On Sat, 4 Sep 2004 10:51:59 +0100, "IMM" wrote:
"Andy Hall" wrote in message
.. .
On Sat, 4 Sep 2004 09:35:12 +0100, "IMM" wrote:
"Andy Hall" wrote in message
.. .
On Fri, 3 Sep 2004 00:24:58 +0100, "IMM" wrote:
Generically, a condensing boiler will have greater efficiency when run
at lower temperatures and when the rate of heat production matches the
characteristics of the load.
And there are various ways of doing that.
Putting a dampening element like a heatbank in the control loop is not
one of them.
It is when appropriate weather compensating control is used, to ensure the
heat bank, or lower section of heat bank, is maintained at the variable
setpoint dictated by the compensator.
The heatbank will always introduce a dampening factor into the control
loop. The element that is intended to be controlled is the room
temperature, not the water temperature of the heatbank. The
weather compensator, if integrated with the boiler should move the
operating curve of the boiler up and down to match the load.
If you switch to maintaining the temperature of part of the heatbank
to maintain a set point according to the outside temperature, you
introduce a double control loop.
In the piece between the heatbank and the boiler, if you use the
typical separate box type of weather compensator, you will have an
analogue sensor for outside and one for inside, plus an input for a
switched room thermostat. The inside analogue sensor would
normally be used on the boiler return, but could be used on the return
from the radiators to the heatbank. Neither is ideal because
connecting it to the radiator return means that the controlled device
(boiler) has the damping effect of the heatbank in between which will
adversely influence the control loop. Having it on the boiler return
means that the temperature being monitored relates to heat use by the
heatbank and not the room space.
Then you have a switched control of the boiler - cycling it on and off
to attempt to maintain the temperature of the heatbank to a set point.
This is inefficient because you are then cycling the boiler.
All in all, a pointless exercise.
Having an effective control loop in
place with the boiler able to sense the operating load properly is the
best way to achieve that.
A good method but there are other ways.
Adding other things will circumvent
the boiler's control system
You have a boiler with a minimum control system.
There's no point. It's overall cheaper, better, more efficient and
simpler to have a modulating boiler with monitoring of its water
temperatures, connected to the radiators directly, and with a weather
compensating sensor able to provide temperature information (not just
on/off cycling).
Within the context of having the boiler in direct control of its load
with its internal control system and using analogue sensing vs. having
simple external sources turn it on and off, the former is clearly
going to be the most effective and efficient.
Not so.
Of course it is. Do you want me to demonstrate the behaviour of
control systems to you or can you provide a properly documented
reference with formulae?
You have to know what you are controlling before you apply control. You
don't.
It is very obvious what is being controlled. One part is the room
temperature. The other part is controlling the boiler firing level
most efficiently to match the room requirement. In the case of a
modulating, condensing boiler, this comes by maintaining the firing
rate at the minimum required rather than turning it on and off.
An external temperature sensor connected directly to the boiler to
provide it with external temperature info. directly will assist with
comfort on the one hand by increasing output above what is suggested
by room requirements when the outside temperature is falling, and
reducing it when the outside temperature is rising to reduce
overshoot.
In a system where the only source of heat input is a gas condensing
boiler, there is no point in putting a heat bank in the middle because
it simply distorts the control algorithm, and offers no advantages.
it also screws up the primary purpose of the heatbank, which is to
provide an energy store to deliver large amounts of energy quickly to
a heat exchanger.
Only the introduction of more sophisticated external controllers with
either PWM or analogue control of boiler modulation would improve on
the boiler internal control system in terms of oprating the boiler in
its most effcient range.
Not so.
For a normal domestic application this would be costly.
Again, not so.
You have already discussed adding in weather compensating boiler
controllers and extra valves and pumps.
1. compenstor
1. 3-way diverter valve
1. extra cyl stat
1. cheaper simpler boiler.
All in all works out about the same, and a superior more efficient system.
Neither statement is true.
There's at least £250-350
cost in that lot. In the context of a boiler costing £500-900, which
would do the job perfectly well on its own, this is adding unnecessary
cost.
..
A BEM5000 costs £188
A diverter valve costs £56
You forgot the extra pump to run the radiator circuit - £56 for a
Grundfos
All prices from Discounted Heating. Total cost is £311.
You can buy a Worcester Greenstar 28HE for £725 from them.
What 28kW condensing boiler you going to buy from Discounted Heating
for £410 that is any good?
Added to which, if the boiler controller is not controlling the boiler
by analogue or analogue equivalent means it cannot possibly do a
better job than the boiler's internal modulation controls.
What tripe!!!
Oh dear. Do you know nothing about the difference between analogue
or pseudo-analogue vs. switched control?
The simple external weather compensator
boxes do not do that. They are
intended as an after-market add-on for
non-modulating boilers and by
definition will not do as good a job as an
analogue sensing internal system.
When combined with heat banks and heating a large mass of water matters are
very different. This you can't understand. This is very sad.
Of course they are very different and I understand the difference
perfectly. The results, if using a condensing boiler as the sole
source of energy, will be inferior, by definition, than the direct
connection of the boiler to the radiators as the manufacturers intend
and design.
The value in a heatbank is in its ability to store energy at high
temperature to run a plate heat exchanger for the hot water. It is
also useful in the introduction of heat from other sources such as
solar.
It provides no value in a path between a modulating condensing boiler
and radiators. Even with all the extra controls that you describe,
the efficiency will be worsened. To suggest that coupling a
switching controller with a simple boiler is an improvement is
laughable.
..andy
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