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I don't think relief valve will help with freezing pipes. Freezing pipes burst because ice is less dense than water, so water expands as it turns into ice. Pipes freeze on the outside first, resulting in remaining water being entrapped in ice. As remaining water freezes, it can't move into relief valve.

I'm not thinking of a mechanical valve - a burst disc essentially is an engineered weak point, designed to fail first. It would have enough clearance for the ice to be forced out through it, and be in a location with sufficient drainage that water damage would be minimised when the ice melts.

It sounds like the answer is that to be effective, you'd have to put too many of them along the length of the pipework.

Water expands about 9% when it freezes. It's not a question of pressure relief, it's a question of the pipe bursting in multiple locations as it freezes end to end if you have lost heat in the house. The pipes don't leak until the ice melts.

PEX handles freezing pretty well