Simple F&E explanation required

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

Tony
Click to expand...

You need to try to understand that!

The pump is circulating water round the system, the potential pressure of the pump can be five or six meters. That is distributed around the system.

So in the short section between feed and vent pipe there is usually a pressure difference of several centimetres of water height. In the traditional layout the vent pipe comes immediately after the boiler closely followed by the feed pipe. That spacing should be the minimum practical ideally not much more than 10 centimetres.

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

Tony
Click to expand...

You need to try to understand that!

The pump is circulating water round the system, the potential pressure of the pump can be five or six meters. That is distributed around the system.

So in the short section between feed and vent pipe there is usually a pressure difference of several centimetres of water height. In the traditional layout the vent pipe comes immediately after the boiler closely followed by the feed pipe. That spacing should be the minimum practical ideally not much more than 10 centimetres.

Tony
Click to expand...
Yes, I get that. If you had vent pipes all round the system, they would be at a head of, say, 8m immediately downstream of the pump ranging down to 3m immediately upstream (allowing for an F&E tank 3m above the pump, which is my case).
My circuit is probably about 15m and my F&E and vent are about 8cm apart. When I am running my pump at, say, 5m head, the pressure drop is not likely to be more than the average value of 5m * 0.08/15 = 0.027m (I say this because this section is smooth with little head loss). 2.7cm is such a small amount that it makes little difference - expansion should go up both F&E and vent and my question remains unanswered. In my case, my F&E pipe gets hotter than my vent pipe by up to at least 1.5m. It puzzles me!
 
Puzzles most people mate. Like previously said it's all to do with positive and negative pressure specifically where feed and vent enter the system. Basically the system is under positive pressure after the pump until the feed. Between the feed and the pump it is under negative pressure. At college my lecturer tried to explain what happens between the feed and vent pipes. But to be honest I got lost in his explanation. And so did everyone else. Including the lecturer. My best advice would be to drop it mate, just put it down to the work of voodoo forces. unless you can get hold of professor Brian Cox. Maybe he can help
 
Last edited:
TCCHeating said:
just put it down to the work of voodoo forces.
Click to expand...
Yes I found carrying a bottle of Holy Water in the van a wise precaution when dealing with similar problems, a few drops in the F&E tank will often exorcise the system completely!:):):)
 
Practical experience can get you through most of the time, but occasionally theoretical understanding may be necessary.
For example, I was checking how much of the F&E pipe could get hot and I estimated that, for an average system, one degree increase in average water temperature should cause a movement of around 150 mm in the F&E (assuming none goes up the vent). So I should not be alarmed at an F&E pipe that is hot for several metres.
I read that the vent pipe should be a maximum of 150 mm away from the F&E pipe - but I could not find out why. This must be related to my original question. To know why this dimension is given would be necessary if, for example, there were constraints on pipe runs.
 
It should be as short as possible.

It is because the longer it is the greater the pressure loss between the two connections. That can encourage pumping over.

I don't know how accurate your calculations are but you seem to have forgotten that the radiators expand as well so most of the water's expansion is accommodated by the increased size of the rads!

I also suspect that you have given the rise of water UP a 22 mm pipe and not the reality that the F&E tank has a large cros section.

Tony
 
Agile said:
I don't know how accurate your calculations are but you seem to have forgotten that the radiators expand as well so most of the water's expansion is accommodated by the increased size of the rads!
Click to expand...
Yes, I need to allow for that. I will have a think about it.

I also suspect that you have given the rise of water UP a 22 mm pipe and not the reality that the F&E tank has a large cros section.
Click to expand...
No, I used 15mm pipe and assumed an internal diameter of 13mm. The size and shape of the F&E tank is irrelevant. It is the rate of passage of water up a pipe of a particular diameter.
 
On a simpler note , if my F&E pipe gets blocked up at the point where it joins into the system ,does that mean the overflow pipe is likely to be blocked as well ?
 
Was that not a reasonable question? Both pipes are tee'd off the same pipe run (in my system) , both have minimal water flow so if cr@p collects in one, why not the other? My system might be running with a blocked overflow
 
MikeAustin said:
Agile said:
I don't know how accurate your calculations are but you seem to have forgotten that the radiators expand as well so most of the water's expansion is accommodated by the increased size of the rads!
Click to expand...
Yes, I need to allow for that. I will have a think about it.
Click to expand...
Water expansion is 12.7 times that of copper. Steel expands less than copper. The maximum effect of pipe and rad expansion is thus about 8% even if everything is copper. I quoted an estimate of 150mm movement up the 15mm F&E pipe for one degree temperature rise. I actually estimated 161mm but rounded down to 150mm. Knocking 8% off the 161mm makes it 148mm. So actually, the original quote of 150mm seems about right.
 
It is irrelevant what the movement up a 15 mm pipe might be.

Its the increase in depth in the F&E tank which matters! Maybe 300 mm x 450 mm!

The feed pipes get blocked because they are only 15 mm whereas the vent pipes are 22 mm.

Tony
 
You must log in or register to reply here.
Back
Top