IMM wrote:

That is why they have back end protection, amongst others.


Nope.
Back-end protection is intended to protect against back-end corrosion,
i.e., prolonged cold start return condensing temperatures of less than
60 degC. It wouldn't necessarily protect from excess dT/ thermal shock
caused by a slug of cold water from switching on a dormant zone,
although it may help. Protection against the possibility thermal shock
should be incorporated in other design details.
Finish.