"Andy Hall" wrote in message
...
On Sat, 4 Sep 2004 17:22:51 +0100, "IMM" wrote:
"Andy Hall" wrote in message
.. .
Not so. It will prevent/reduce boiler cycling that is for certain.
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.
Nonsense. Thew boiler doesn't
need to know the return temp of the rads.
It does if it is going to be involved
in an efficient control system
where the heat output to the radiators
and the heating space are
measured
They are measured by the weather compensator, with any room temp influence
to drop the slope if need be.
and controlled without lag,
then it most certainly does.
No lag. A mass of water the temperature required is available.
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.
You are getting there.
I said *may*, and it will only be in so
far as a difference between an
analogue sensor on the cylinder
and a thermostat with substantial
hysteresis on the boiler.
Thermostat on the boiler is on full, and will rarely cut out on this.
The whole thing is then wrecked by
the external controller driving the
boiler in on/off mode.
However, since the
burner is either full on or off,
it is a) cycling
Failed.
So are you saying that the external
controller doesn't control the
boiler by turning it on and off.
No.
You'll be telling me that it
operates a motor on the gas tap next.
Nope I will not.
and b) not operating
at the optimum temperature
Failed again.
If it is a condensing boiler, on/off control with full output or
nothing will not operate it efficiently.
You fail to understand condensing boilers, or boilers at all. A boiler will
only raise water a certain temperature with a given flowrate through the
heat exchanger. The flow through it can be constant, as no TRVs used. If
say at x flowrate it raises the water 25C, if the return is 30C only 55C
come out of the flow. Way below the boiler stat set to 82C. When the mass
of water is heated the boiler stat would have never have cut in to cycle the
boiler, so one long efficient burn.
- 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.
Failed again. When on it heats a large
mass of water. This confuses you.
That is sad.
The trouble is that it doesn't.
You are rarther silly now.
On the one hand you are saying that you are going to have part of a
heatbank operating at a set point and being maintained that way by
controlling the heat going in. By definition, if you do that with a
non-modulating boiler with power output level larger, probably
substantially larger than the rate of use by the radiators, then you
will have to cycle the boiler.
The boiler does not cycle when re-heating the mass. Operation: heats the
mass of water mass cools - mass is reheated all at once with "no" boiler
cycling - heat extracted from the water mass by the rad circuit - mass is
reheated all at once with "no" boiler cycling .
That is unless you know some law of
physics whereby heat can be stored
in a different dimension.
You are mad.
On the other hand, you say that you want to have long burns on the
boiler to improve efficiency (which it only does by not having
cycling,
Mor failure. You have droped from 2/10 to 1/10.
not because of running temperatures). If you do that, you
are going to have to let the heatbank (or part thereof in use)
discharge all of its heat before running the boiler again.
This will result in cooling and heating of the radiators, a
fluctuation in heat output and a variation in room temperature.
Any fluctuation would be minimal. the mass of water takes time to cool. It
is not like hot water running though a rad circuit return pipe. very little
mass and less mass in the whole rad circuit than in the stored water mass by
at "least" half as much. The water mass stabilises operation. think hard
about that. Ermmm well don't bother.
You can't have it both ways.
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.
Failed again. It is right it is just above.
As I said, you can't have it both ways.
Failed again.
There is nothing to be
gained with partitioning a heatbank
when a gas condensing boiler is
the sole source of energy.
Lots to be gained. A cheaper simpler boiler
and vastly reduced/elimination
of cycling.
There's nothing to be gained.
failed again.
You have added a bunch of complexity and achieved poorer efficiency.
Less complexity, as simple cheaper boiler is used.
You haven't saved any money either
No expensive 1.5K bpoioer to be bought.
Please provide a make and model
number of one of these "simpler boilers"
at around £400 Discounted
Heating prices, that is anywhere near the
quality of a WB Greenstar.
You have a thing about £400.
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.
A heat bank a neutral point. A great advantage.
Only in circumstances where
a) multiple heat sources are being combined or
b) when there is a need to store heat because of the requirement of
large amounts to heat water, or
c) when the boiler, because of its sizing and limited or no control
range would naturally cycle.
