On Sun, 21 Sep 2003 01:16:20 +0100, Sapient Fridge
wrote:
Sorry if this is off topic for the group but several people in cam.misc
suggested you would be able to give better advice that they could. I do
a lot of my own DIY (which means I'll probably keep this group
subscribed now I've found it) but putting a new boiler in is too big a
job for me (I wouldn't have a clue as to what needs connecting to what)
so I'll be getting someone in to do it.
I'm about to have a new boiler in and the plumber is trying to persuade
me to put in a condensing-combi boiler. Apparently they heat up the
water on demand and are quite efficient but I've heard that they can be
slow to get going. Anyone know if that is a problem?
Currently we are looking at Vaillant.co.uk boilers:
http://tinyurl.com/o1er
Has anyone got one of these (or something similar) and if so then is it
any good?
Do a search in Google Groups for Vaillant in this news group and you
will find that people have good experiences of their products.
There are some general points if you are changing from a system with a
tank in the roof and a hot water cylinder to a combi.
- These do heat the water on demand and save the space of the tanks.
If you have a small house this can be an advantage.
- Because you are using water directly from the mains, there is the
potential for greater water flow and pressure and being able to have
showers without pumps.
However there are some issues to check.
- Make sure that the mains water supply is adequate. Do this by
timing how long it takes to fill a container of known size at the
kitchen cold tap. It is not so much the pressure when everything is
turned off that matters, but the combination of pressure and flow
under running conditions. If you are getting less than about 20
litres/minute then the supply may need to be upgraded from the main in
the road. This can be inexpensive, can be costly, but you will have
to pay. Bear in mind, that for a decent shower you need to have 15
litres/minute, and that if somebody else turns something on, the
supply has to deal with that. There have been posts here where
people have done this change, not checked the water supply and been
very disappointed. It is even more important if you have a larger
house and/or 2 or more bathrooms in simultaneous use.
- Make sure that the boiler is adequately sized to give the hot water
flow required. Since the water is heated instantaneously, a lot of
heat is required. Combi boilers are specified for hot water by a
flow rate for a certain temperature rise - normally 35 degrees. For
most of the time this is reasonable. However, in the winter, the
mains water supply can drop below 7 or 8 degrees. If you consider
that a shower is normally run at about 40 degrees, and the boiler will
be producing water at 43 at its specified flow rate, very little cold
will be being mixed in and you will effectively be limited by what the
boiler can do. Tank and cylinder systems don't have this issue since
the water is heated over a longer period and stored normally at 60
degrees. This is not necessarily a problem, but if you go for a
small boiler at 11 litres/min, that may be all you will get in the
winter, and it will also take a very long time to fill baths. The
type of boiler that you mention varies its heat output for central
heating purposes, so you can use a boiler that is larger than the
heating requires in order to get the hot water performance needed.
Again, consider the usage requirements in the house. If it's 2
bathrooms, this will need a lot more output.
Another thing I don't understand is the difference between the "high"
efficiency (ecoMAX) and the "standard" efficiency ("turboMAX"). More
efficient looks better on the face of it but what's the disadvantage?
There are explanations of this in the UK.D-I-Y FAQ as well as on
numerous internet web sites.
There is a system used in the UK for measuring this which takes
account of the climate and pattern of use. You can look up the SEDBUK
web site for more details, but it involves more factors than just
measuring energy in and energy out.
Older types of boiler of 10 or more years ago had efficiencies in the
50-65% range using heat exchangers of various technologies.
Various improvements have been made to the designs, such as larger
heat exchangers, fan operation and so on, which have taken the
efficiency of conventional (meaning not condensing) boilers up to
around the 80+% mark. There is now a requirement as a result of
Building Regulations that requires all new boilers being fitted to
have an efficiency of at least 78%
Condensing boiler technology achieves even greater efficiencies by
controlling the boiler operating temperatures where it can to run at
lower temperatures - principally for the central heating. By doing
this, they deliberately arrange for the water (effectively in the form
of steam) in the flue gases to condense inside the boiler, either to
water or to water vapour (remember that what you see from the kettle
is water vapour - steam is invisible). At any rate, the change of
state from steam to water/water vapour cause a release of energy (the
latent heat of condensation). This energy is effectively added
back to what the boiler is producing. Thus, you get an even greater
efficiency - of the order of 90% on the SEDBUK scale. There is a
table of different boilers on the SEDBUK web site. Fairly soon,
there will be changes to the minimum requirements in the Building
Regulations and condensing boilers will effectively be mandated
anyway.
The UK boiler manufacturers, through conservatism in the industry in
general, were slow to produce condensing boilers. They have been in
use in the rest of Europe for 15 years at least. It is necessary to
design the boiler with heat exchangers and other components able to
handle the condensate water produced. This is mildly acidic and the
early attempts of UK manufacturers were not good, leading to
reliability issues. You do find a lot of installers not wanting to
recommend condensing boilers because a) they don't understand them and
b) might have had bad experiences in the past, or more likely heard
old wives tales from the other old women at the plumber's merchants.
Manufacturers from the rest of Europe have been making these for a lot
longer and are on their 3rd or 4th generation, while the UK
manufacturers are typically on their 2nd. Therefore, in that sense,
purchase of a German product like Vaillant is a good move.
There are a couple of issues to be aware of with condensing boilers.
- A condensate drain has to be provided. This is usually plastic
pipe (e.g. overflow pipe) led to a drain inside or outside the house.
It should go to a drain and not just onto a patio.
- When the boiler is in condensing mode, which won't be all the time,
there is a plume of water vapour from the flue. This needs to be
directed sensibly away from the building. Sometimes it is referred
to as a "nuisance plume". Personally, I haven't found it to be an
issue. In any case, the flue gases from conventional boilers, when
the weather is really cold and they are running at full blast will
condense to form water vapour as soon as they hit the cold air. The
condensing boiler will do it even on a summer afternoon.
..andy
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