Perplexing heating / water problem

[doitall]

The vent and cold feed are always connected on the suction side of the pump in a fully pump installation never above on the delivery side, and in which case I would expect it to blow over the vent, if the cold feed is blocked how does the water return to the primary flow a few inches further on.

Oh dear....
'Above' and 'suction side' are NOT contradictions! Either you haven't been around enough or you didn't read what I wrote! IMHO, in the MAJORITY of traditional installations with a pump and motorised valves beside the HW cylinder upstairs in a two storey house, the cold feed and the open vent are BOTH connected ABOVE the pump on its SUCTION side. The pump then pumps DOWNWARDS towards the zone valves. (the 'pump pressure' I referred to before acts all the way round the system and back to the point immediately above it, where the OV and CF are located!)
Clearer now?

Yes other than a few minor points, the pump should never pump downhill or at least not recommended by the manufacturers, before the pump would have been a better word than above,.

The cold feed and vent are positioned close together to balance the system head and avoid the problem of sucking air into the system, block the cold feed and see what happens with a jam jar test.

If however your suggesting that the circulating pump will blast the water round the system and out the vent on a correctly installed system, then this argument is by no means concluded because I don't believe you, and in 46 years I've yet to see it happen.

 

[kevplumb]

46 years

who you kidding

 

[RedAl]

Can I ask for a summary of the pages of discussion here of a suggested set of actions ?

My initial plan is to:
1. Drain the system.
2. Remove the section of return pipe back into the boiler and replace
3. Refill and test.

If this doesn't work then I guess the next thing is to rework the pump & valve layout.

Given the layout and space available as shown in the pictures what is the best approach ?

Cheers for you interest and help so far fellas.

 

[PEDANTICVINDICTIVEMAN]

I would alter pipework and fit a Myson airjec, fit new pump (15/50 not 15/60) and valves while drained down treat with X400 for a week and then X100 when finished and not bother with all this physics stuff.

 

[felix]

By all means clean out that common return pipe if you can get in there easily. It might just work though the problem will eventually return. Don't waste time and money with your pump and valves. There's nothing wrong with them. The problem is on the return side where the feed pipe goes in. Disconnect the feed from that single return and bring it in as close as possible to the vent attachment point. In your case this means teeing it into the side of the vent just above the boiler. You can bring it down a little way past the new tee then back up again if you like. This will ensure that any air or steam bubbles go up the vent and not the feed.

 

[RedAl]

Well that's my weekend planned.

Thanks for the help.

 

[ChrisR]

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?

 

[ChrisR]

RedAl
Thought I'd start again because your eyes are probably glazing over!.

Drain to a bit below the level of the T where the return comes in. If you want you can turn all the rads off first to reduce volumes. You can of course undo the connections to the boiler and that gate valve (which technically shouldn't be there!) but it looks like you'll have to cut both the Return vertical pipes before they join. It lools like the return pipes are 22 and 28mm? You may find it easier to put back together if you use some "slip" couplings. BES.LTD.UK do them, or a BIG plumbers' merchant.
You can sometimes make your own by hammering pipe though a normal coupling if it only has a small indentation to "stop" the pipe going through.

Do tell us what you find!! Picture?
The feed pipe might have a lot of scale in it - best to replace the pipe back to where you can scrape the grot out. It'll go along the pipe to the boiler but not backwards much if at all.

When you replace the pipes, it would be best if the feed joined the main pipe from the bottom (needs 4 elbows) to avoid convective circulation in the pipe. The bit going down should be a few inches long.