Isolated zone pressure

Techman

Coming off of a Primary loop is a zone for a backup FanCoil set up. This zone is isolated by ball valves . What happens to the expansion/contraction of the water in that zone?

Zman

The only time it would be an issue would be if the temp changed significantly.I also doubt it would be an issue a PEX loop as the tubing is pretty pliable.
So if you filled the loop of copper or steel with 180 degree water, and let it cool to room temp, the water would contract. I think it would eventually equalize through the isolation valve or other minor leak points.
You see this kind of problem sometimes when a zone valve is installed on one end of loop and check valve on the other. You get some undesirable water hammer when the valve opens the next time and the pressure equalizes.