Hot water flowing from vent pipe into loft tank

[BlueLoo]

Curiosity maybe.

The forums taken a very sour turn of late inundated with diyers showing so much vitriol towards some of the regulars. What's your problem?

You really are a knob, if you can't help people , why bother replying with drivel,wish I was as hard as you on your play station

Perhaps it's not the DIYers

 

[Dereekoo]

Thats no answer

 

[Dereekoo]

Transam, you are wasting your time.
Screwfux forum must have been purged

No he's not, he's adding to the discussion with some really in depth thought provoking statement which needs clarification for the benefit of others with lesser knowledge/understanding.
"Purged" another thought provoking inputi as its hard to understand its context within the structure of your sentence.

 

[Agile]

There are a couple of points which might be useful.

The vent pipe shows corrosion over the last 30mm as if the F&E tank has been at too high a level.

The cold depth in an F&E tank should only be about 100m. This will rise to about 150mm when the system is hot.

If the level is too high and the vent pipe dips into it then a slow circulation will run through it.

A system which is left pumping over can become virtually completely blocked with iron oxide within a very few days. So it is a very serious condition indeed.

Sometimes the pipework where the vent pipe joins the system can become well gunged up with very hard compounds which are quite whitish and obviously contain calcium. I have not yet worked out why this can happen sometimes. It is not very common and being so hard can be quite a problem to clear.

 

[BlueLoo]

Why

Because water in the system will look to return to a static (ie steady) balance. Water is lazy.

A pump generated a positive and negative head of which the sum is zero.

This point of zero is called the neutral point. It is the point where the pump stops pushing and starts pulling. ( Ignoring semantics).

This neutral point is always at the point where the circuit is open to the static head provided by the head tank.
In these cases, the Feed and expansion pipe.

So, if the vent is after the pump, the pump will add its own head to work against the static head of the fluid in the vent. If the pump head is greater than the static head, your system will "pump over", that is, the pump will be to powerful for the static head to stop it.
If the vent is before the pump on the negative side, (remembering the position of the FE pipe) then the pipe is under negative head from the pump and the pump can't reverse that to overcome the static head to cause pump over (usually, although if you have a very big pump, you will eventually not have enough free flow in the pipe and the neutral point moves).

You can also dead head the pump by blocking the line, thus nothing moves as overall dp is zero. However,bif you do this after the vent line and before the pump return as in this instance, the return flow has nowhere left to go but up the point of least resistance. The vent.

It's all about having the pump push and pull in the correct location.

 

[denso13]

tank

Cistern

 

[Dereekoo]

Because water in the system will look to return to a static (ie steady) balance. Water is lazy.

A pump generated a positive and negative head of which the sum is zero.

This point of zero is called the neutral point. It is the point where the pump stops pushing and starts pulling. ( Ignoring semantics).

This neutral point is always at the point where the circuit is open to the static head provided by the head tank.
In these cases, the Feed and expansion pipe.

So, if the vent is after the pump, the pump will add its own head to work against the static head of the fluid in the vent. If the pump head is greater than the static head, your system will "pump over", that is, the pump will be to powerful for the static head to stop it.
If the vent is before the pump on the negative side, (remembering the position of the FE pipe) then the pipe is under negative head from the pump and the pump can't reverse that to overcome the static head to cause pump over (usually, although if you have a very big pump, you will eventually not have enough free flow in the pipe and the neutral point moves).

You can also dead head the pump by blocking the line, thus nothing moves as overall dp is zero. However,bif you do this after the vent line and before the pump return as in this instance, the return flow has nowhere left to go but up the point of least resistance. The vent.

It's all about having the pump push and pull in the correct location.

A bit like post 7 on page 5 then, thanks for the clarification

 

[Gasguru]

The vent would never be after the circulator unless the circulator was on the return side of the boiler and that would be bad practice (it was common 50 years ago for other reasons).
On a modern system having the vent after the pump would be non compliant, the vent must always connect to the boiler flow pipe side.

BS6798 states....
5.4.1 Open systems
For open systems, an open vent pipe shall be provided from the circulating system to discharge over the feed and expansion cistern above the level of the overflow connection. The internal diameter of the pipe
shall be not less than 19 mm. The vent pipe shall rise continuously and be connected in such a position as to prevent discharge of water or ingress of air in all normal conditions of service. There shall be no valves
or components other than full-bore pipe fittings between the boiler and the discharge point of the open vent pipe unless specified in the manufacturer’s instructions.

COMMENTARY AND RECOMMENDATIONS ON 5.4.1
Except where the boiler manufacturer’s instructions specifically state otherwise, the open vent pipe should be fitted to the flow connection or flow pipe from the boiler. The vent pipe may be used as part of the
circulating system. Except where the boiler manufacturer’s instructions specifically state otherwise, a close coupled cold feed and open vent configuration may be fitted provided that there is a cold water feed path
available when all automatic valves are in the closed position. When this configuration is used the cold feed and open vent connections should be not more than 150 mm apart. Except where the boiler manufacturer’s
instructions specifically state otherwise, a combined feed and vent pipe may be fitted provided that the boiler incorporates an overheat cut-off device.

