"Andy Hall" wrote in message
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On Fri, 12 Nov 2004 00:39:49 -0000, "IMM" wrote:
With condensing boilers headers
reduce efficiency. With a condenser
you don't want very hot water feeding
right back into the return. This raises
the return temp which reduces efficiency.
You want the return temp as low as
possible. Best use a heat bank or
thermal store, which is a great neutral
point, and have the boiler heat that directly.
For most of the re-heat the return temp will be low.
Here is Andy still attempting to justify an expensive purchase. He goes
on....
Not if you are using a condensing boiler
with wide modulating range
such as the MAN Micromat or Viessmann.
All that does is lower the flow temp in the header. If a secondary circuit
kicks in it may require hotter water than what is in the header, then the
boiler has to react, which can take far too much time. Fan coil units
require high temps of around 80C pretty instantly, so having a header stting
there at 40C is no use. Also the primary and secondary pumping has to be
setup correctly. If not you may have problems.
Putting a heat bank in the middle
will do one of two things.
a) If the heatbank is used for HW as well as CH, and there is a
thermostat arrangement on the heatbank, the effect will be to prevent
the boiler from operating at the lower end of its temperature range
and will cause it to cycle on and off.
Not if anti-cycle stats are fitted. Simple thing to do. Putting a MAN
boiler on a heat bank will do the same thing as if it is on a header, except
the return temp will be cooler most of run time as speaders are inside the
heat bank to ensure hot water goes to the top and stays there, preventing
mixing of the store, and the heat bank heats up top down. It is possible
for the water at the top of the heat bank to be 80C and 20C at the bottom.
Which is not the case with a header which sends heated water from the boiler
directly back to the return.
In commercial systems cylinders are used as neutral points to replace
headers, similar to Dunsley neutralisers and heat banks. The Germans tend to
use these. Cylinder neutral points, similar to a heat bank, are better when
using condensing boilers. The cylinder water is stratified with say hot
water for fan coil units and DHW at the top and cooler water at the bottom
for background rads circuits controlled weather compensators. When the
boiler reheats the neutral point cylinder, the return temp is invariable
lower than using a header promoting efficiency. Also not having to set the
primary and secondary pumps up is great bonus.
In is usually better to divide and rule and have dedicated boilers heating
to the temperatures of the various circuits. One boiler heating only high
temp DHW, one high efficiency condensing boiler only heating a low temp
background rad circuit etc. Boilers can then be matched for maximum
efficiency. I have seen one demostic boioer heat only the fan coilo battery
on an air handfling unit. There misy have been 20 or 30 of these in the
place. They had a cuborad of spare for the bopioer type and they were never
down for long if part were replaced. No expensive mixing valves were used
and large harders and long large expensive pipes. Quite cheap to do they
told me. The problem is redundancy in commercial systems, that is why
boilers are sequenced together. If open drops out the other(s) will cope.
Then headers, neutral points, mixing circuits etc, are incorporated to make
it all work.
Many commercial boilers are frames with smaller boilers on the frame, called
modules, each with its own burner. The size of the whole "boiler" can be
specified by adding modules. Having many modules reduces large boiler
cycling as smaller burners (boilers in effect) are brought in to match the
heat demand. If one module drops out the others are still operative. The
"modules" are switched in to demand by a controller similar in operation to
the two stage Landis & Staefa one I mentioned, only it will have stages to
suit the number of burners.
So, dividing and ruling when boilers are cheap enough makes a lot of sense
in a small comercial large domestic setup.
The whole point of this type of
boiler is to have high output for a
short period to heat the heatbank/cylinder
quickly for DHW use, and to
drive the heating system directly to
allow modulation down to low
output levels.
Is it? Look at what I write and take note. Nah, don't bother.
or
b) If the heatbank is used solely for CH
No DHW?
and no thermostat, it will
effectively create an equilibrium
situation with heat in and out and
no real advantage over a much smaller header.
There is. The return is cooler most of the run time compared to a straight
through header.
It is far more efficient to run this type of boiler continuously at
lower output than in bursts of high output.
So, you use a cheaper and simpler boiler. The MAN type of boilers will be
more efficient on a heat bank rather than on a straight through header,
because of the top down heating of the heat bank.
Most commercial system are still non-condensing and use headers. Where
commerical condesners are used they still use the same temps as in non
condensers, and gain about 5-7% because of the larger heat exchangers.
Headers are commercial and are intended as:
1. A neutral point where different circuits may take off hot water to suit.
The different circuits then mix to lower temps to suit.
2. Headers also are used to keep the return temp up on the boiler to
prevent back end condensation.
3. A central point for all circuits.
All headers are neutral points for circuiots
to tee into. A heat bank gives
you that and a DHW store with instant high
pressure DHW. You only need cheap
simple boilers with a heat bank, one of its
selling points.
I'll try and come up with some alternative systems.
This could be entertaining.....
It will be and you will be educated somewhat. Imagine allowing people like
you to design a system? The thought of it.....
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