Years ago I read an article on painting engine blocks and valve covers. The color, if it made any difference at all, was so negligble as to not warrent engineering consideration. The amount of scale on the block prior to painting, and the thickness of the paint had more influence; and even these...
But I should add, there are differential pressure valves that can be used to open at preset pressure differences (or close at preset conditions). Call up a good boiler supply house and they can assist with selection.
You may want a check valve in the system for flow reasons, but the boundry valve must be a block valve. And, there is a very practical reason for this. In some designs, the is the point that separates a shop manufactured item, wherein the the pressure testing was done in the shop; from the field...
