On Sun, 5 Mar 2006 01:06:20 -0000, "Dave Baker" Dave
wrote:

Sorted

I've just been up into the loft and airing cupboard and mapped out all the
pipe runs. If you look at this basic diagram.

http://www.diydata.com/planning/cent...ing/boiler.htm

Then my pump would be on the blue line immediately adjacent to the boiler,
pumping into it and then back out into the line that feeds the vent and then
the rads and cylinder. The vent is therefore in the wrong place, before the
rads rather than after them and hence seeing all the pump pressure. Turning
the pump round wouldn't help because then the feed from the bottom of the
loft tank would see this pressure instead thus similarly reducing flow
through the rads.

So at present I only get normal flow because the vent is blocked and the
expansion/feed pipe does any venting required. When I get round to fixing
all the niggles in the system I'll reorganise everything to correct this.

As I say though, whether vented, for the first 15 years here, or unvented
for the last 4 I've always had piping hot rads and hot water and I guess
just the pressure head from the loft tank has sufficed to keep the system
running ok. Black mark to the guy who fitted all this before I moved in I
suppose.


If your pump is on the blue line adjacent to the boiler, it's the pump
that's in the wrong place relative to the other pipes.

It's not so much an issue of whether the pump is before the heating
circuit or after it, but rather that there is a pressure differential
between the feed/expansion pipe and the vent.

If your set up is as shown on the web site except that the pump is
adjacent to the boiler, you have a classic pumping over situation
because the two pipes to the tank are on opposite sides of the pump.

Actually, the setup in the diagram is not a lot better because the F/E
pipe and the vent have the boiler between them. Since there is a
level of flow resistance through the boiler there will be a pressure
differential so in the case illustrated there is a possibility of air
being sucked down the vent if the boiler flow resistance is high -
e.g. with pump on high speed. If you imagine the flow through the
boiler stopped (for the sake of illustration) you can see that the
effect would be that the water level in the tank would rise and air
would enter through the vent. In normal operation it would depend on
the pump setting, the height of the tank and the flow resistance of
the boiler and heating/HW circuits.

A better design would have the flow from the boiler going to an air
separator - e.g. www.bes.ltd.uk part 11334 - and the F/E and vent
pipes separately joining it. The pump goes after that followed by
motorised valve(s) and the heating and HW circuits. The returns of
those would be commoned and go directly to the boiler.

This achieves two key objectives:

- The F/E and vent pipes are close together. This means that there is
very little pressure differential and no chance of sucking down or
pumping over. Where a separator isn't used, but the pipes join the
circuit, it is recommended that there is not more than a 150mm space
between the two connection points.

- There is a clear path from the boiler flow to the vent. This helps
with venting but also the safety purpose of an easy path for the
escape of steam in the event of control or other failure.


An additional benefit of the air separator is that the system becomes
very easy to vent and little or no air should collect in the radiators
after the initial filling and expulsion of dissolved air.


--

..andy