When the booster (circulator) pump starts, what happens in the hot water heating system?

When the booster (circulator) pump starts in a hot water heating system, the flow control valve opens. This is essential as the purpose of the pump is to circulate hot water throughout the system, and for that to happen, the flow control valve must be engaged to allow the movement of water. The opening of the flow control valve ensures that there is a clear path for water to flow from the boiler to the various heating elements, such as radiators or baseboards, allowing for effective heating distribution throughout the space.

The other options do not directly relate to the immediate action prompted by the operation of the booster pump. The pressure reducing valve and relief valve are concerned with maintaining system pressure and safety but do not engage solely due to the pump operation. The backflow preventer serves to prevent the reverse flow of water into the system and would not close simply because the pump starts. Hence, the opening of the flow control valve is the critical response that supports the primary function of the booster pump in enhancing circulation within the hot water heating system.