"Christian McArdle" wrote in message
. net...
Whilst I agree with many of the advantages, I'd say not everything is
entirely true.
- High mains water pressures at up to 10 bar, compared to 3.5 bar with
unvenetd cylinders.
DPS seem to want a pressure reducing valve if static pressure is above 6
bar.
It is all down to what the plate heat exchanger can take. Most are about 10
bar, max. This is another advantage in that no pressure reducer is fitted.
- Very high water flow-rate - The high-end
heat-banks have a flow-rate up to 45 litres/min.
Whilst very good, unvented cylinders will supply more. (i.e. Megaflo
claims
72 lpm)
Just add plate heat exchangers (not very big) and it will give you more than
the Megoflo as no pressure reducer is fitted.
- Cold water storage eliminated - No
need for cold water storage tanks.
A header or expansion tank is needed for the vented heat bank store. This
is
frequently combined with the store itself in a combination cylinder, but
this requires additional head room.
Not an issue really.
- Lower water temperatures with fast flow-rates - As with Combi boilers,
fast flow-rates through the plate heat-exchanger results in lower water
temperatures. (...) This also applies to unvented cylinders (...)
Could you explain how increased flow
rates decrease water temperature with
an unvented cylinder? The limit on flow
rate with an unvented cylinder is how
much water you can shove through the
thing,
That is true. A good heat bank with a well sized boiler can virtually heat
the store faster than what is extracted. Another advantage: a directly
heated bank can take any sized boiler as long as the flow and return pipes
are large enough. In fact two cheapish 80,000 Btus boilers can be fitted
directly to the store with their own flow and returns for a zippo heat up (A
Worcester 82,000 Btus heating boiler can be had for £534.63 Including VAT
each, £1065 for two. Or a Glow Worm 30HXI condensing boiler for £663.88
Including VAT each, £1328 for two). That is, if you draw off water below
approx 24 litres/min, the boilers will be re-heating faster than you
draw-off - great for two showers as you "never" run out of hot water. Then
you can downsize the heat banks cylinder size, saving money on the store
costs to compensate.
An unvented cylinder is limited by the coil size. Some unvented cylinders
can take nearly 200,000 Btus of boiler heat, but they are usualy very large
and very expensive.
No so. They can operate up to 10 bar pressure.
Unvented cylinders can only
operate up a typical 3.5 bar.
Again, the only heat bank I've got details
for (DPS Pandora) claims 6 bar.
However, the pressure reduction isn't as
bad as it seems. Obviously, as
water is drawn off, the pressure falls
massively. So even if your static
pressure is 6 bar, drawing 75lpm will
cause the pressure to collapse below 3
bar, meaning the pressure reducing valve
is fully open, and only providing a
restriction in line with the reduced bore
of its opening, probably equivalent to a
90 degree elbow or a few metres of pipe.
But if you have 10 bar to spare it is not utilised. All academic, as 40
litres/min is more than adequate for most homes, filling a typical 100 litre
bath with 43C water in about 2-3 mins or so, which no one would complain
about.
---
--
Checked by AVG anti-virus system (
http://www.grisoft.com).
Version: 6.0.507 / Virus Database: 304 - Release Date: 04/08/2003