On Sat, 4 Sep 2004 13:14:02 +0100, "IMM" wrote:
"Andy Hall" wrote in message
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On Sat, 4 Sep 2004 10:51:59 +0100, "IMM" wrote:
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.
Not so. It will prevent/reduce boioer cycling that is for certain.
In the DHW case where either the water in the heatbank is cold or
where the rate of use of heat via the plate heat exchanger massively
exceeds the boiler capacity, boiler cycling will obviously not occur.
That much is obvious.
In the case where the heatbank is sitting between a modulating boiler
and the radiator load, it will screw up the control loop and reduce
the efficiency of the boiler, simply because the boiler is unable to
monitor the radiator load directly.
If you use a non modulating boiler and control it in some way based on
the heatbank temperature, that may be better than a non modulating
boiler relying purely on its internal thermostat. However, since the
burner is either full on or off, it is a) cycling and b) not operating
at the optimum temperature - unless you are talking about a
non-condensing boiler as well, of course.
If you use a modulating boiler and attempt to control it in an on/off
fashion as an external simple weather compensator will try to do, then
the efficiency will be worsened as compared with allowing the boiler
to monitor the radiator water temperature directly.
The element that is intended to be
controlled is the room
temperature, not the water temperature
of the heatbank.
Your comprehension is poor. The boiler/heat bank maintain the lowers section
of the heat bank at the temperature that the rads require for optimum
performance. That is very simple.
It is very simple, and you have it wrong. There is nothing to be
gained with partitioning a heatbank when a gas condensing boiler is
the sole source of energy. Further to that there is nothing to be
gained by having a heatbank in the radiator part of the load at all in
this instance.
The weather compensator, if integrated
with the boiler should move the
operating curve of the boiler up and down
to match the load.
This weather compensator is NOT integrated with a boiler. This you can't
understand.
Oh good grief.
I am fully aware that this weather compensator is not integrated with
the boiler. That is why using this type with this application is
completely pointless.
There are three useful applications for weather compensation:
a) An external, simple one of the type you describe, like the BEM5000,
which has analogue input sensors and a switched output which cycles
the boiler on and off. This is useful wih conventional, especially
older and possibly overpowered boilers where the hysteresis of the
built in thermostat is large and the boiler has a tendency to cause
the room temperature to overshoot; and the thermal characteristics of
the house tend to cause the same. This type of device reduces the
average radiator temperature by switching the boiler on and off rather
than letting its thermostat do the job. It's a crude system and may
make an improvement in this environment.
b) An external temperature sensor connected to a modulating,
condensing boiler's microprocessor. This type of controller already
has analogue inputs from the flow and return temperatures and can
control burn rate and even the pump. This additional sensor gives
additional data from the environment outside the house which will
affect the internal temperature after a time. The boiler can then
adjust accordingly as well as maintaining minimum power level and
temperature.
It isn't possible to achieve the same levels of control and efficiency
using method a) as this.
c) A commercial system, where there is an external energy management
system having multiple analogue sensors inside and out, and able to
monitor the water temperatures. This type of controller is able to
control a boiler or boilers which are able to modulate, but by the use
of some kind of digital signal (PWM or coded), or via an analogue
control voltage or current (e.g. 4-20mA, 0-10v etc.)
Your proposal is for something that is basically a type a)
arrangement, but you are claiming that it is equivalent to or better
than type b) or c). This is patent nonsense.
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.
Nope.
Of course you do. One loop is from boiler to heatbank, the other
from heatbank via radiators to room.
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
Yes.
and one for inside,
?? One to sense the temp of the heat bank
plus an input for a
switched room thermostat.
Uh! Switched? room temp trimming is analogue unless to have a cheapo.
take a look at the BEM5000. This is the common weather compensator
and is hardly cheap.
It has a switched input for the room thermostat.
Contrast this to the Worcester Bosch Greenstar, which can have a
modulating room thermostat as an option for £42.
The inside analogue sensor would
normally be used on the boiler return,
Or boiler flow, which most operate on. Used on the heat bank here.
Er no. The important temperature for condensing efficiency is the
boiler return temperature.
For other control applications this is also commonplace because it
gives an indication of what the thermal load is doing.
The BEM5000 uses the sensor on the return.
but could be used on the return
from the radiators to the heatbank.
Could be on the cylinder not far from the boiler/rads returns pipes.
That makes it even worse. The temperature of the cylinder wall is
only vaguely related to the return pipes and will have a time lag.
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.
It is clear you do not understand.
It's clear that I do, and that your scheme gets more an more
complicated and less useful/
The rad circuits are off the bottom of
the heat bank and have their own pump. Easy.