This is not any of those cases.
It is. Even a modulating boiler cycles when demand is below the minimum kW.
With a heat bank, no cycling.
The boiler and rads operate
independently of each other. Constant flow can be through the boiler at
all
times.
That's clearly nonsense.
Failed again. The boiler and rads operate independently of each other.
The only way that they can operate truly
independently is by having two heat banks.
Are you advocating that now?
failed again.
The heat requirement into the cylinder will be quite different for DHW
vs. CH. In the former case, it will be maximum transfer after water
has been run. In the latter, either the heat will have to be fed in
at the rate of use (less than the boiler maximum output) or at full
power for short periods.
failed again. Full power for the re-heat time in one burn.
Where you have got the idea of constant flow from, goodness only
knows. If you mean constant water flow, regardless of demand, it is
going to be wasteful when neither CH nor DHW are demanding heat.
More failure. Boilers have an optimum flow through the heat exchanger.
No silly pressure diif valves needed. Rads with TRVs can be
virtually off and the boiler doesn't know or care.
There aren't anyway. If the pump is integral to the boiler,
Heating boilers don't have them integrated, well most don't.
the boiler's controller can match
it to the heat load.
Heating boilers with minimum control are used.
An exterior one can sense the flow
and pressure conditions and also reduce output.
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.
Sounds encouraging.
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.
Failed again.
I'm not repeating the obvious, and I'm certainly not about to educate
you about control systems.
Be sure, you will not educate me on control systems.
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.
It is. When using heat bank.
Not if the controller is operating
the boiler by turning it on and
off, which it will have to do in the
case of a non modulating one.
Switches on - one long bur to setpoint - switches off. Simple.
This is how the BEM5000 works.
All that the heatbank is doing is to
add a lag in the control path.
failed again. not so.
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.
None of your points has a heat bank incorporated, so squewed.
Again. All that the heatbank does is to
introduce a lag in the control system.
You can either choose to keep it topped up with heat
on a tight setpoint window, in which case the boiler will have to be
cycled a lot, or you can let the heatbank temperature drop by the use
of a wide window in which case it will cycle less and the radiator
temperatures will drop.
drop by not much at all.
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.
Only one loop. Boiler to heat bank, dictated to by outsoide waether
conditions.
There is also the room thermostat or TRVs as an outer loop.
room stat? no there is not. TRVs are not in the boioere control loop.
The heatbank is not representative of the characteristics of the
radiator heat load.
It provides a mass of water at the temperaure the rads want. The heat bank
doesn't care about the rad heat load, it just provides the mass of water for
the rads to use.
All that you achieve at best is a rather poor
attempt at providing a heat source to the
radiators, the temperature
of which is varied according to outside conditions. THe boiler energy
manager, will attempt to maintain the heatbank temperature by cycling
the boiler on and off.
more failure
There is no value in doing that in comparison with a true modulation
of the boiler.
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 is not the only model. It saves the cost of a 1.5K boiler.
Please provide an example
of a suitable weather compensating
controller at the same or lower price
than the BEM5000.
All the big makers make compensators at low prices.
Please also explain how you believe that on/off controlling a simple
boiler is more efficient than a modulating, condensing one.
Please read all of this thread again.
Please explain where the figure of £1.5k comes into this.
The discussion is around something like a WB Greenstar with price
point of £725. It's not good trying to justify your argument by
adding in £800 of cost which is irrelevant.
biolers with built-in load compensation modulation and waether compensation
are well north of 1K
THe arithmetic is between a good quality modulating boiler at £725 vs.
the cost of some alleged "simple" boiler at £400 plus your controls at
£300.
You are making things up.
It has a switched input for the room thermostat.
Not good if room influence is incorporated.
Exactly.
Contrast this to the Worcester Bosch Greenstar, which can have a
modulating room thermostat as an option for £42.
Only does one zone. A heat bank can have two zones fed from it. Each
zone
hits a neutral point.
This is irrelevant.
failed again. It isn't
The issue is still one of energy in vs. energy
out.
In and out of what? less energy into the boiler for energy out.
Microbore can be used when both go back to one
neutral point.
