On Sat, 13 Nov 2004 23:36:44 -0000, "IMM" wrote:
"Andy Hall" wrote in message
.. .
On Sat, 13 Nov 2004 22:19:34 -0000, "IMM" wrote:
These people mainly deal with commercial applications, where headers are
the
norm. You intend to use UFH, contact the UFH people and just about
everyone
of them will say use a thermal store. So their weight of opinion would
be
greater, as there are more of them.
That is not a sound argument.
It is. You have never dealt with boiler manufacturers.
I have actually - several - and thoroughly tested their technical
departments.
At times their
knowledge of systems is minimal. They after all only make a water heater,
which is the new name for a boiler. Say boiler and you fail the corgi test.
Headers with a condensing boiler, UFH and rads in a domestic system is
ludicrous. I can them suggesting that if two boilers are used, or a regular
boiler, but a header which raises the return temp is ludicrous.
I contacted Ferroli about combining the output of two combi's. They said
no. I rang their top techie, who wasn't very good at all, and he was going
on about back pressure to the heat exchanger. I said a non-return valve
would be fitted on each and one also on the main in, so it highly unlikely
this would ever occur. I told him I had seen two Ferroli's connect this way
in France. He murmured and said no. They want people to fit them as per the
instructions as they can deal with that. Any deviation and they are
screwed.
I rang Worcester Bosch about doing the same. They said yes sir, and sent a
coloured diagram on how to do it. I mentioned what Ferroli said and they
confirmed my view that there is no reason at all why any two combi's of any
make cannot have their draw-offs combined as long as non-return valves are
used and there is enough flow.
Even the makers can't agree.
I'm not surprised. If I find an issue like this and there are signs
of disagreement or I am not convinced of the soundness of the
argument, I would probe deeper.
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.
Using a header will lower a condensing
boilers efficiency, that is certain. Ask
the boiler people about
connecting the boiler to an integrated
thermal store with the UFH and rads
taken off the store. Not one will say no.
You also have to take into account that
the tech depts have people who read
from crib sheets. Did you ask them about
a header specifically? Or did you
ask them about running a UFH and rad
system off the boiler?
Look at this:
http://www.heatweb.com/techtips/unde...orheating.html
Then scroll down to Twin Zone Thermal Store Injection System
A dedicated UFH thermal store shown with the old fashioned type of
waether
compensation of stats. Cheap and effective. Of course an outsise
weather
compensator can be fitted to maintain the UFH section to what the outside
temp dictates.
Rads can be taken off the top of the store.
Also go to:
http://www.heatweb.com
scroll down to underfloor heating and see the solutions to underfloor
heating.
There are quite a number of muddled points on their underfloor heating
page.
For example, they say:
"Reducing cycling in itself will improve
efficiency, however gains are
also to be achieved by keeping return
temperatures to the boiler low
at all times.
That is so.
Without a thermal store this is very difficult to
achieve, unless the boiler has built in electronics. "
That is so.
This is erroneous. If you have a
modulating, condensing boiler,
which most are nowadays,
What sort of modulation? Load compensation as yours is. Few have that.
Most modulation is maintaining the boiler temp setpoint. The burner lowers
when reaching setpoint. Very different.
Their description and justification is based on boilers running in
on/off mode between 60 and 80 degrees, and the argument is that a
heatstore is needed to smooth out that effect and keep the boiler
running for longer periods. I don't have any argument with that.
I also realise that there are different algorithms used in modulating,
condensing boilers, but the effect is still that the output is
reduced as the load requirement falls simply because the temperatures
will be maintained if that happens. 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.
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. DPS are intimating that cycling is going to be a
big issue needing a thermal store *unless* there are some electronics
and modulation. Most condensing boilers are modulating types so they
are really trying to make a selling point out of what has become a
corner case.
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.
They don't present a convincing argument
for the case of most new boilers which
modulate.
They do.
It doesn't convince me. Once somebody does this kind of thing, then
I go through the rest of their claims with a degree of scepticism.
Their whole "buffer store" argment is
based on the idea that the boiler will otherwise
cycle when driving the load, and this argument
is flawed with a modulating boiler.
Not so. They say that a low temperature will be maintained from a heat
bank/thermal store. This because of stratification and re-heating in one
pass of the water through the boiler. A tall heat bank that has dropped to
say 25C at the bottom and 55C at the top, and most of the centre 30C will be
reheated in "one pass" of the stores water. This is important. The boiler
will dump heat into the top of the heat bank, without mixing the stores
water, heating it up from top down. It may be 75C at the top and 25C at the
bottom. For most of the reheat the stores return temp to the boiler will be
very low and high efficiency follows. They also say once the heat bank
water starts a second pass the return temp is substantially higher.
How can that happen? If you are taking water from the cylinder at
25 degrees and passing it through a heat exchanger that can manage a
dT of typically 20 degrees or 25 if you're lucky, then the flow
temperature can't be more than 45- 50 degrees. I assume that you are
talking about a directly heated store where the bulk water goes
through the boiler. This means that the water will need to go
several times through the boiler to reach 75 at the top.
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? 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.
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. 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 and this effectively knocks that argment on the
head.
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. 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.
--
..andy
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