What can cause excessive pressure in the supply header of a two-port zone valve system?

In a two-port zone valve system, closed zone valves can cause excessive pressure in the supply header. When a zone valve is closed, it restricts the flow of water in that particular zone, leading to an increase in pressure as the water attempts to continue moving through the system. Since the fluid cannot flow freely through the closed valve, it leads to an accumulation of pressure behind the valve.

In scenarios where multiple zone valves are closed, this effect can be compounded, resulting in significantly higher pressures in the supply header. If the pressure becomes too high, it can potentially damage the system components or result in a failure of the piping or valves due to stress. Therefore, managing the operation of zone valves and ensuring that not too many are closed simultaneously is crucial to maintaining appropriate pressure levels in the header.

The other options may contribute to system behavior, but they do not directly lead to excessive pressure in the same way that closed valves do. Open zone valves allow for flow and should relieve some of the pressure, faulty pressure relief valves can malfunction but are typically designed to prevent excessive pressure rather than cause it, and high water temperatures can influence the overall dynamics of a system but do not specifically lead to high pressure in a straightforward manner like closed valves do.