This is irrelevant as well. Microbore is simply a size of tube.
There is nothing magic about it.
Look up how microbore operates.
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.
Er yes, depending on application.
Sigh.....
The important temperature for condensing efficiency is the
boiler return temperature
Lower the flow and the return follows suit.
Well obviously. Most modulating boilers that I have seen have both
flow and return sensors.
For other control applications this
is also commonplace because it
gives an indication of what the thermal
load is doing.
Using a heat bank the thermal load is irrelevant. The boiler doesn't
care
about it.
Oh dear. Of course it's relevant.
Not as far as the boioer is concerned.
THe DHW load is an overdraft on the heatbank of anything up to 200kW
for short periods of time. Boiler requirement to recharge is full
power, for a short time.
The CH load is likely to be 20-80% of full load according to demand
and is continuous during short to medium time periods.
The BEM5000 uses the sensor on the return.
Just an option they use.
It's pretty critical to the operation of the operation of the unit.
From the instructions:
"The ESMA must be mounted on a pipe connected to the boiler return
tapping, between the bypass and the boiler, using the clamping band
provided, in a position where it will sense the temperature of the
water returning from the HEATING circuit"
ESMA is the return sensor. It doesn't say *may* but *must*.
Regarding the room thermostat it uses the words
"If a room thermostat is used....."
i.e. optional.
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.
failed. makes it better as it sense the mass of water, which is at the
temp
the building requires.
The mass of water will be at some temperature. The controller should
be trying to measure the radiator return temperature in your example
because that represents something derived from the heat load into the
space. The temperature of the heatbank is not well related to that.
The temperature of the cylinder wall is
only vaguely related to the return pipes
and will have a time lag.
The boiler doesn't want to know the rads return temperature. I doesn't
care.
In the case of your external controller and full power switched boiler
set up, obviously not. The closest would be that the controller
wants to know the return temperature from the radiators.
However the exercise is pointless because it will be worse than
letting a controller in the boiler do the job.
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,
No. It is clear you do not understand.
It's behind you.......
and that your scheme gets more an more
complicated and less useful/
It is simple. It also works very well indeed.
Have you implemented and measured this in comparison to a modulating
condensing boiler connected to radiators directly?
If so, which boiler and controller did you use for each?
The rad circuits are off the bottom of
the heat bank and have their own pump. Easy.
An extra pump.
That will break the bank.
None of this breaks the bank.
The point is that you are proposing an unnecessarily complicated
system, which patently can't perform as well as one with analogue
sensing of water temperatures and modulation of the burner.
You are unable to provide types and prices of components for this that
will come in at under the price point for a modulating system.
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.
yep.
Do you think that the Greenstar at £725 is expensive?
Not bad for what it is. Others are cheaper. This is besides the point.
Not really. You can get modulating, condensing boilers for a bit
less than this, or a bit more.
What is the make and model number of your alleged "simple" boiler,
with equivalent build quality to this?
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 a simpler boiler with less components.
They are all external in various boxes, valves and extra pumps.
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.
failed again.
You would be trying to match a (say) 30kW boiler into a
(say) 10kW load. The boiler cannot run efficiently like that.
You fail to understand. A mass of water is heated to a temp a
compensator
dictates. The mass of water is heated and the rads take off as much heat
as
they need. Wheh the mass of water needs heating the 30kW boiler reheats
the
mass. The boiler know or care about loads of rad circuit.
No you fail to understand.
The equations are simple.
In the modulating, condensing boiler case, if the load to the
radiators needs to be 10kW, the boiler will drop down to that output
level and a low running temperature, continuously. There will be no
cycling at all and the boiler will be running well into into the
higher efficiency part of its operating curve.
In your simple boiler case, the 10kW load match will have to be
achieved (assuming a 30kW boiler) by running it at full power for a
third of the time. This is less efficient, a) because the boiler
is going on and off and b) because it is not operating on the most
efficient part of the curve.
You can make those on/off periods shorter or longer by how large you
make the allowable temperature drop in the heatbank, but it will
average to a 3:1 off/on.
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.
It doesn't, it depends on what compensator you use.
Oh really. What kind of compensator do you have in mind?
