On 02/02/2011 19:03, Kostas Kavoussanakis wrote:


Thanks again, Roger (and harry too).

I was unaware of the details of the two different cold tanks; do I
really have a small F&E tank in the attic? I can't see it, but it's
probably on the unfloored side of the big tank.


You *might* have a sealed (unvented) primary system - in which case
there wouldn't be an F&E tank. Have you got an expansion vessel anywhere?

I have no idea which circuit I am (they are) talking about

Sadly, neither have they!

, all I wanted
to figure out is what kind of boiler I should be looking at. I am now
guessing this is about the primary circuit, shall we establish? Let me
use Vaillant terminology, as per the following:

http://www.vaillant.co.uk/installers...ncy-boilers-2/

The terminology used by Vaillant in that reference is very confusing and
ambiguous - so I'm not surprised that you are confused!

The primary and secondary circuits can each independently be either
vented or non-venting - giving four possibilities - whereas Vaillant
give the impression that it's all or nothing. The most common current
domestic arrangement is a non-vented primary and vented secondary
circuit. You could almost certainly achieve that with either of
Vaillant's so called Heating Only or System boilers. The only difference
would be that the system boiler would have the pump and pressure vessel
inside the boiler casing whereas the Heating Only boiler would require
these to be installed external to the boiler. The system boiler saves
space - and some plumbing - but the pump and pressure vessel are
probably then proprietary, and more difficult and expensive to replace.


I am discounting combis.


Good!


I like the fact that I know nothing about this F&E tank and have nothing
to maintain. Does this mean that I am looking at open-vent boilers? What
is the benefit of a vented primary circuit?


No. A vented primary circuit DOES have an F&E tank. An unvented system
has a pressure vessel and filling loop instead. You would still have to
maintain *that* in terms of checking the pressure periodically and
topping up if necessary - just as you have to check the level in an F&E
tank.

Others have already explained the pros and cons in terms of adding
inhibitor, pumping over, etc. Generally, non-vented primary circuits are
preferable.

Re pressurised secondary circuits: am I right that they are possible
with both open-vent and system boilers?

Yes.


Also, I had a look at prices for
the Vaillant tanks and the eyes watered, even without the big job you
and harry mentioned. What does one get in return? You suggested "mains
pressure hot water, and you don't have to accommodate a cold tank." I
don't mind the latter; can one get a similar effect to the former by
installing a wee pump close to a gravity-fed HW tank? Any other benefits?


Because non-vented cylinders have to sustain mains pressure, they are
built like the proverbial brick s**thouse - and probably last for ever.
But they *do* require expert installation and regular maintenance - and
you get no hot water if there's a mains water supply failure.

You can indeed fit a boost pump on the hot water outlet (NOT the cold
water inlet!) of the hot cylinder, increasing the flow rate to the taps.
This would usually be controlled by a flow switch so that it would only
operate when you open a hot tap. It wouldn't be silent, of course, but
shouldn't be too intrusive.
--
Cheers,
Roger
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