The vent is always the first connection on the flow pipe and modern practice and regulations dictates the feed pipe will connect after it and within 150mm. As mentioned before this short length of pipe is effectively operating as the neutral point through its entirety by virtue of the minimal head loss (the water flow friction between the connections is negligible and hence no pressure difference)...the pressure here remains at the same static pressure with the pump on or off.

Provided the boiler manufacturer allows the vent and feed pipe can be combined into a single pipe however IMHO it's poor practice and venting can be slow. Like all corner cutting in the building trade it was no doubt done under pressure to reduce costs in the new build market (new build installs can be dire).

Connecting the pump after the close coupled Feed & Vent ensures the negative inlet side of the pump operates close too or a little below the static system pressure (the neutral point pressure). How far below depends on the head loss (pipe friction) between the close coupled F & V and pump.
This layout has the advantage of maximising the pressure around the radiator/cylinder circuit (the positive pump pressure is added to the static pressure) at the pump outlet side and in so doing helps to reduce air ingress through radiator valve stems etc.

If you take a look at sealed systems the expansion vessel connection to the system equates to the neutral point and you can almost guarantee that on any modern boiler it will also be on the inlet side of the pump whether that be a combi/system boiler or a heat only boiler layout diagram. Again it's to maximum the positive pressure to the system (and the heat exchanger in the vast majority of combi/system boilers...it also aids bleeding).

150 years of heating has arrived at this situation but if you've got an old system then all manor of configurations have been used, even as little as 20 years ago 4 pipe heat exchangers were available with feed/vent/flow/returns etc.

If you want to understand the close coupled F&V being limited to 150mm take a look at Primary/Secondary systems (aka Closely Spaced Tees) more widely used in the US.
An enhancement to that are Low Loss Headers and Buffers used here on large systems.

 

[fixitflav]

On a modern system having the vent before the pump would be non compliant, the vent must always connect to the boiler flow pipe side.

??? Did you mean after the pump? Should be boiler - vent - cold feed - pump.

 

[Gasguru]

Fixed it Ta

 

[Dereekoo]

The vent would never be after the circulator unless the circulator was on the return side of the boiler and that would be bad practice (it was common 50 years ago for other reasons).
On a modern system having the vent after the pump would be non compliant, the vent must always connect to the boiler flow pipe side.

BS6798 states....
5.4.1 Open systems
For open systems, an open vent pipe shall be provided from the circulating system to discharge over the feed and expansion cistern above the level of the overflow connection. The internal diameter of the pipe
shall be not less than 19 mm. The vent pipe shall rise continuously and be connected in such a position as to prevent discharge of water or ingress of air in all normal conditions of service. There shall be no valves
or components other than full-bore pipe fittings between the boiler and the discharge point of the open vent pipe unless specified in the manufacturer’s instructions.

COMMENTARY AND RECOMMENDATIONS ON 5.4.1
Except where the boiler manufacturer’s instructions specifically state otherwise, the open vent pipe should be fitted to the flow connection or flow pipe from the boiler. The vent pipe may be used as part of the
circulating system. Except where the boiler manufacturer’s instructions specifically state otherwise, a close coupled cold feed and open vent configuration may be fitted provided that there is a cold water feed path
available when all automatic valves are in the closed position. When this configuration is used the cold feed and open vent connections should be not more than 150 mm apart. Except where the boiler manufacturer’s
instructions specifically state otherwise, a combined feed and vent pipe may be fitted provided that the boiler incorporates an overheat cut-off device.

The vent is always the first connection on the flow pipe and modern practice and regulations dictates the feed pipe will connect after it and within 150mm. As mentioned before this short length of pipe is effectively operating as the neutral point through its entirety by virtue of the minimal head loss (the water flow friction between the connections is negligible and hence no pressure difference)...the pressure here remains at the same static pressure with the pump on or off.

Provided the boiler manufacturer allows the vent and feed pipe can be combined into a single pipe however IMHO it's poor practice and venting can be slow. Like all corner cutting in the building trade it was no doubt done under pressure to reduce costs in the new build market (new build installs can be dire).

Connecting the pump after the close coupled Feed & Vent ensures the negative inlet side of the pump operates close too or a little below the static system pressure (the neutral point pressure). How far below depends on the head loss (pipe friction) between the close coupled F & V and pump.
This layout has the advantage of maximising the pressure around the radiator/cylinder circuit (the positive pump pressure is added to the static pressure) at the pump outlet side and in so doing helps to reduce air ingress through radiator valve stems etc.

If you take a look at sealed systems the expansion vessel connection to the system equates to the neutral point and you can almost guarantee that on any modern boiler it will also be on the inlet side of the pump whether that be a combi/system boiler or a heat only boiler layout diagram. Again it's to maximum the positive pressure to the system (and the heat exchanger in the vast majority of combi/system boilers...it also aids bleeding).

150 years of heating has arrived at this situation but if you've got an old system then all manor of configurations have been used, even as little as 20 years ago 4 pipe heat exchangers were available with feed/vent/flow/returns etc.

