In an earlier contribution to this discussion,
Jason wrote:

There have been a number of posting recently about gravity HW systems
which I have read with interest, having put off the job of upgrading
the current layout year after year. After doing a good search on
google I think I have all the concepts, and think that the way to go
is just to stick a valve on the cylinder.

Can anyone help me out, and let me know if there is a
simpler/easier/better/more efficient etc etc option before I buy the
bits.

I have stuck details on my system on a webpage, so you can see how it
is all plumbed together, any thoughts, comments, recommendations
would be gratefully recieved. I rather be 110% clear on what I am
doing, and get it right first time, than fiddle and have no heating
at all!

Details at:

http://www.freewebs.com/jase12345/Fu...%20Heating.doc

If this were my system, my first priority would be to add boiler/pump
interlocks to prevent the boiler from cycling when neither CH nor HW require
it.

I would thus be looking at either a C-Plan system or an S-Plan system. I
would *not* go for a Y-Plan because it would greatly complicate the venting,
and 3-port valves are best avoided anyway.

The choice really hinges on whether or not the HW performance is currently
adequate. If it is, go for a C-Plan - it's a lot easier. You will need a
2-port valve in the HW circuit right next to the cylinder, *after* the vent
pipe tee - so as to maintain an un-interrupted path from the boiler to the
vent pipe. You will, of course also need a cylinder stat and a room stat.
The radiator nearest to the room stat *shouldn't* have a TRV on it. I
presume that you don't get any unwanted gravity circulation in the CH
circuit when the pump is not running? If you *do*, you will need an
anti-gravity valve - which only opens when the pump generates a bit of urge.

If the hot water takes too long to heat with gravity, you need a fully
pumped system - so I would go for an S-Plan. Put a 2-port valve in the HW
circuit next to the cyliner, as for C-Plan. Put another one in the CH
circuit. The current pump location may be a convenient position. Move the
pump to a position in the common bit of pipe before the circuits split. In
order to prevent pumping over, move the fill pipe connection into the HW
flow pipe - very close to where the vent pipe connects.

I would go for a programmable room stat for several reasons:
* It gives you completely independent control over the timing of HW and CH -
which most ordinary programmers don't. [Set your main programmer to HW timed
and CH constant - so that the room stat conrols the CH]
* It adapts to the characteristics of your system/house - and maintains the
set temperature with less overshoot
* With 'Optimum Start' it decides when to turn the heating on in order to
achieve the set temperature by the required time
* In the 'Off' position, it acts as an automatic frost stat - and still
turns on the heating if needed to prevent freezing

I doubt whether you need a by-pass circuit - even if you go for an S-Plan
system. These are needed when you have a boiler which only holds a small
quantity of water and which will overheat due to the residual heat in the
metal bits if the water isn't carried away quickly enough. Such boilers are
only used in fully pumped setups - and have pump over-run stats which keep
the pump running after the boiler stops firing until it has cooled down a
bit. The bypass circuit provides somewhere for the water to go if all the
valves are closed.

If a boiler works ok on gravity without the pump being on at all (when
operating on HW only), it can pretty certainly take care of itself.

Not sure whether I have answered *all* of the questions. HTH, anyway!

--
Cheers,
Set Square
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