On Sun, 14 Nov 2004 02:28:55 -0000, "IMM" wrote:
It is. You have never dealt with boiler manufacturers.
I have actually - several - and thoroughly tested their technical
departments.
Once.
Actually several times for myself and for others as well.
However, to be fair to them, what proportion of Ferroli's target
customer base is likely to combine two combis in the way
you describe? One in a thousand? It's not going to be one in ten.
Not the point. They are supposed to know their product and systems in
general. They don't.
Why? They are there to support the requirements of the vast majority
of their customer base. For a domestic boiler, this is going to
mainly be installers of single boilers in houses. How often do you
imagine they get questions on how to hook two of them together? Once
a year? Twice?
..
The ouput of the boiler falls, not the temperature given to the building.
If the output falls it's as a direct or indirect consequence of the
building requiring less heat and does not imply that the burner is
cycling to do it.
It may not be the same mechanism
as load compensation, but the effect
is crudely the same - the boiler
does not cycle in the same way as
one with full power on/off would.
You have lost it. It reduces cycling.
That makes no sense. If the output is reduced, it results in less
heat transfer. If the heat requirement of the load is matched by
the boiler running at lower output, why would the boiler cycle.
Your argument only holds true if the boiler output exceeds the load
requirement.
they have temperature sensing and
electroniics to optimise behaviour.
Only the up market expensive boilers.
Modulation is still based on sensing *a* temperature somewhere and
modulating on the basis of it. My point was that the argument that
a heatstore is needed to stop cycling is really not true for this
type of boiler.
An expensive load compensation boiler, not flow setpoint modulation.
The issue is that the output is reduced - the boiler modulates down,
not off.
If the monitored point is maintaining the flow temperature set point,
then inevitably if the load reduces, the burn rate will be reduced to
match it - otherwise why have modulation?
DPS are intimating that cycling is going to be a
big issue needing a thermal store *unless* there
are some electronics and modulation.
Correct.
Most condensing boilers are modulating
types so they are really trying to make
a selling point out of what has become a
corner case.
You don't understand the various types of modulation.
Actually I do.
However the point is that the power output is being reduced.
Their argument and yours is based on an assumption of cycling.
Where does this arise unless the heat required is less than the
minimum that the boiler can do?
With a heat bank high efficiencies can gained from a very simple cheap
condensing boiler. A great bonus.
It becomes an equation between
a) simple boiler (and there seem to be few new ones coming onto the
market like that) plus heatbank, or
b) boiler with modulation.
The only bonus in a) for the customer is if he has to use a simple
boiler for other reasons or is the cost of a) is less than b)
Obviously a simple boiler with on/off
burner control will operate more
effciently with a buffer.
And one with flow setpoint burner modulation.
Are you saying that this form of modulation
would cause the burner to
cycle on and off rather than up and down?
I'd find that hard to believe.
The burner modulates down when approaching setpoint reducing, not
eliminating, cycling.
If the burner is turned down and not off, then cycling hasn't
happened.
A heat bank can eliminate cycling and produce high
efficiencies.
If the boiler would otherwise cycle. Where is the mechanism for that
if the boiler modulates down such that it is producing heat at the
rate required by the load?
They don't present a convincing argument
for the case of most new boilers which
modulate.
They do.
It doesn't convince me.
because you don't know much.
So you believe that this can help a modulating boiler that is not
cycling off because its output matches the load?
How can that happen? If you are taking water from the cylinder at
25 degrees and passing it through a heat exchanger
No direct.
I assume that you are
talking about a directly heated
store where the bulk water goes
through the boiler.
Yes, and so are they.
This means that the water will need to go
several times through the boiler to reach
75 at the top.
No. One pass and it heats up top down and the heated water from the boiler
does not mix with the water already in the store.
How can that happen? You can't have water entering the heat
exchanger at 25 degrees from the bottom of the store, going through a
heat exchanger with dT of 25 degrees max and delivering water into the
top of the store at 75 degrees. That can only happen when the
return temperature reaches 50 and that won't be in one pass.
So, the secret is:
- Have a tall thin cylinder to aid stratification.
- Controls to ensure store water is heat in only one pass through the
boiler
- Stats to ensure the store is re-heated when most of water has cooled.
This
also prevents boiler cycling.
If the boiler is a modulating type, what is the mechanism for it
cycling?
uh! ???
It was a simple question. What is the reason that a modulating boiler
would cycle without the store if the production rate matches the load?
This will only happen if the rate of heat production by the
boiler exceeds the rate of use. If the burner has modulated down and
stays lit, then it will be running in an efficient range.
If it has load compensation control, it may drop the flow temp tom
unacceptable levels.
That's only an issue if you put the store there. If you are driving
the heating load of radiators directly and the UFH via a blending
valve, the issue doesn't arise. There is only a need to heat the
store to 75 degrees if it's driving radiators and they need this
temperature to give required output or if it's running a DHW exchanger
as well. If it's only driving a DHW exchanger and that is handled
by a priority arrangement and motorised valve, then the whole thing is
simple.
I get the distinct impression, that as a vendor of thermal stores,
they are bending the situation to justify buying their product.
They are telling it as it is. Also with thermal stores/heat banks, a
cheaper simpler boiler can be purchased.
They are selling a thermal store, partly
on this argument.
Only one of the benefits.
It would be on boilers that exhibit cycling. Other than that it's a
hollow argument.
There are cases where a simpler
non-modulating boiler may be what has to be
used (e.g. oil). However, most modern
condensing boilers are at
least modulating types
Depends on what type of modulation.
Why do you believe that that affects whether or not it will cycle if
driving a heating load directly and matching the requirement with
lower burn rate?
and this effectively knocks that argment on the
head.
You fail to grasp.
Then please explain. I want the mechanism by which you believe that a
boiler modulating on flow set point and running with output at or
below load requirement will cycle the burner on and off.
The sales argument is a reasonable
one as a sales pitch, and results
in the customer buying a heatstore
to make a simple boiler more
efficient - revenue for DPS.
Heat banks are not big sellers, but getting better, as most plumbers fail to
understand them.
With embellishments and claims that are stretching reality it's not
surprising.
What I believe not to be reasonable is
the impression given that condensing
boilers *need* this unless they
are in some way sophisticated when the reality is that it's nowadays
teh exception that condensing boilers are not modulating.
While not untrue it's at least misleading.
Nothing misleading, you just fail to understand. That is sad.
Then please explain the mechanisms as requested
I am specifically asking why you believe that the method of modulation
has an impact on whether a boiler will cycle when running at or below
the heat load requirement when directly connected to the load -i.e.
radiators and via a blender valve to UFH.
--
..andy
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