On Tue, 26 Aug 2003 23:43:19 +0100, "IMM" wrote:
"IMM" wrote in message
...
"Ed Sirett" wrote in message
...
Andy Hall wrote:
On 23 Aug 2003 05:46:20 -0700, (robf) wrote:
Aside from my other post re. repairing existing boiler. I am tempted
by the higher efficiencies of the condensing boilers but are there
hidden problems in upgrading an existing installation.
I do not need a combined boiler - I have HW cylinder and pump /
valves.
Concerns I have are -:
Cleanliness of existing system - any prerequisites?
Will existing rads be ok - presuming that water temp is lower????
TIA.
Rob
There are a few areas to be considered, but overall, I think it is
worth the effort for the improved efficiency.
Most boiler manufacturers will, in any case recommend ot make it a
warranty condition that the system is thoroughly cleaned and ideally
an in line strainer is put on the return to the boiler.
If you do a Google search, I have a suggested a few times a very
effective way of thoroughly cleaning a system involving flushing at
each radiator and taking each radiator outside for a thorough flush
through.
A large proportion, though not all condensing boilers require to work
on a sealed system. This is no big deal and it is very easy to
convert an existing open vented system and to install the expansion
vessel and filling and pressure relief equipment.
One possible impact is that there may be seepage at the radiator
valves. If these are of significant age then it is worth changing
them and that is a cheap exercise.
You do need to properly provision for the condensate drain. The
condensate is acidic and must be delivered from the boiler through
plastic pipe to a suitable drain inside the house or if outside making
arrangements such that the condensate can't freeze.
The acid level is that of Coca Cola.
Approximately pH4, and metal pipe can't be used.
There is a myth and much confusion in the heating and plumbing supply
industry (especially at the merchants) about the efficiency and
behaviour of condensing boilers.
The misinformation is that they must always operate below 52 to 54
degrees on the return side in order to realise efficiency, as though
some nirvana is reached at this temperature and that the radiators
must be organised to achieve this.
It isn't the case. The main difference compared with a conventional
boiler is that the heat exchanger is more substantial and the boiler
is designed to operate with a flow/return temperature difference of 20
degrees instead of about 10-12 as for conventional systems.
Heat exchangers can be one piece downward or upward firing Some are boilers
are conventional rail burners, some are basic system boilers with an extra
condensing heat exchanger.
Heat exchangers can also be cylindrical like the Econox/Spiranox
burner/heat exchanger.
One piece heat-exchanger downward firing burners are self cleaning.
This does not necessarily mean that no cleaning is needed in the
burner/heat exchanger chamber. The manufacturer's instructions should
be followed.
Conventional systems are (supposed to be) designed to work with 82
flow and 70 degree return temperatures and radiators sized
accordingly.
The reality of the efficiency of a condensing boiler is that it
increases with reduction in return temperature. It is the *rate* of
increase of efficiency with reduction of temperature that drops below
the dew point (54 degrees) - there is not some Holy Grail that is
discovered at this point.
So...... to maximise boiler efficiency, it follows that the
temperature should be made as low as possible.
Without exceeding the flow/return delta T.
However..... condensing boilers are able to modulate their output (by
controlling the fuel burning rate over a range. 10kW to 25kW is
very typical; some will do 4kW to 30kW.
Some are fixed rate burners. "most" modulating burners have load
compensation control. In some boilers this can be disabled to give a fixed
rate output. Fixed rate outputs are beneficial to some thermal store and
UFH applications.
Some boilers have a DHW mode which give full unmodulated power when DHW is
calling and modulation on CH.
Some boilers have a high temp output for DHW and rads, and a low temp UFH
output.
During quite a bit of the heating season, a lower than maximum output
is enough. The effect of this is that the radiators will warm up but
perhaps only to 50 to 60 degrees flow and the return could be at 30 to
50. This is well down the curve of good efficiency.
Known as part load.
When it is really cold, and full power is needed, the radiators may
have to warm up to their original design temperature of 80 degrees and
then the boiler won't be running quite as efficiently. However it is
for only part of the year and in spring and autumn it will be running
very efficiently.
Once a house is up to temperature the burner will modulate to the more
efficient lower temp part load.
With weather compensation it may well also start at part load.
You can improve the efficiency by increasing radiator sizes to provide
the same amount of heat at lower water temperatures. Thus you could
work with 70 degree flow and 50 return if you like.
I did this when I converted my system and achieved most of what was
needed by moving radiators around and replacing only three.
It is not mandatory, however.
It is madatory in Sweden and Denmark since 1980 to have 60 flow 40 return.
That is irrelevant since this is a UK group.
No mention that super long flue lengths can be obtained using cheap plastic
drain pipe. This means boiler may be fitted anywhere in the house, not just
on outside walls.
That depends on what the manufacturer offers or specifies.
I vote this goes in a condensing boilers FAQ.
I bit of work required though.
Correction. A hell of lot of work required.
The objective was a simple and concise piece which covers most of the
salient points without introducing irrelevancies. One or two of
your points may be useful, once the spelling and grammar are
corrected, but that is all. The intent was not to make this into a
diatribe about peripheral issues.
---
..andy
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