what ever lights your candle m8
Perplexing heating / water problem
- Thread starter RedAl
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i'm not getting in a bath with you bonny lad 
I think it was Sherlock Holmes who said that when you've eliminated every other possibility then whatever's left, no matter how improbable, must be the truth.
Bernoulli's equations apply to non-turbulent flow of an inviscid fluid so they have little relevance to real plumbing - most of the time. But in this case the feed is attached at a point of high water velocity whereas the vent is up in a 'dead' corner of the boiler where the velocity is low. There is also very little pressure drop across the very short pipe between them.
If we ignore viscous effects and assume that the vent sees static water then a flow rate of 1m/sec in the pipe is enough to raise the water in the vent 5cm above the water in the tank. If that doesn't sound like very much then think about this. The pressure difference is proportional to velocity SQUARED. A flow rate of 2m/sec will push that water up 20cm and 3m/sec will raise it 45cm. At some point it's going to go over the top!
Bernoulli's equations apply to non-turbulent flow of an inviscid fluid so they have little relevance to real plumbing - most of the time. But in this case the feed is attached at a point of high water velocity whereas the vent is up in a 'dead' corner of the boiler where the velocity is low. There is also very little pressure drop across the very short pipe between them.
If we ignore viscous effects and assume that the vent sees static water then a flow rate of 1m/sec in the pipe is enough to raise the water in the vent 5cm above the water in the tank. If that doesn't sound like very much then think about this. The pressure difference is proportional to velocity SQUARED. A flow rate of 2m/sec will push that water up 20cm and 3m/sec will raise it 45cm. At some point it's going to go over the top!
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Bernoulli's Principle, in physics, the law that as the speed of a moving fluid (liquid or gas) increases, the pressure within that fluid decreases. Formulated in 1738 by the Swiss mathematician and physicist Daniel Bernoulli, but earlier by Leonhard Euler, it states that the total energy in a steadily flowing fluid system is a constant along the flow path. It can be shown that as a consequence an increase in the fluid's speed must be matched by a decrease in its pressure.
Bernoulli's principle is applied in nozzles, where the flow is forced to accelerate as the tube diameter decreases, and the pressure consequently drops. It is also applied in orifice or Venturi flow meters, where measurements of the pressure difference between low-speed fluid in an approach pipe and high-speed fluid at a smaller orifice diameter are used to determine flow velocities and thus to meter the flow rate. Bernoulli's principle is sometimes mistakenly used to explain the net force in a system that includes a moving fluid, such as lift on an aeroplane's wing, thrust of a ship's propeller, or drifting of a spinning ball. The principle only applies to systems that do not produce a net force.
It stands to reason that more cannot be put in than taken out, the return will only return what the pump takes out in the first place, in this case the top is the least line of resistance, and the consequenses is it will suck not blow
Bernoulli's principle is applied in nozzles, where the flow is forced to accelerate as the tube diameter decreases, and the pressure consequently drops. It is also applied in orifice or Venturi flow meters, where measurements of the pressure difference between low-speed fluid in an approach pipe and high-speed fluid at a smaller orifice diameter are used to determine flow velocities and thus to meter the flow rate. Bernoulli's principle is sometimes mistakenly used to explain the net force in a system that includes a moving fluid, such as lift on an aeroplane's wing, thrust of a ship's propeller, or drifting of a spinning ball. The principle only applies to systems that do not produce a net force.
It stands to reason that more cannot be put in than taken out, the return will only return what the pump takes out in the first place, in this case the top is the least line of resistance, and the consequenses is it will suck not blow
I've done a check with a magnet as someone suggested and found that the magnet sticks to the horizontal return pipe at the bottom of the inverted T where the feed pipe joins.
From what i've read of the mind bending discussion that's been going on here this means the pipe contains some sort of contamination which would be restricting the flow and accentuating any potential bernoulli effect described.
I guess the next step is to clean this pipe out.
Has anyone got a straightforward suggestion of how this is done ?
From what i've read of the mind bending discussion that's been going on here this means the pipe contains some sort of contamination which would be restricting the flow and accentuating any potential bernoulli effect described.
I guess the next step is to clean this pipe out.
Has anyone got a straightforward suggestion of how this is done ?
Depends how bad it is, the boiler could be descaled provided theres a circulation through the water jacket, (do a search on the forum) local pipes are easier to just cut out and renew, you could even modify the pump-valve layout at the same time and fit an auto by-pass valve.
Sure it stands to reason - but YOUR reasoning is WRONG!
And we can tell when it's YOUR words instead of the ones you copy from elsewhere by looking for the mistakes in the English...
Are you gettin better by the way? I mean your cold..
Thankyou.
Mechanical means are more appropriate than chemical here. You may only have a few centimetres of significant scale, but in some cases it goes on for a metre or two. Makes the pipe wierdly heavy.
You can measure the effect of the Berni thing by temporarily increasing the height of the water level in the f/e tank. The level in the vent will also rise - if you can measure the difference at spill-over between the water level and the top of the bent-over bit, you have the measure. It would be interesting to try it before and after the pipe is cleared. If it's marginal you could try to take the vent pipe higher into the roof- you could use plastic of course.
Berni's effect works whether the flow is turbulent or not - which is why aeroplane flaps and air brakes work.
Its back again, you are of course correct in what you say Chris but the logic is not.
As per my email, with any positive pressure flow the vent will always suck, if for example the cold feed is blocked which now seems likely the system will become unbalanced , this is proved by the admission of air in the system.
It is also the biggest problem with a normal fully pump S or Y plan
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.
As per my email, with any positive pressure flow the vent will always suck, if for example the cold feed is blocked which now seems likely the system will become unbalanced , this is proved by the admission of air in the system.
It is also the biggest problem with a normal fully pump S or Y plan
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.
Absolutely not true!
In situations where the open vent and cold feed are connected SEPARATELY into the primary flow just above the pump (thousands of typical 'inverted H cylinder cupboard' setups), the blockage often occurs between the two. Then pump pressure forces water UP the open vent into the F&E, from where it runs back DOWN the cold feed into the primary flow a couple of inches further on. As a result, the loft gets full of warm fog rising from the F&E and condensation all over the roof timbers.
I've personally repiped at least 3 of these in the last 6 months! (with 100 percent success)
croydoncorgi said:
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.
And in any case it's irrelevant to 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, another perhaps problem which could be what you and Chris are getting at in a roundabout sort of way.
Is; if the return was blocked before the cold feed connection, the pump could possibly suck water from the tank and dump it back via the vent
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?
chill girls
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