On Fri, 13 Apr 2012 20:25:24 +0100, John Rumm wrote:

On 13/04/2012 18:50, Lieutenant Scott wrote:
On Fri, 13 Apr 2012 18:01:34 +0100, John Rumm
wrote:

On 12/04/2012 23:35, Lieutenant Scott wrote:
On Thu, 12 Apr 2012 18:33:14 +0100, John Rumm
wrote:

On 12/04/2012 15:15, Lieutenant Scott wrote:






That's a vented system. The alternative is a sealed system, with no
tank, a filling loop and pressure gauge, and an expansion vessel.

I asked because I only have ONE pipe top the expansion tank. Not "feed
and expansion pipes".

You need a minimum of two pipes on a vented system, however exactly how
they are configured can vary.

The Feed and Expansion pipe is *one* pipe, (it serves both purposes -
allowing water to feed into the system, and accommodating expansion by
allowing water to be pushed back up into the F&E tank). It will connect
to the base of the F&E tank - and its open end should always be
submerged.

The vent pipe should loop over the top of the F&E tank so that its
highest point is higher than the water level in the tank, the top is
then typically bent around such that it discharges back into the tank.
This provides a *low resistance* path for steam to escape should the
primary water in the system ever boil for any reason.

Traditionally both these pipes attached to the system at different
places - often wither side of the pump. However you can attach the F&E
pipe to the system at one place, and then tee the vent pipe off that a
little higher, as this reduces the chance of drawing air into the
system, and if placed on the suction side of the pump can reduce the
chances of pump over (i.e. forcing water up the vent pipe so that it
spews back into the F&E tank, mixing in air into the process)

Hence the pump could not suck air down it, it would need to suck the
whole tank of water down first.

See above.

I have one pipe coming from the suck side of the pump (between the pump
and the boiler), leading along then up to the expansion tank. It goes
straight into the bottom of the tank, and there's a T just at the tank
with the over the top one you describe.

I fail to see why that pipe is less resistance. Water pressure is
measured by the height of water above you, which is identical inside the
tank and in the loopy over the top pipe.

What about the resistance offered by a lump of congealed gunk and hard
water scale wedged into the outlet of the F&E tank?

Tis the thing about fail safe systems, they are there it cope with
situations when other stuff goes wrong.

I would think gunk would offer less resistance than the pipe connections in the system.

Anyway surely water frequently moves up and down into the expansion tank, so the gunk will shift.

And in Scotland, we don't have scale :-P

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