An extra pump.
?The boilers flow and return
is off the heat bank, at the bottom of the heat bank. The compensator has
a probe on the bottom section of the heat bank producing a mass of water at
the temp the rads require. The lower temp at the bottom on part load will
ensure low temp return for high efficient condenser operation, with an
expensive complex boiler.
I presume that you mean without an expensive complex boiler.
Do you think that the Greenstar at £725 is expensive?
If you take off the £300 or so needed to implement your scheme, plus
the poorer long term reliability because of the extra components, you
have about £400 to spend. Are you saying that you can get a good
quality condensing boiler for this.?
Since the external box weather compensator is going to turn the boiler
on and off to maintain the temperature, it will not run at its most
efficient. You would be trying to match a (say) 30kW boiler into a
(say) 10kW load. The boiler cannot run efficiently like that.
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 room temp influence only trims.
This depends on the thermal characteristics of the building.
The other part is controlling
the boiler firing level
most efficiently to match the room
requirement.
No. Get the boiler to heat a mass of water all at one time, to what
temperature the room requires.
This is why your approach is nonsense.
If you were to connect the modulating boiler directly to the
radiators, an average flow temperature will be in the 50 degree area.
If you try to do this via part of a heatbank and set that temperature,
there is relatively little energy storage - the typical volume of
water is perhaps half of the radiator capacity.
Very soon, the water temperature in the heatbank will have fallen and
the boiler has to come on again. In effect, you are making it cycle
by having regular relatively short full power burns.
This is not as efficient as letting the boiler run continuously at the
temperature required for the radiators.
In the case of a
modulating, condensing boiler,
No need for a modulating condensing boiler./
There are very few quality ones that don't.
Most do modulate, but heating
boilers modulate on flow setpoint temp.
.... and also account for the return temperature.
As the boiler will be on full, when
heating the mass of water for CH very rare will it modulate.
It will be cycling inefficiently instead.
Not only that, the radiator temperatures will be fluctuating as well.
Fundamentally, you can either let the heatbank cool right down and
reheat it at full power which causes the boiler to run less
efficiently and radiator temperatures to vary, or you can attempt to
maintain a setpoint. If you do the latter, then having the boiler
going on and off is not the best way to do it.
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.
You still don't understand.
I think that you don't or are simply being obtuse.
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.
Primary?
And to prevent boiler cycling and giving on-demand mains pressure hot water.
Eliminating cold water storage tanks.
That is implicit in what I said.
With a heat bank the DHW and CH
circuits don't care about the boiler.
They have totally different characteristics.
The DHW application needs large amounts of heat for relatively short
periods of time. That is why it is called a heat *bank*. It
allows overdrafts relative to the capabilities of the boiler.
The CH application requires much smaller amounts of heat on a
continuous basis with quite slow rates of change of demand.
The boiler can be controlled more
efficiently by having it heat one mass of water very quickly and in one long
efficient burn.
That is only true for the DHW case. For the CH case it is not. We
have already established for a condensing boiler that it will run more
efficiently at lower temperatures and power levels continuously than
by switching on and off.
For the DHW application, the main purpose of a long burn at high power
level is to transfer as much energy as possible back into the cylinder
as fast as possible.
Taken further it can heat two masses of water at different
temperatures.
So now we have two heatbanks?
High for DHW and low for CH. Taken further again, the CH mass
of water can be at variable temperatures to what the outside weather
dictates. When called to heat the CH mass of water it does it in one long
burn no matter what temperature the CH water mass is.
The burn and cycle time will vary with the rate of use of energy and
the amount of temperature drop allowed before reheating is initiated.
This is far from efficient.
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?
But you save a wedge on a 1.5K load compensating boiler and musch simpler.
Where did you get £1.5k from?
I mentioned the WB Greenstar at £725 with all these features.
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.
Not so.
Sigh.....
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.
You clearly do not understand. "by definition, than the direct connection of
the boiler to the radiators as the manufacturers intend and design.". I am
not on about one of theses type of boiler.
So you mean a non modulating boiler? These are not as efficient as
modulating types by definition, and all that you are proposing is to
put a simple switching controller and a damping element in front in an
attempt to make them so. This is a nonsense.
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?
It is also useful in the introduction of
heat from other sources such as
solar.
..and a condensing boiler.
Not if that is the only source.
Even with all the extra controls that you describe,
the efficiency will be worsened.
You clearly do not understand.
I clearly do.
To suggest that coupling a
switching controller with a simple
boiler is an improvement is
laughable.
You clearly do not understand.
ditto. etc.
..andy
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