In article . com,
Niall Smart writes
Hi all...

I'm replacing the boiler and DHW system in my 3 bed semi-d that I'm
renovating.. so far so good - thanks to all the regular posters! I've
putting the boiler and DHW cylinder in the loft (attic) space to save
room downstairs. Hoisting the boiler up was fun!

Anyway, I've run into one issue which I haven't seen covered here
much. I live in Dublin, where the mains water supply / pressure can
be quite erratic, especially during summer. That pretty much rules
out a mains pressure system. The local water bye-laws also mandate
cold water storage for cisterns, showering, etc. I guess this is to
stop our mains distribution system getting overloaded at peak times.

So I can't get rid of tanks in the loft, and if I want a pressurised
(power) shower I'm going to have to use a pump. The two options seem
to be either (a) regular vented / unpressurised system where the pump
is on the appliance side of the DHW cylinder or (b) use a pump to
pressurise an unvented cylinder from a cold water storage tank.
These diagrams illustrate what I mean:

a - http://www.lycrisbyrne.com/images/combination1.jpg
b - http://www.lycrisbyrne.com/images/glasslined2.jpg

All the kit will be in the attic, so if I go with (a) I'll need to use
a combination cylinder to get a positive head to feed the cylinder.

Has anyone installed (b) the pumped unvented system?

As far as I can see the only pro is that I can have a larger DHW
cylinder as I don't need the cold water directly overhead, but with
the drawbacks that I will have pump noise on any hot water draw off,
no hot water if the pump fails, and more complicated pipework.
Advantage combi cylinder?

I suppose the advantage with b) is that the pump is simpler (single) and so
may cost less, but the twin pump is a standard retrofit for those wishing
higher pressure so economies of scale may have eliminates the price
difference.

Bear in mind that with a) you can use the cheapest (pressure) grade of
cylinder available whereas with b) you may require a more expensive one,
plus grade 3 (lowest) cylinders are on the shelf in every plumbers'
merchants.

Remember also that in a) there is no requirement for the hot and cold
tanks/cylinders to be co-located, the cold could be a coffin tank high up in
the apex of the roof or elsewhere.
--
fred
Plusnet - I hope you like vanilla