On Thu, 17 Feb 2005 10:57:18 +0000, Tim S wrote:

On Thu, 17 Feb 2005 09:55:45 +0000, Andy Hall wrote:

On Thu, 17 Feb 2005 08:06:19 +0000, Tim S wrote:


Ah, my apologies. "Modulating" condensing boilers is a new one to me.


snip
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.

Hi Andy

Yes - thanks for the excellent explanation.

It makes sense - though I have an overriding reason to use a heat bank in
my setup. Not a problem as we already have a not-very-old back boiler
which should be good for 20 years.

Do they make condensing back boilers?

I haven't seen any.

Your scenario is a good reason to put in some thermal buffering.



I suppose that it would be possible to have an intelligent controller
between the heatbank and boiler, that would "learn" the characteristics of
the bank and based on various measurables (rate of temp drop at various
vertical points for one) and could make a decision to run the boiler at
partial output. Be quite a nice little project with a PC, then turn it
into a micro-controller package.

That could be worth doing, although I fear that you might have
relatively little control over the boiler behaviour. Really it has to
be hot or off. Nevertheless I think that you could probably do some
predictive stuff to make it more responsive in anticipation of demand
and to turn it off in good time rather than overheating the store.
I presume that you are going to fully pump it. That will also give
you some good opportunities for control by timing the pump on off
ahead and after the burner.

Given what you have, I think it's definitely worth a go.


If modulating condensors are indeed becoming the norm, I'm sure that one
of the more advanced controller manufacturers will think of doing this.

Tim


--

..andy

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