In an earlier contribution to this discussion,
don wrote:

Set Square wrote:

Does your loft have lots of headroom? Assuming that you're not going
for a mains pressure hot water system, you'll need to get your cold
header tank *above* the level of the hot cylinder. This will need
supporting *very* well - water is heavy stuff!

The intention is to try and get everything mounted above a supporting
wall. The loft has enough head room for the cylinder plus a bit more.
I was planning to get the water line for the header tank a few inches
above the cylinder ... should I really be aiming to get the whole tank
above the cylinder?

Yes! Consider the situation where you're running a bath and the header tank
is less than full, because the mains feed can't keep up with the rate of hot
flow. Once the level gets below that of the top of the cylinder, you'll be
relying on syphoning to get any further flow.

If you're going fully pumped with a single pump, you'll need both HW
and CH to share a common flow pipe until after the pump - where the
circuits then split using zone valves (S-Plan) or a 3-port
mid-position valve (Y-Plan). [See
http://content.honeywell.com/uk/homes/systems.htm]

At the moment, the pump is directly above the boiler. My original plan
was to put a 3 port valve directly after the pump and run a new 22mm
pipe run straight upto the cylinder in the loft. This seems the
simplest approach as it involves the least modification to existing
pipe runs.

If you currently
have separate circuits (4 pipes into the boiler) this will need
changing.

Yes, it has 4 pipes. Is it a simple case of capping off the existing
pipe for the gravity cylinder? Or does it need to be a circuit with
water in to stop the boiler over heating?

Disconnect it at the boiler itself, and blank off the "hole" in the boiler.

If your boiler can stand it, you might consider converting to a
non-vented (sealed) primary circuit - using a pressure vessel and
filling loop, and getting rid of your small F&E tank.

Is the advantage here that you're losing the tank or is there an
additional benefit?

If you *don't* do this, you will need to raise the F&E tank to a fair height
*above* the highest point in the primary circuit - which is now the coil in
your hot cylinder. With the limited headroom available, you're in danger of
having the primary circuit "pumping over". Also, you'll have to run a vent
pipe all the way from the boiler side of your zone valves to the new F&E
location. A sealed system would prevent pumping over, and would do away with
the need for a vent pipe. It's also much better at keeping air - and the
resultant corrosion - out. [The downside is that if you've any grotty old
radiator valves, they may start leaking with the increased pressure.] Maybe
a good time to replace all the valves with TRVs (on all rads bar one) and
drainable lockshields at the other end.
--
Cheers,
Set Square
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