Ooh missed this thread for a while.
Dia, saying things like
The vent and cold feed are always connected on the suction side of the pump in a fully pump installation
Isn't doing you any good. It's nonsense. There's a system 5ft from where I'm sitting which has been fine for 20 years thankyou very much, which disproves what you say and I've seen loads more like it and so have you.
You seem to refuse to acknowledge that the explanation for the facts, which your assertions cannot explain, could be to do with the Bernoulli effects, exacerbated by a buildup of scale.
Sorry mate but I can only believe that you're in the camp of the miserable old gits who think they've seen everything and know everything, so can't learn anything.
with any positive pressure flow the vent will always suck
not "always"- wrong if the pump's in the return, wrong if Mr B is very active.
this is proved by the admission of air in the system.
You don't know there's air in the system, it could be corrosion products, but of course we would all expect some of both - the thing's got a fountain in it!!
Finally the only way water can exit the vent is through a boiler overheat, or a circuit between the vent and cold feed or the vent and system.
We all know there's a circuit, because the water coming out of the vent is clearly re-entering the system. No need to state the obvious.
the current problem, which looks most likely to be a cold feed blockage, and a simple drain down will prove the tank is not emptying
Of COURSE the cold feed is not completely blocked, ya prune! Water keeps coming out of the vent, but the tank hasn't overflowed(!) so where do you think it's going?!!
another perhaps problem which could be what you and Chris are getting at in a roundabout sort of way.
What's roundabout? We're saying there could be a pressure set up, because of the water speeds, between the feed and vent pipe which would make water come out of the vent , contrary to the pressures you would expect from looking solely at the pressures resulting from the flow resistances. OK?
A complete blockage in the feed pipe is impossible here because water's coming out of the vent. A partial restriction in the feed pipe would have no effect as there's normally no water flow in the feed pipe.
A restriction after the feed pipe, in the main (return) pipe, would normally have the effect of reducing the pressure at the base of the vent pipe relative to that at the feed pipe, so there would be a tendency to suck air down the vent.
But in this case there must be another factor at work, and the only thing suggested is the Bernoulli effect, which would create pressure between the feed and vent pipes in the negative sense to the normal flow derived pressures at those pipes.
Those B effects do not normally cause a problem because of the sizes of the pipes/boiler cavity, and the height of the "dry" part of the vent pipe (which is limited ny head-room in this installation).
But it is quite possible, as has been seen elsewhwere (even if not by DIA in 46 years) , that the B forces are sufficient to raise the water level in the vent pipe to the position where it falls into the tank. The magnitude of the B force depends on the speed of the water, which would it turn be strongly affected by scale build up at the base of the feed pipe. If the scale narrowed the pipe (likely) or created anomolous flow, including turbulent, the suction in the feed pipe relative to the vent, would increase.
Come on then DIA, where's the error in logic?