Dripping Overflow from unvented system (w/pics)

[bernardgreen]

Do you have G3 qualifications BG??

You have been told I do not hold a qualification. But many people know having a qualification in a subject is not the same as being competent in that subject.

Free of all commercial pressure a candidate sits and passes an examination ( multitple choice answers maybe ) then he or she does a couple of "perfect" installations for the assessor to look at and there is the qualification. The qualified ( registered ) person now has to earn a living in the real competative world. He or she may forget some of the things that were learnt for the exam and he or she may intentional dismiss some ""trivial"" things if by dismissing them the profit margin can be increased.

The problems come when the qualified person considers something to be trivial when it is not trivial. It is not their fault, the training system has not explained to them why something that appears to be trivial is not trivial. Too many training companies are driven by the commercial ( profit ) need to process as many students as possible. One learns best by experience and that comes first by working alongside a skilled tradesperson who is prepared and willing to pass on his or her knowledge and skills to the next generation.

he's now turned his Googling ability to unvented cylinders!!!

Long before the Internet and Google came into being I was trusted to work on the monitoring and control systems of industrial sized pressurised water systems ( 6 bar and over 200°C )

 

[Hufty]

Hang on, bernardgreen, didn't you did leap in and misdiagnose the situation only a few short posts ago, getting the chemistry wrong for good measure (hint: it's not sulphuric acid)

The reason I suspect it is the condensate drain is the bright blue deposits on the end of the pipe which appear to be the same colour as copper sulphate.

Copper sulphate would be produced by the action of sulphuric acid in the condensate on the copper of the pipe. "clean water" would produce copper oxide which is green rather than blue

 

[bernardgreen]

(hint: it's not sulphuric acid)

Almost all fuels contain some sulphur, some domestic heating oils have sulphur compounds added to lubricate fuel oil pumps. ( moving to low sulphur fuels resulted in shorter life times of some oil pumps )

Sulphur dioxide is produced when the sulphur ( and its compounds ) are burnt
Water is produced when the hydrogen in a hydrocarbon fuel is burnt
Nitrogen combines with excess oxygen to produce nitrogen oxide and nitrogen dioxide
These combustion products react in a chain of reactions and one of the end products is sulphuric acid H2SO4
The nitrogen oxides are regenerated in the chain of reactions and can be considered as enablers of the reaction chain to the sulphuric acid.
The chain of reactions is affected by too many variables to allow any prediction of how much sulphuric acid will be produced.
Some nitric and / or nitrous acid is produced
Some carbonic acid may be produced ( it is unstable and reverts to by mostly CO2 gas dissolved in water )

Basic school boy chemistry 1960 ( but with a bias to my then interest in coke ovens and blast furnaces )

but I've noticed that every evening/morning when the boiler is running DHW, one of the overflow pipes drips.

If it was a pressure relief valve letting by then why would it only happen when the boiler was running ( supplying ) DHW. It would be more likely to let by when there was no DHW flowing ( hint :- static pressure is higher than dynamic pressure if flow is controlled by tap(s) downstream from the cylinder )

An over temperature relief valve might let by if the DHW temperature was too high but it would be more than just a drip if the temperature was to be reduced with replacement cold water.

 

[Hufty]

Yeah but how can you assume enough nitrogen oxides are being regenerated to enable the reaction chain to produce sulphuric acid, assuming it's a standard domestic pressure jet burner?
What mechanism do you propose to explain the pattern of the sulphuric acid attack - ie only the bottom inch of copper condensate pipe, mercifully no corrosion of the copper pipe next to it.
Finally, if you heat a sealed system, what happens to the pressure?

 

[Steelmasons]

Someone has been pizzing and splashed all over the D2..

 

[bernardgreen]

how can you assume enough nitrogen oxides are being regenerated

In a domestic pressure jet burner there is an execess of combustion air so regeneration of the nitrogen oxides is not essential to continue the reactons ( unlike a coke oven where the air ( oxygen ) is restricted to prevent the coke ( carbon ) burning.

the sulphuric acid attack - ie only the bottom inch of copper condensate pipe,

because it runs down the inside of the pipe, surface tension holds it to the end of th pipe.

if you heat a sealed system, what happens to the pressure

when heated the fluid in the system will try to expand its volume.

