On Thu, 17 Feb 2005 08:06:19 +0000, Tim S wrote:
On Wed, 16 Feb 2005 17:17:14 +0000, Andy Hall wrote:
On Wed, 16 Feb 2005 16:52:31 +0000, Tim S wrote:
I can think of other reasons to operate a heatbank - mains pressure hot
water without using a combi for one.
Yes of course and I didn't suggest otherwise. My point was
specifically in regard to modulating, condensing boilers. If you were to
use a conventional one then a long run is sensible.
Ah, my apologies. "Modulating" condensing boilers is a new one to me.
Most of the newer ones do do this, and the general mode of working is
that when they do a hot water cycle (i.e. reheat the cylinder or
store) that they go to full power - the philosophy being to get the
maximum amount of heat as quickly as possible into the cylinder.
However, condensing boilers operate more efficiently the lower the
return temperature. When below the dew point (at which condensation
actually begins to take place) they become even more efficient
(although realise that there is not a sudden Nirvana). In effect
what happens is that the rate of change of efficiency increases below
the dew point. Therefore the lower the return temperature the
better. One aspect of this of course is that the lower the operating
temperature, the lower the flue gas temperatures and hence less heat
simply going out with the flue gases.
The other aspect is that there is always an inefficiency involved with
starting the burner whatever the type of boiler. Therefore the less
times it goes on and off the better.
All of this takes you to the point that it is most efficient to run
this type of boiler continuously at lower temperature than to force it
into a situation of reheating a cylinder in bursts. For the hot
water, you have to do this, but for the space heating you don't.
If this type of boiler is connected directly to the radiators and you
have TRVs, as the room temperature reaches the set point, you have
effectively balanced the building heat loss with the radiator outputs.
THe TRVs will begin to close. If the boiler is directly connected to
them, it is able to sense the reduced heat demand and modulate down
accordingly. Thus, in the typical UK autumn/spring situation, which
is a lot of days of the year, and you need *some* heat, then you can
have the boiler operating in this equilibrium mode at low temperature.
If you put a store in the way, you will be emptying it at a low
continuous rate of (e.g.) 8kW, but then when most of the energy has
been used, replenishing it in bursts of perhaps 25kW - all controlled
by a thermostat on the cylinder. Therefore you prevent the boiler
behaving as it was intended to do - you force it to run at the high
temperature end and even though it will probably be one continuous
burn to replenish, there will still be more of those than if the
boiler had been left running continuously at lower output.
THe two exceptions that I made were a) if it's a conventional or
non-modulating boiler - there you would make an improvement by having
it recharge the cylinder once as opposed to driving the radiators
directly and cycling to match effective power output to load; or b) if
you want to introduce other sources of energy such as solar. Since
these change the economics, it may be that the overall equation works
better by having a store in the middle.
Perhaps a good reason not to bother with a condensing boiler then (yes, I
know about the supposed law mandating them, more b*ll*cks interference in
my home by the government blah blah...)
I think that this is one case where it is worth doing something despite
silly government meddling I've certainly saved considerably on gas
consumption using a condensing boiler.
I'll probably get a significant gain just by fixing the roof
insulation(!). Which is slightly non-trivial as a dormer conversion is
involved and I don;t want to do anything to bung up the already limited
airflow between roof and upper ceiling. Still thinking about that.
The roof is generally the biggest loser of heat in a house and usually
the easiest and cheapest to fix. However, it's worth doing the heat
loss calculations. For example, if you already have 100-150mm of
insulation in a roof space and nothing in the walls, you will probably
get a better result from doing something about the walls than adding
another 100mm in the roof. As you say, definitely avoid doing
anything that reduces ventilation. This is asking for trouble.
If you are tight for space, you could consider sliding sheets of
Celotex into the awkward spaces. It comes in various thicknesses and
has about four times the insulating property for a given thickness
than glass fibre mats.
Tim
--
..andy
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