croydoncorgi said:
The force of the fluid on the O ring pushes it ALONG the pipe towards the 'low pressure' end (ie. where the pipe enters the area of the joint where the O ring is).
Perhaps on JG, but AFAIK not on Polypipe, and certainly on Hep2o.
The pressure has the effect of centering the pipe (overcoming any side-force) and making sure the ring is not sitting at an angle on the pipe.
Focussing on Hep2o, which is more familiar to me than the other brands of pushfit, the thing you describe just isn't possible, because the end of the pipe is held in place, laterally, by a well inside the fitting, beyond the O ring seal, and the O ring has only negligible movement along the axis of the pipe centre.
Depending on the joint design, there may be a flat end for the O ring to push against (Hep2O) or not (Speedfit).
I still don't see why you think that there's a force acting to push the O ring along the pipe.
I don't know whether the O ring housing is exactly cylindrical or slightly tapered towards the open end of the joint (but it would make logical sense if it tapered, to tighten the ring against the pipe).
That, to me, would be an illogical design, verging on the appalling. I've never seen any O ring joint where the mating parts are tapered, and I imagine that this is because you can't force an O ring to have a smaller internal diameter. To do so would pucker it and result in a leak.
If you've ever tried to remove a pushfit joint under pressure, I think you'll have to agree that there IS a powerful force acting to tighten the ring onto the pipe that is not there at zero pressure.
You're right - I do agree. But my understanding of the cause is different to yours. The pressure of fluid within a pipe acts in the direction of forcing the pipe out of a fitting. The result of this is to draw the teeth of the grab ring inwards, which increases the force of the teeth cutting into the wall of the pipe.
Simple answer - install it properly such that it isn't tight or stressed under no/low pressure
Much of the point of using plastic pipe is that it DOES bend. Always keeping bend radii out of the 'too much' zone is easier to say than to do!
I wholly disagree - it's very easy to do, by clipping the pipe near each fitting. This eliminates the undesirable shearing force at the entrypoint to the fitting. You don't even need to keep coiled pipe straight - just let it settle where it wants to be and clip it there.
The pump created a pressure difference across the Flow and Return that reduced the pressure in one (or more)of the joints in the loft to the point where it admitted air.
Intriguing. Which edition of Speedfit fitting was it? If the new locking type, then was it locked to reduce (or even prevent) pipe movement? And were the best type of inserts used, the ones with the O rings on the internal pipe wall? Or, if the previous type of fitting, was a collet clip fitted?
I take your point, if I've interpreted it correctly, that the effect of heat, pipe expansion, fluid pressure and general movement all conspire to increase the risk of a leak, but my point is that it's never safe to allow oneself to be lazy with plastic and pushfit in the belief that it's a doddle to assemble. Just as with any type of fitting, it responds to careful handling of materials, sensible design of installation, and correct deployment and fitting. There's always an optimum way to use it, depending on the circumstances, but a great many more crap and incompetent ways.
