"Ed Sirett" wrote in message
...
Andy Hall wrote:
On Tue, 26 Aug 2003 18:44:55 +0100, Ed Sirett
wrote:
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.
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.
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.
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.
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.
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.
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.
I vote this goes in a condensing boilers FAQ.
It has come up a few times, hasn't it.
Maybe if Phil is reading, he could pick this up.
Also, I think this is closely allied to your sealed system FAQ and it
would be worth having that or a pointer to it as well.
When yours is up, we can add xref links.
The only thing I feel called to query in the above was...
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.
This is not the province of condensing boilers per se. Firstly modern
non-condensing boilers can and do modulate and there control systems
exploit this.
The difference is that by using forced premix burners (cf atmospheric
burners), they can modulate over a deeper range.
pre-mix burners are used in all boilers.
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