Simple F&E explanation required

[MikeAustin]

A theoretical question here. Why should the expansion go up the E pipe in preference to the F pipe?
At system start, both have the same head of water. They are just a few centimetres apart upstream of the pump, so the pressure should be similar when running. The 15mm vs 22mm diameter would make little difference with the low expansion flow rate.
I am just curious because, when I have pump over, the F pipe gets hotter than the E pipe (both pipes are free from obstructions).

 

[Agile]

I don't know what you mean by F and E pipes.

The pump produces a flow and so there is a pressure differential between the feed and vent pipes.

Pumping over is a very serious situation and leads very quickly to pin holing of the rads. Sometimes one each day!

Tony

 

[JohnD]

A feed and expansion tank has a feed and expansion pipe. As the water in the system cools and warms, contracts and expands, the water goes down and up this pipe.

The other pipe (if fitted) is the vent pipe. This rises above the top of the F&E tank, then bends and comes part-way down. The top of the loop, and the open end of the vent, are both above the water level in the F&E pipe.

Water will only rise up the vent pipe if it is at a pressure that will push it.

However the water level in the system is the water level in the F&E tank. So the pressure can only rise if the F&E pipe is blocked or there is another source of pressure up it. And the vent pipe is provided to allow escape if that happens. It will not happen unless something is wrong, perhaps a design fault but more likely a partial blockage.



Edit: too slow

 

[MikeAustin]

A feed and expansion tank has a feed and expansion pipe. As the water in the system cools and warms, contracts and expands, the water goes down and up this pipe.

The other pipe (if fitted) is the vent pipe. This rises above the top of the F&E tank, then bends and comes part-way down. The top of the loop, and the open end of the vent, are both above the water level in the F&E pipe.

OK, my understanding seems to be quite wrong. I took the vent pipe to be the expansion pipe, rather than the feed pipe to be both feed and expansion.

Water will only rise up the vent pipe if it is at a pressure that will push it.

However the water level in the system is the water level in the F&E tank. So the pressure can only rise if the F&E pipe is blocked or there is another source of pressure up it. And the vent pipe is provided to allow escape if that happens. It will not happen unless something is wrong, perhaps a design fault but more likely a partial blockage.

So my question is still a valid one. Theoretically, during normal operation, the level of water in the vent pipe will be the same as in the F&E tank, and the pressure at the connection to the system will be the same as the F&E pipe and the vent pipe are adjacent, just upstream of the pump. So, during normal operation, the vent pipe can act as an F&E pipe and should also get hot?

 

[JohnD]

no. Nothing should go up the vent pipe when the system is working correctly. Something is wrong. As Agile told you, this is serious. It needs fixing.

Sometimes the pipe may get a little warm due to convection currents, but there should be no water flow out of it.

 

[MikeAustin]

no. Nothing should go up the vent pipe when the system is working correctly. Something is wrong. As Agile told you, this is serious. It needs fixing.

Sometimes the pipe may get a little warm due to convection currents, but there should be no water flow out of it.

Nothing is going up my vent pipe. My question, for the reasons I gave, was that it should go equally up F&E and vent. So I wondered where my reasoning was wrong.

 

[MikeAustin]

no. Nothing should go up the vent pipe when the system is working correctly. Something is wrong. As Agile told you, this is serious. It needs fixing.

Sometimes the pipe may get a little warm due to convection currents, but there should be no water flow out of it.

Nothing is going up the vent pipe. My question, based on the reasons I gave, was that it should equally go up the vent pipe as the F&E. So where did my reasoning go wrong?

 

[footprints]

Because even if they are on the same side of the pump there is a neutral point at the first pipe when the pump stops pushing and starts sucking.
All about suction and blowtion
On a sealed system pressure will remain the same throughout but a open system will not, leave it at that. We wound up endless instructors at college about it Just let it go it will do your head in I can tell you! I have seen combined vent and feed systems pump over or draw in air and in theory that's nigh impossible!

 

[JohnD]

Unless there is a fault, it can't travel up the vent pipe any higher than the water level in the F&E tank. Nor can it travel any lower than the water level in the F&E tank. It can't flow into or out of the tank because the pipe is higher.

The water in the F&E pipe, however, can flow into and out of the tank, raising and lowering the water level by perhaps an inch or two.

You said earlier that your system was pumping over, so you have a fault.

 

[MikeAustin]

Because even if they are on the same side of the pump there is a neutral point at the first pipe when the pump stops pushing and starts sucking.
All about suction and blowtion
On a sealed system pressure will remain the same throughout but a open system will not, leave it at that. We wound up endless instructors at college about it Just let it go it will do your head in I can tell you! I have seen combined vent and feed systems pump over or draw in air and in theory that's nigh impossible!

So, the pressure at the bottom of the F&E pipe and the bottom of the vent pipe, although on the same level, are different? The level in the vent pipe is slightly higher than the F&E tank causing it not to accept any expansion?

Unless there is a fault, it can't travel up the vent pipe any higher than the water level in the F&E tank. Nor can it travel any lower than the water level in the F&E tank. It can't flow into or out of the tank because the pipe is higher.

The water in the F&E pipe, however, can flow into and out of the tank, raising and lowering the water level by perhaps an inch or two.

That is what I mean - up the F&E pipe, raising the F&E tank level and the vent pipe level goes up with it (although it cannot get into the F&E tank from the vent pipe without overcoming the extra half a metre head above the tank).

You said earlier that your system was pumping over, so you have a fault.

That was my error. Because hot water was going up the F&E to quite some height, I thought it was equivalent to going up the vent pipe, so I wrongly called it "pump over". This is a problem that I am getting advice on those in another thread. My question here was just about the theoretical behaviour of an open vented system.

Thanks guys for trying to help me out on this. I can get quite tenacious in my attempts to understand, so thanks for your patience. I am developing quite a pressure head here!

 

[JohnD]

So, the pressure at the bottom of the F&E pipe and the bottom of the vent pipe, although on the same level, are different? The level in the vent pipe is slightly higher than the F&E tank causing it not to accept any expansion?

no, the pressure is the same, and the water level is the same, unless there is a pump interfering or a blockage between them

 

[vulcancontinental]

So, the pressure at the bottom of the F&E pipe and the bottom of the vent pipe, although on the same level, are different? The level in the vent pipe is slightly higher than the F&E tank causing it not to accept any expansion?

no, the pressure is the same, and the water level is the same, unless there is a pump interfering or a blockage between them

???

 

[JohnD]

!!!

 

[vulcancontinental]

It means there's a question mark over your post. It is incorrect.

 

[MikeAustin]

So, the pressure at the bottom of the F&E pipe and the bottom of the vent pipe, although on the same level, are different? The level in the vent pipe is slightly higher than the F&E tank causing it not to accept any expansion?

no, the pressure is the same, and the water level is the same, unless there is a pump interfering or a blockage between them

Would that not mean equal resistance to expansion and thus equal usage of both pipes for expansion?