Agreed, provided the posters were commenting on the OP's latest post; but only DP has done this. All the other recent contributions are related to the original problem which, according to the OP's latest post, has been solved.
Has our plumber put us in danger?
- Thread starter vexation
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Except the "solution" is still in dispute
As a practical matter, there is no doubt here that changing the vent from 22mm to 28mm (not a huge difference in CSA) has, for what ever reason, eliminated pumping over (or the same effects as pump over). That is to say that the level in the vent no longer climbs up past the top of the inverted U.DP- The cold feed and pipework to pump was not at all obstructed when the vent was redone and the two are right next to each other. I understand your explanation about the negative pressure in the pipe between boiler and pump - and the absence of surge on that side but I've been seeing much evidence of "springiness" (no doubt due to air or other gasses) despite great efforts to bleed rads. etc. With compliance in the primary circuit I think we're getting energy storage and release during on/off transitions of the pump. I first suspected this when I noticed the pump-over effects were diminished with lower pump speed settings. There's potential for a vicious circle where the effects of an air pocket cause surges that introduce more air here.
Although others on here never suggest it I am reasonably convinced that many of those problem are in fact caused by the "springiness" as a result of the apparently small amounts of air which is left in the top of rads after bleeding which together can add up to quite a fair volume.
This always arises after draining a system. Thats why experienced people prefer as far as possible to keep rads full of water and just drain the pipework. We often seem to read that DIYers love draining and refilling systems as if thats an advantage.
Air left at the top of rads will eventually dissolve in the water, oxygen quite quickly and nitrogen much more slowly.
Tony
This always arises after draining a system. Thats why experienced people prefer as far as possible to keep rads full of water and just drain the pipework. We often seem to read that DIYers love draining and refilling systems as if thats an advantage.
Air left at the top of rads will eventually dissolve in the water, oxygen quite quickly and nitrogen much more slowly.
Tony
I'm glad to see the effects of air traps being discussed. I know there's a temptation to view hydraulic systems as "perfect" transmission systems but that's not the case in practice. Like you say Tony, in our case with 16 (mostly twin panel) rads, there's still always going to be a significant amount of compliance after re-filling and bleeding until the gasses are fully dissolved once more.
If however, in the meantime, this causes pump over effects that introduce even more air then we're in that viscous circle again. So while the bore of the vent is theoretically less important (although I note that the textbooks do discuss constraints on minimum diameter) I think I've found that it's necessary to cater for exceptional surge while a re-filled system settles down. Maybe.
If however, in the meantime, this causes pump over effects that introduce even more air then we're in that viscous circle again. So while the bore of the vent is theoretically less important (although I note that the textbooks do discuss constraints on minimum diameter) I think I've found that it's necessary to cater for exceptional surge while a re-filled system settles down. Maybe.
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Does the flow pipe rise from the boiler and become the vent pipe with the feed continuing as a branch Teed in to the flow/vent pipe? i.e.Except the "solution" is still in dispute
[code:1]
Vent Feed
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Flow[/code:1]
This could be the problem, there is less resistance to flow in the vent pipe so water will prefer to travel that way rather than round the bend to the pump.
The preferred way is like this
[code:1]
Vent Feed
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====== ===== ====To Pump
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Flow[/code:1]
I haven't read all of the posts for this one, just the last few. There is obviously something wrong with the operating heads on your new boiler. By installing a larger vent you have created a surge arrestor which is frequently recommended by the manufacturers when a product is being installed in a low-head situation, eg in a flat, bungalow or any application where operating head and dry vent requirements can not be met.
Certain boilers can require in excess of 1m operating head.
Certain boilers can require in excess of 1m operating head.
just what I saidDoes the flow pipe rise from the boiler and become the vent pipe with the feed continuing as a branch Teed in to the flow/vent pipe? i.e.Except the "solution" is still in dispute
[code:1]
Vent Feed
|| ||
|| ||
||========= ======To pump
||
||
Flow[/code:1]
This could be the problem, there is less resistance to flow in the vent pipe so water will prefer to travel that way rather than round the bend to the pump.
The preferred way is like this
[code:1]
Vent Feed
|| ||
|| ||
|| ||
====== ===== ====To Pump
||
||
||
||
Flow[/code:1]
I was taught BE BOP a LOO LA vatory. and " stick to 1in40 the solids won`t get caughty" But that`s drainage
I like the word Inertia- yep,, it's Inertia!.
D_Hailsham said:
No, there's less tendency for the water to climb up the vent we way we have it: (also see photo half way down page one of thread)
[code:1]
Vent Feed Pump
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============| |
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Flow ^[/code:1]
Not as sensible as it could be but pretty common I think.
heatingman - its two-storey so the F&E tank in the loft is 3 or 4 meters above boiler on the ground floor. In a network of identical pipe diameters, "Surging" would not be an issue as the velocity of fluid in the network would be the same everywhere. It's only when we introduce changes in diameters that we introduce changes in velocity.
newgas - I can't honestly tell if you're being sarcastic or not.
We'll you were adamant it was inertia of the water after pump stopped, and you also thought of a problem to solve it, but no one seemed to pick up on it, sssooooo I thought I would give it a mention.newgas - I can't honestly tell if you're being sarcastic or not.
And yes there might have been a little sarcasm in there
I was'nt being nasty though, it's not my style.I had a problem similar to this which could have been pump overrun stat. caput, but at the time I managed to change 3 port/pump/overrun stat. and cylinder stat/ oh and I re-wired the system cos it was bodged by someone, but it sorted it.
I never fully found out what it was but my theory is the pump overrun as the boiler was still firrin after pump went off and cut out on overheat and the water whoooshed up the vent as it kettled in the boiler.
I hope this helps
OK newgas I know Inertia is a big issue in large-scale industrial pumping systems where fluids sometimes have the kinetic energy of express trains! The fluid dynamics inside your average CH system are no less complicated though. And a lot of good problem solving has to do with thought experiments - the one I started with was what would happen with a bicycle pump filled with water. A short push on the piston would squirt water a far greater distance because of the change in CSA. The bigger the ratio of the differences, the greater the distance amplification effect. Using a smaller CSA for the vent gives the same effect. Amplifying the surge (creating an artificially higher head momentarily) is OK if the surge is small enough - so the water in the vent doesn't get over the top of the inverted U. But in our case, it looks like the surge is unavoidably greater with the new boiler (less water content?).
I know Inertia is a big issue in large-scale industrial pumping systems where fluids sometimes have the kinetic energy of express trains! The fluid dynamics inside your average CH system are no less complicated though. And a lot of good problem solving has to do with thought experiments - the one I started with was what would happen with a bicycle pump filled with water. A short push on the piston would squirt water a far greater distance because of the change in CSA. The bigger the ratio of the differences, the greater the distance amplification effect. Using a smaller CSA for the vent gives the same effect. Amplifying the surge (creating an artificially higher head momentarily) is OK if the surge is small enough - so the water in the vent doesn't get over the top of the inverted U. But in our case, it looks like the surge is unavoidably greater with the new boiler (less water content?).so what is the operating head on your system?
Could it be a badly manufactured pump. Impeller misshaped or spinning not central to chamber. Drawing in air (supposed to be self venting)
For the sake of the cost of a new pump, put back a Grundfos as before.
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