Pip, I think many of the previous posters are forgetting that the prv valve is spring loaded, and will open when the pressure exceeds the pressure setting, and release enough pressure to to get it down to the pre-set pressure, and then re-close, it will not remain open and drain the whole system, I do not recall ever seeing a prv get to the stage where they actually blow off! they usually just slowly dribble off the pressure, usually unnoticed.
Terrier.
Thanks Terrier
Yes I agree, I think the size of the vessel only needs to be a few litres in reality. However care should be observed if the vessel would over flow and cause a problem, say in the roof space.
From experience I have had PRVs stay open when testing. I think the test knob on many are designed to turn past a point and let the valve “Click down” or in other words, the test knob will lift the valve open to a certain high, then let it thump down to form a seal when the knob is turned past a predefined point. I have found on some valves gently opening by turning clockwise, then gently releasing by letting it go back anticlockwise against the spring can sometimes cause a partial close. Also on first operation if debris should get caught on the valve seating the same thing can happen. (note to self; always flush the system, including the PRV).
I am thinking in an over pressure situation, then the valve would most likely would open and close slowly thus potentially not seal fully, hence the need to be careful should the loop empty down due to this problem.
Not that I have ever observed this scenario. I wonder if anyone in the forum has experienced of PRV not closing properly?
Cheers
Pip
