it will lift to release steam or water when the pressure surpasses the safe threshold.

This mechanism is crucial because maintaining proper pressure levels is vital for the safe operation of the boiler. If the expansion tank cannot perform its intended function of accommodating thermal expansion, safety features like the safety valve will activate to prevent potential damage or hazards associated with overpressure situations. Other options do not lead to the lifting of the valve in the same way. For instance, water being too cold does not create the conditions for excessive pressure, and blocked vents or low pressure typically do not cause the safety valve to lift.