If the volume cannot expand then :-
(1) if the fluid is a gas then the gas will be compressed and pressure of that gas will increase.
(2) if the fluid is a liquid which cannot be compressed then the containment will be forced to expand by deforming the vessel possibly to the point of rupture.

Hence when a liquid is intended to be heated in a sealed system an expansion vessel is provided where the expanding liquid can expand into the vessel and thus reduce the volume of a gas ( most often air ) held in the vessel. It is the reduction in the volume of the gas that increase the pressure in the system.

The DHW system is not sealed when a tap is opened, it is not heated until a tap is opened.

The part of the sealed system that forms the CH ( central heating ) system is likely to be heated when the boiler fires up for DHW. The expansion of water in the sealed CH system is allowed for by the use of an expansion vessel.

If that expansion vessel has become full of water then expansion is not possible and the expansion of the water will create mild deforming of the components in the system. ( the diaphram in the expansion vessel should be the only component that deforms ) It will also increase the pressure in the pressure release valve which will ( should ) open and allow water to leave the system. The amount of water that leaves will be the extra volume created by the expansion. When the temperature of the sealed system has reached it's operating temperature expansion will stop and any release through the pressure relief valve will cease. It will not be a continuous release.

When the system has cooled down the water will have contracted to its original volume. This will leave some void in the sealed system without water. This void might contain a vacuum with saturated water vapour or it will contain air that has bled back through the pressure relief valve. The next time the system heats this void of vacuum and vapour ( or air ) will become filled with water due to the expansion of the water. In effect it is a pseudo expansion vessel

If it is air then the pressure in the system will increase as the air is compressed but will not increase as much as it did when there was no air in the system. Some water and / or air may be let out via the pressure relief valve but it will be less than was released in the previous heating cycle. After several heat and cool cycles the amount of water left in the system will be reduced and when this volume of water does expand the pseudo expansion vessel wil absorb the expansion without the pressure rising high enough to open the pressure relief valve.

 

[PullerGas]

how can you assume enough nitrogen oxides are being regenerated

In a domestic pressure jet burner there is an execess of combustion air so regeneration of the nitrogen oxides is not essential to continue the reactons ( unlike a coke oven where the air ( oxygen ) is restricted to prevent the coke ( carbon ) burning.

the sulphuric acid attack - ie only the bottom inch of copper condensate pipe,

because it runs down the inside of the pipe, surface tension holds it to the end of th pipe.

if you heat a sealed system, what happens to the pressure

when heated the fluid in the system will try to expand its volume.

If the volume cannot expand then :-
(1) if the fluid is a gas then the gas will be compressed and pressure of that gas will increase.
(2) if the fluid is a liquid which cannot be compressed then the containment will be forced to expand by deforming the vessel possibly to the point of rupture.

Hence when a liquid is intended to be heated in a sealed system an expansion vessel is provided where the expanding liquid can expand into the vessel and thus reduce the volume of a gas ( most often air ) held in the vessel. It is the reduction in the volume of the gas that increase the pressure in the system.

The DHW system is not sealed when a tap is opened, it is not heated until a tap is opened.

The part of the sealed system that forms the CH ( central heating ) system is likely to be heated when the boiler fires up for DHW. The expansion of water in the sealed CH system is allowed for by the use of an expansion vessel.

If that expansion vessel has become full of water then expansion is not possible and the expansion of the water will create mild deforming of the components in the system. ( the diaphram in the expansion vessel should be the only component that deforms ) It will also increase the pressure in the pressure release valve which will ( should ) open and allow water to leave the system. The amount of water that leaves will be the extra volume created by the expansion. When the temperature of the sealed system has reached it's operating temperature expansion will stop and any release through the pressure relief valve will cease. It will not be a continuous release.

When the system has cooled down the water will have contracted to its original volume. This will leave some void in the sealed system without water. This void might contain a vacuum with saturated water vapour or it will contain air that has bled back through the pressure relief valve. The next time the system heats this void of vacuum and vapour ( or air ) will become filled with water due to the expansion of the water. In effect it is a pseudo expansion vessel

If it is air then the pressure in the system will increase as the air is compressed but will not increase as much as it did when there was no air in the system. Some water and / or air may be let out via the pressure relief valve but it will be less than was released in the previous heating cycle. After several heat and cool cycles the amount of water left in the system will be reduced and when this volume of water does expand the pseudo expansion vessel wil absorb the expansion without the pressure rising high enough to open the pressure relief valve.

