On 27 Mar 2007 06:04:18 -0700 someone who may be "Clive"
wrote this:-

I have bought a small two and a half bedroom end-of-terraced
Victorian house. It has gas central heating with a floor-standing
boiler that does the job. The boiler could be 20 - 25 years old. The
system has a hot water tank with a jacket that seems as thick as a
cardigan. The system supplies loads of hot water and some of the rads
are absolutely scorching. I'm sure the rads could do with a "re-
balance". I feel it is all very inefficient in fuel terms but it works
and will cost a pretty penny to replace.

Yet you go on to suggest an expensive to buy and complicated boiler.

At most, the house will be occupied by one or two people. So here is
the idea. Replace the boiler with a condensing combi boiler. Then I
get hot water "on-demand" and at mains pressure.

You can get mains pressure hot water, heated on demand, by using a
thermal store.

I don't heat a tank
and then let it cool for hot water I never use.

Yet you go on to suggest using a hot water cylinder for
"pre-heating".

I had a Vaillant combi
boiler in a one-bedroom ground floor flat and (apart from the
occasional wobbly) it would chuck out hot water at mains pressure
within seconds.

A thermal store will do that.

I want to retain that and avoid shower pumps and
electric shower heaters.

No shower pumps or electric heaters with a thermal store. Note also
that gravity hot water systems don't need them either, as one can
fit a venturi shower.

The "clever" bit would be to have a hot water tank but only use it
for the central heating. It would have two supply circuits, one from
the wet solar panels on a south-facing roof - this would "pre-heat the
water. The second supply circuit would be from the boiler. The idea
being I could collect as much heat as possible on a sunny day on the
solar circuit and top it up with gas-heated water. The tank would need
to be well-insulated to store the heat from a sunny day and release it
in the evening.

Solar heating is necessarily a relatively slow process, as solar
radiation is diffuse. Thus it involves capturing heat over
relatively long periods. This then needs to be stored.

In this situation I would consider using a thermal store which is
heated by a solar panel on a separate circuit and the existing
boiler heating the shell water (until it dies, then replace with a
condensing boiler). Heating would be taken from the shell by a
separate pump. Hot water would be generated from a coil in the
thermal store, or plate heat exchanger if you are in a hard water
area.

Cost depends on how much you do yourself.
http://www.navitron.org.uk/solar_collector_panel.htm is a reasonable
place for some DIY equipment.



--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54