I was all set to replace the gravity-fed HW system in the house I am
about to move in to with a megaflo (or some other unvented system), but
research on the net on "mains pressure hot water" brings up lots of
mentions of thermal stores (aka heat banks), and these would appear to
be a recent development, with many advantages over megaflos (as
described in
http://www.gasapplianceguide.co.uk/M...r_Systems.htm).

If these are so good, and a recent development, have they made megaflos
obsolete (at least for domestic use)? Would somebody like to argue the
case for a megaflo and against a thermal store?

On Thu, 14 Sep 2006 09:42:53 -0700, richardclay09 wrote:

I was all set to replace the gravity-fed HW system in the house I am
about to move in to with a megaflo (or some other unvented system), but
research on the net on "mains pressure hot water" brings up lots of
mentions of thermal stores (aka heat banks), and these would appear to
be a recent development, with many advantages over megaflos (as
described in
http://www.gasapplianceguide.co.uk/M...r_Systems.htm).

If these are so good, and a recent development, have they made megaflos
obsolete (at least for domestic use)? Would somebody like to argue the
case for a megaflo and against a thermal store?

For a system with a sealed primary circuit (as is usual with new boiler
installations these days) an unvented cylinder only involves one heat
exchanger between the primary (boiler) circuit and the DHW, whereas a
thermal store system requires two (one from primary to thermal store,
another from thermal store to DHW). This results in higher temperature
drops or bigger, more expensive heat exchangers. Bigger temperature drops
is worse news for high efficiency (condensing) boilers which work more
efficiently at lower return temperatures.

However this difference is slight in the overall context of domestic Ch &
DHW systems, and the choice for or against unvented is usually based on
other factors such as the specifier's or installer's familiarity with one
or other technology.

Contrary to John's note, in my experience the water in the thermal
store is part of the primary circuit and is heated directly by the
boiler. Claimed advantages include the boiler being fully loaded while
heating the store.

--

On Sat, 16 Sep 2006 15:37:55 +0100, richard wrote:

Contrary to John's note, in my experience the water in the thermal
store is part of the primary circuit and is heated directly by the
boiler.

In the bit of my post that you clipped (i.e. all of it) I said; "For a
system with a sealed primary circuit...". What you're describing is a
system with a vented primary. Some thermal stores are like this,
including the one I have built for myself. However since CH installations
increasingly tend to have sealed primary circuits these days thermal
stores for use with them have to have two heat exchangers.

... Claimed advantages include the boiler being fully loaded while
heating the store.

This is less of an advantage with modern boilers which can modulate their
outputs to match the demand so the use of a thermal store as an
efficiency-improving buffer between the boiler and the heating system is
perhaps less relevant.

However re-heat times with vented primary systems are very good and are
IMO a good argument for such systems in the right situations.