Final answer:
True. Systems with water-heating appliances and backflow-prevention devices require a pressure control mechanism to prevent dangerous pressure build-up, which can be a safety hazard.
Step-by-step explanation:
True. A device for controlling pressure should indeed be installed in a system that has a water-heating appliance and a backflow-prevention device or check valve. In the context of water heating systems or any system that could be sealed and subject to thermal expansion, a pressure control mechanism, such as a pressure relief valve or an expansion tank, is an essential safety feature.
Without it, as the water heats and expands, the pressure could build up dangerously since the backflow-prevention device or check valve would prevent the excess pressure from being relieved back into the main water supply. Moreover, understanding from thermodynamic systems, like a pressure cooker or a pressurized water reactor, shows how critical pressure relief systems are for safety and functionality.
Similarly, just as the membrane in a plumbing system helps maintain the water pressure, a pressure relief valve or expansion tank would serve to prevent over-pressurization and potential system failures or hazards.