On Wed, 5 Jan 2005 10:31:27 -0000, "Jim Alexander"
wrote:
"Andy Hall" wrote in message
.. .
On Tue, 4 Jan 2005 13:21:27 -0000, "Jim Alexander"
wrote:
Several things here.
1) There are to be some exemptions which will basically operate if
replacement of an existing conventional boiler with a condensing one
will be too disruptive. This will be cases related to flue
arrangements, condensate drain etc., and a points scoring system is
used. Thus if you have a backboiler with conventional flue in the
middle of the house, then you may score sufficient points not to need
to replace with a condensing model. However, if it's a wall mount in
the kitchen, you would be pretty unlikely to meet exemption criteria.
Guess we have both read the rules but for the education of anyone who hasn't
my gripe is that although the guidance claims to avoid situations which are
"too costly" the cost is NOT taken into account, only theoretical
disruption. Not only that but the acceptability of the lowest cost assessed
position to the householder and existing furniture and fittings are totally
ignored in the assessment. Not a gripe about the installer but he will get
his money. Absolutely a stealth tax on the hard-up.
A stealth tax on everybody - just like all the rest.
An election looms does
it not?
Yes indeed. Whether all of these intrusions into people's lives and
the stealth taxes will be sufficient to influence how they vote is
another thing though.
2) There are some typical curves for condensing boilers on the Keston
web site.
Thanks for the Keston link. Seems to me that the main efficency advantages
acrue from the larger heat exchanger.
Until you get down below the dew point.....
3) It doesn't matter if you have an existing radiator design for the
more typical 82/70 boiler arrangement. When the weather is really
cold, the boiler will wind up to these temperatures. When it's
warmer, less heat will be needed and it will modulate down and try to
operate at as low a temperature as possible.
How exactly does it do that? Maybe I have got this wrong but I thought the
modulation worked by reducing the gas rate to control the flow temperature
to its setting. I would have to manually adjust the temperature according
to the weather to achieve lower flow temperatures?
There are a couple of ways.
If you have TRVs, they will respond to the rooms becoming warm and
reduce the flow. The boiler is monitoring the resulting changes in
temperature of the water and will respond to it by adjusting output.
Since the heat required to be put into the room to maintain a given
temperature varies with outside temperature (plus a lag), there is
going to be a correlation between burn rate and outside temperature.
Some boilers have outside temperature monitoring and weather
compensation. The effect here is that the boiler now knows directly
what the heat requirement is going to need to be.
You can always adjust the water temperature down. Obviously, the
upper limit needs to be enough to provide adequate heat under cold
conditions. Other than that, cooler water means longer warm up time.
So for much of the year,
you would still be operating in the more efficient range below the dew
point. Obviously if you take the situation over the whole year it
won't be *as* good, but still worthwhile.
OK, I've turned the temperature down, doesn't the water heating take an age
now and give me legionnaires disease?
It depends on the boiler and its connection arrangements. If it has
a way of knowing that the heat demand is for a water heating cycle,
then it can go to a higher temperature for that. If not, even if the
flow temperature is reduced to 70 degrees, it will be enough to
maintain a cylinder at 60 degrees.
Not if its a condensing combi but I'm not going there.
Quite. The point is though that the boiler does then know the
difference between HW and CH.
Jim A
--
..andy
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