It is possible to run a modulating boiler purely based on outside
temperature measurement, but the effect will be that the inside
temperature will tend to fluctuate.
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.
failed again. See above
Sigh.
If you were to connect the
modulating boiler directly to the
radiators, an average flow temperature
will be in the 50 degree area.
Once the buildings heat demand is below the minimum of the boiler cycling
starts.
Obviously. However it is far better if it cycles between 0 and 20%
of full load than 0 and 100%
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.
You make sure the storage is at least x 2 of rads.
So let's see. My radiators and pipework have a capacity of about
100 litres.
So now you are saying that I need a 200 litre heatbank for this; and
the one for the hot water in addition to that?
Plus I get worse performance and efficiency than a boiler connected
directly to them?
Why would I want to do that?
Very soon, the water temperature in
the heatbank will have fallen and
the boiler has to come on again.
Getting it.
In effect, you are making it cycle
by having regular relatively short full power burns.
No. short power burns at lomng intervals.
The energy still has to be put in.
With a 200 litre cylinder that will be about 30 mins on - I wouldn't
call that short.
It is also not a practically good example because the heatbank has
been run to zero in the intervening time.
This is not as efficient as
letting the boiler run continuously at the
temperature required for the radiators.
failed again.
Do you have figures which demonstrate that running a condensing boiler
at full power with cycling is more efficient than running it on part
power continuously?
If your case were more efficient, why do you imagine that the
manufacturers of modulating boilers arrange them to drop the power and
temperature as the load reduces? Do you imagine that British Gas
sponsors them to use more energy??
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.
failed again. Not so.
Not only that, the radiator temperatures
will be fluctuating as well.
Fluctuation of rad temps would be minimum.
Not if you run the temperature in the heatbank down.
What do you believe that the operating window of the heatbank should
be and why?
Fundamentally, you can either
let the heatbank cool right down and
reheat it at full power which causes
the boiler to run less efficiently
failed again. the boiler runs efficiently. as the mass of water is to
the
temp the building requires.
So are you saying that you want to maintain the heatbank at close to
the setpoint temperature or allow it to fall?
If so, by how much?
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.
Which is a great advantage. Divide and rule.
That's your best argument so far. It's a pity that it bears no
relationship whatever to the subject matter.
The DHW application needs
large amounts of heat for relatively short
periods of time. That is why it is
called a heat *bank*.
failed again. It is called a heat bank because it is a bank of heat.
That's helpful :-)
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.
failed again. applicable to CH.
This depends on the temperature range that you are going to use.
Are you going to try to maintain the set point or allow the
temperature to fall?
We have already established for
a condensing boiler that it will run more
efficiently at lower temperatures
Good. getting better.
and power levels continuously than
by switching on and off.
Failed again. Continuous power levels when burning in one long efficient
burn.
It will be more efficient at lower temperatures and power levels.
With your proposed simple boiler, this doesn't happen.
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.
Good.
Taken further it can heat two masses
of water at different
temperatures.
So now we have two heatbanks?
One split into two separte temperature zones.
With what? Some kind of divider or just relying on stratification?
If it's the latter, you are going to get a big interaction between the
performance requirement of the hot water vs. your desired control for
the CH part. Since the characteristics are completely different, you
will end up with a poor compromise.
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.
Efficient for the boiler usage.
Whatever that means, which is not a lot.
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 much
simpler.
Where did you get £1.5k from?
A load compensating modulation, outside waether compensating boioer is
about
1K
You introduced the idea of weather compensation as a means to control
the heatbank temperature, just so that you could hook up the radiators
to it - a completely pointless exercise.
Using a heat bank load compensating modulation is not required.
You can equally argue that with a modulating boiler with a room
temperature analogue sensor that outside weather compensation isn't
required.
You should find out the advantage of weather compensation.
So you mean a non modulating boiler?
Yes.
These are not as efficient as
modulating types by definition,
failed again. They are. It depends on how it is controlled and what
conditions it is run in.
Certainly with your suggested method of on off control it is not going
to run at anything like the efficiency of a modulating model using its
own temperature sensing.
failed again. more efficient.
Poor show only 1/10.
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