On Apr 14, 6:37*pm, "Lieutenant Scott" wrote:
On Sat, 14 Apr 2012 17:20:27 +0100, harry wrote:
On Apr 14, 12:55 pm, "Lieutenant Scott" wrote:
On Sat, 14 Apr 2012 07:19:10 +0100, harry wrote:
On Apr 13, 6:50 pm, "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:
On Thu, 12 Apr 2012 13:14:15 +0100, John Rumm
wrote:

On 11/04/2012 20:16, Lieutenant Scott wrote:

(With vented systems, you also need a little care in selecting the
position relative to the feed and expansion pipes since you don't want
the pump sucking gobs of air into the system down the vent pipe on
startup. (the reason a combined feed and vent works better))

Not sure what you mean by "vented". Mine has an expansion tank fitted
which is full of water (and refills from the main with a ballcock should
you lose any water to leaks or over bleed radiators).

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.

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The reason there should be two pipes is in case the boiler thermostat
fails.
If the water boils, a convection loop takes the excess heat away.
Water and steam goes up the expansion pipe and cool water comes down
the cold feed.
This hopefully prevents damage to the system.

Circulation pumps were quite likely to fail in days of yore if the
water boiled.
This was another reason they were on the return pipe. (Event of
thermostat failure)

Sealed systems have a safety/relief valve for this function.
However if that system boils dry, you could have major problems.

I'm not convinced that convection loop could remove the excess heat from a boiler going full bore, especially if most of the radiators have switched off their TRVs.

It can. I have first hand knowledge.

Would the small expansion tank not eventually boil and be unable to remove as much heat as the boiler could put in? *My tank is about a third of the size of the boiler.

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Most of the energy is carried away as steam. Cold water goes down the
feed to replace it.