On Mon, 10 Jan 2005 15:36:03 -0000, "Jim Alexander"
wrote:
"Andy Hall" wrote in message
.. .
On Sun, 9 Jan 2005 16:12:26 -0000, "Jim Alexander"
wrote:
Its useful feedback as input to my planning. Monitoring my return temps
don't think I will get much condensing so I'm assuming 15% efficiciency
gain.
It's hard to do a meaningful comparison using a conventional boiler
for this because the whole principle of control is different.
In the conventional boiler environment, the room thermostat will cause
the boiler to fire up and the temperature to rise to the design 82
flow, 70 return or thereabouts. One control loop is the boiler
cycling on and off to maintain approximately those temperatures.
With the condensing boiler, the boiler modulates and is striving to
maintain the operating temperatures as low as possible consistent with
providing adequate heat for the radiators.
I'm talking mainstream domestic condensing system boilers that would be used
for replacement in a smallish system. Yes they modulate just like combis
have for a few years but your explanation is misleading.
Not really.
Fixed pump speed
is the norm in the smaller boilers.
True on many system boilers, but a number even in smaller capacity
have at least a switched pump.
If a self adjusting pump such as the Alpha is used then variation as a
result of TRV behaviour is accounted for.
The set point is the same old
temperature limit as in older boilers, a limit which Potterton for example
says set to max in winter. The modulation can only try to keep the
temperature at the setpoint, not down to a lower setpoint. The boiler has
to achieve the heat requirement of the radiators or heating effectivness is
poor.
This depends on the season, the system and what one is trying to
achieve. I wouldn't take anything from Potterton as indicative of
anything. They are living on former glory and not a lot more.
On a conventional boiler, you should not really be turning the boiler
thermostat down below the 82/70 figure too much since it will induce
condensing and in this environment that is deleterious.
In a condensing boiler, the boiler thermostat can be turned down when
the weather is warmer. This is because the heat loss from the house is
lower and required radiator output lower. It's a fallacy to suggest
that the system needs to be run at 82/70 for most of the time unless
the radiators are marginally sized or the weather exceptionally cold.
It's true that more sophisticated condensing models have weather
compensation and take this into account to control the water
temperature, but this does not mean that the manual control that can
be applied to a simpler condensing boiler to operate it in a lower
temperature range rather than having it cycle is not valuable.
The radiator system may reduce the flow as a result if TRV action.
Perhaps that's why some condenser advocates now recommend removal of TRVs.
A flow reduction causes the return temperature to rise, the modulation only
goes so low, typically 50% of rating.
Actually not. If you look at products from Vaillant at the low
capacity end they will modulate from 4.8 to 14.2kW - essentially 3:1
The small Worcester Bosch is similar.
Crucially in a system boiler the
setpoint has to be high enough for safe DHW. In the mainstream domestic
system boilers there is not currently a separate control loop for DHW. Yes
if there was things would be better.
I'm not arguing against bespoke condensing design, only the dubious
economics of compulsory replacement in some small systems.
Since the issue is one of compulsion through legislation, there are
more issues to consider than the cost of your gas bill.
I suppose that a max power level could have been set below which
fitting of condensing boilers was not mandatory (notwithstanding the
other exemptions), but it's then adding another level of complexity to
the whole thing. Since condensing combis, for right or wrong have a
significant market share and in general have a minimum power level of
20-25kW to do anything remotely useful, it would be difficult to
structure and operate rules to specifically exempt just small heating
boilers.
--
..andy
To email, substitute .nospam with .gl
|