This has got to be one of the biggest loads of drivel I ever read on here!!!
The water, any water, be it in your HW system or Primary CH system, heats it expands & in any sealed system the expansion vessel absorbs that expanded water.....simple, very simple. A pressure relief valve at a set pressure, will relieve that water if it goes above the set pressure.

Hot water systems with an unvented cylinder normally have a pressure reducing valve set at 3-3.5bar on the cold inlet, so if you have a faulty expansion vessel or pressure relief valve it'll discharge water from the blow-off pipe.

Work on any unvented cylinder should only be carried out by a qualified Plumber holding a G3 unvented ticket. It should also be serviced annually by said Plumber.

Some types of cylinder - like Megaflo for example, have an internal bubble that also needs replenished annually.

No chemistry just physics.

 

[Hufty]

Loving the pseudo expansion vessel, and the idea of a vacuum filled with something, in this case water vapour. Maybe there's loads of water vapour in space - that's a vacuum.

But I have to take issue with the idea that the DHW is not heated until a tap is opened. Obviously not correct.

Another thing I'm not getting, probably because of my weak grasp of school boy physics, is that you have this vacuum in the cylinder, and you also have a supply of mains pressure water being held back by a diaphragm in a pressure reducing valve. Why doesn't the diaphragm drop between heating cycles, allowing water past to fill the vacuum?

Any guesses as to what the plant is in the photograph that seems to be thriving in all that pipe-rotting sulphuric acid by the way? We've got quite acid soil round here and I'm bored of begonias...

 

[dilalio]

We've got quite acid soil round here and I'm bored of begonias...

 

[PullerGas]

Loving the pseudo expansion vessel, and the idea of a vacuum filled with something, in this case water vapour. Maybe there's loads of water vapour in space - that's a vacuum.

But I have to take issue with the idea that the DHW is not heated until a tap is opened. Obviously not correct.

Another thing I'm not getting, probably because of my weak grasp of school boy physics, is that you have this vacuum in the cylinder, and you also have a supply of mains pressure water being held back by a diaphragm in a pressure reducing valve. Why doesn't the diaphragm drop between heating cycles, allowing water past to fill the vacuum?

Any guesses as to what the plant is in the photograph that seems to be thriving in all that pipe-rotting sulphuric acid by the way? We've got quite acid soil round here and I'm bored of begonias...

BG is best ignored Hufty, he's a crackpot or is it crackhead??!!

 

[bernardgreen]

Work on any unvented cylinder should only be carried out by a qualified Plumber holding a G3 unvented ticket. It should also be serviced annually by said Plumber.

Totally agree with that.

Hot water systems with an unvented cylinder normally have a pressure reducing valve set at 3-3.5bar on the cold inlet

I was talking about the water in the CH circuit expanding and contracting. That circuit is ( or should be ) only connected to the mains water supply by the filling loop and manually operated filling valves when the pressure gauge shows the pressure is too low.

Maybe there's loads of water vapour in space - that's a vacuum.

There is no water vapour in space because if there were it's temperature would be way below dew point and water droplets would be formed which eventually would be pulled into the atmosphere of some planet ny that planets gravity.

 

[barki]

OK guys,

Cheers very much for the help. I learned a couple of things;

1) I'll need to get someone in to do this one
2) BernardGreen likes having a rant


Will post back results for the benefit of future readers. If anyone is in the Norwich area and would like this job, PM me

Tom

 

[bernardgreen]

BernardGreen likes having a rant

Just trying to put some science into the black art of plumbing

 

[barki]

Finally had a qualified plumber round (took week to find someone who would actually turn up...the usual).

Anyway - he put my own footpump on the expansion vessel and pumped it up. Sorted in 5 mins. Cost £42.

If it goes flat again immediately i'll call them back to switch out the faulty component. If it takes a year or more i'll certainly do it myself next time, that to be honest, was £42 wasted.

 

[PullerGas]

Finally had a qualified plumber round (took week to find someone who would actually turn up...the usual).

Anyway - he put my own footpump on the expansion vessel and pumped it up. Sorted in 5 mins. Cost £42.

If it goes flat again immediately i'll call them back to switch out the faulty component. If it takes a year or more i'll certainly do it myself next time, that to be honest, was £42 wasted.

So did he service the cylinder also???