Richard Clay wrote:

- I don't quite understand why I need the tank - purely because the
boiler
isn't designed to be fed mains-pressure water? All the "unvented
systems"
diagrams I have read up on omit tanks of any description.

Clearly you have not seen diagrams for the correct layout of an unvented
HWC as opposed to boilers with unvented primaries.

The HWC (HW cylinder) is unvented, that is there is no way for the water
to leave the cylinder except via the taps (or when there is a problem
via the releif valve). This contrasts with the normal HWC often found in
a first floor airing cupboard where the top connection leads to a vent
pipe which goes up into the loft higher than the cold water tank. Were
the water in a vented cylinder to boil (which can happen if an immersion
heater fails) then water/steam escapes via the vent pipe.

The water in the boiler and rads is sperate from the HW used a the hot
taps. This water and its circuit is known as the primary. This water
comes form a small open header tank (vented) or is sealed (unvented) see
the FAQ below. You have a vented primary circuit fed from the small well
lagged tank.


Thanks for your advice. Could you just clarify the bit above please? What
exactly is the difference between the 2 setups you mention? What's a
"primary" in this context? (I assume "HWC" is "HW container"?)



--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html