What must all closed type hot water heating systems, not under continuous attendance, be provided with?

In closed-type hot water heating systems that are not under continuous attendance, it is essential to equip them with a pressure relief valve. The primary function of this valve is to automatically release excess pressure that may build up in the system due to thermal expansion or other factors.

When water is heated, it expands, which increases the pressure within a closed system. If this pressure exceeds the safe operational limits of the system, it can lead to equipment failure or even catastrophic incidents. The pressure relief valve acts as a safety mechanism that opens when pressure exceeds a predetermined level, allowing steam or water to escape safely, thereby protecting the integrity of the heating system.

The other options, such as a thermostatic valve, check valve, and expansion tank, serve different purposes. A thermostatic valve helps regulate temperature, a check valve prevents backflow, and an expansion tank absorbs the expansion of water in systems that incorporate it; however, they do not fulfill the critical role of managing high pressure in the same way that a pressure relief valve does. This makes the pressure relief valve a vital component for ensuring safety in closed hot water heating systems.