If you want to understand the close coupled F&V being limited to 150mm take a look at Primary/Secondary systems (aka Closely Spaced Tees) more widely used in the US.
An enhancement to that are Low Loss Headers and Buffers used here on large systems.

Now this is the sort of input you want facts and explanations and relevance, not single line comments stating this should be done or things going off at a tangent

 

[BlueLoo]

A bit like post 7 on page 5 then, thanks for the clarification

Yes.
Absolutely.

 

[BlueLoo]

The vent would never be after the circulator unless the circulator was on the return side of the boiler and that would be bad practice (it was common 50 years ago for other reasons).
On a modern system having the vent after the pump would be non compliant, the vent must always connect to the boiler flow pipe side.

BS6798 states....
5.4.1 Open systems
For open systems, an open vent pipe shall be provided from the circulating system to discharge over the feed and expansion cistern above the level of the overflow connection. The internal diameter of the pipe
shall be not less than 19 mm. The vent pipe shall rise continuously and be connected in such a position as to prevent discharge of water or ingress of air in all normal conditions of service. There shall be no valves
or components other than full-bore pipe fittings between the boiler and the discharge point of the open vent pipe unless specified in the manufacturer’s instructions.

COMMENTARY AND RECOMMENDATIONS ON 5.4.1
Except where the boiler manufacturer’s instructions specifically state otherwise, the open vent pipe should be fitted to the flow connection or flow pipe from the boiler. The vent pipe may be used as part of the
circulating system. Except where the boiler manufacturer’s instructions specifically state otherwise, a close coupled cold feed and open vent configuration may be fitted provided that there is a cold water feed path
available when all automatic valves are in the closed position. When this configuration is used the cold feed and open vent connections should be not more than 150 mm apart. Except where the boiler manufacturer’s
instructions specifically state otherwise, a combined feed and vent pipe may be fitted provided that the boiler incorporates an overheat cut-off device.

The vent is always the first connection on the flow pipe and modern practice and regulations dictates the feed pipe will connect after it and within 150mm. As mentioned before this short length of pipe is effectively operating as the neutral point through its entirety by virtue of the minimal head loss (the water flow friction between the connections is negligible and hence no pressure difference)...the pressure here remains at the same static pressure with the pump on or off.

Provided the boiler manufacturer allows the vent and feed pipe can be combined into a single pipe however IMHO it's poor practice and venting can be slow. Like all corner cutting in the building trade it was no doubt done under pressure to reduce costs in the new build market (new build installs can be dire).

Connecting the pump after the close coupled Feed & Vent ensures the negative inlet side of the pump operates close too or a little below the static system pressure (the neutral point pressure). How far below depends on the head loss (pipe friction) between the close coupled F & V and pump.
This layout has the advantage of maximising the pressure around the radiator/cylinder circuit (the positive pump pressure is added to the static pressure) at the pump outlet side and in so doing helps to reduce air ingress through radiator valve stems etc.

If you take a look at sealed systems the expansion vessel connection to the system equates to the neutral point and you can almost guarantee that on any modern boiler it will also be on the inlet side of the pump whether that be a combi/system boiler or a heat only boiler layout diagram. Again it's to maximum the positive pressure to the system (and the heat exchanger in the vast majority of combi/system boilers...it also aids bleeding).

150 years of heating has arrived at this situation but if you've got an old system then all manor of configurations have been used, even as little as 20 years ago 4 pipe heat exchangers were available with feed/vent/flow/returns etc.

If you want to understand the close coupled F&V being limited to 150mm take a look at Primary/Secondary systems (aka Closely Spaced Tees) more widely used in the US.
An enhancement to that are Low Loss Headers and Buffers used here on large systems.

I got fed up of air, sludge, banging, clanging, overflow, etc on my CH system and reconfigured it to the most contemporary approach i could find.
I still have a combined FEV and a pump before boiler configuration but the effect if getting the neutral point correct has been nothing short of staggering.
Moving the pump and decoupling the FEV is a task for the summer, but in drawing out my system and planning for it, i cannot fathom what on earth the original installer was thinking.
This isn't a one of house either, my plumbing is template with hundreds in the same locale.

Don't ask me about the boiler and control wiring, it's worse.

How do they get this stuff so wrong?
SMH.

 

[Gasguru]

Because new build may be done by the housebuilders specifier (who knows b all) and the Suprima manual only shows the combined F & V so he goes along with it. Of course the manufacturers reps do everything to get him to use their gear however carp it might be.

 

[BlueLoo]

Because new build may be done by the housebuilders specifier (who knows b all) and the Suprima manual only shows the combined F & V so he goes along with it. Of course the manufacturers reps do everything to get him to use their gear however carp it might be.

About the manual, it only shows a feed on the boiler outlet. The "vent" is in the cylinder feed.

I found that a bit of poor draughtsman ship tbh because the manual clearly states that the vent should be independent of any valving.

I have took this to mean the vent is actually just a AAV and the open vent isn't shown. Which should be quite wrong.

A bit messy really.