On Sat, 4 Mar 2006 11:00:40 +0000, mark
wrote:
In message , Andy Hall
writes
If you mean that SEDBUK says 22kW for a house of a given type, that's
pretty meaningless. It's about as useful as the leaflets in B&Q
which tell you to measure the room, add a fiddle factor and multiply
by aunty's telephone number.
I did wonder..........
There are a number of web sites around that purport to do the sums -
these seem highly variable.
The only two methods I would trust are
- Radiator manufacturer calculator programs (e.g. Myson, Barlo - I
have the Myson one if you want it) but check the U values they use
with those in the Building Regulations Approved Documents and other
sources.
I think I do have one of these somewhere; pretty tedious as I recall.
Perhaps I will impose on you if I can't find my copy ?
- Calculator or spreadsheey and do the sums by hand.
I could do them as well 
Again it gets pretty tedious
Either way, you do need to make sure that you have correct U values
for the different materials used in the construction of the house -
especially external walls and roof - these make the largest difference
to the outcome usually. The other is to make reasonable allowances
for air changes. There are standard figures for different room types.
If you have an older property or a draughty one, then allow a bit more
there.
If you want the Myson program, drop me a mail. (andyh at hall dot gl)
I plan to have a y plan type diverter and I'd put up with the odd
occasion that I'd need the cylinder re heated from scratch for 28
minutes.
28 mins reheat is not super-fast and you could get better than that if
you wanted it.
Hmmm.I shall look into that...
Albion is one vendor, but most have one.
Remember that if the boiler is reheating the cylinder then the house
will begin to cool - more rapidly if not well insulated. If you have
any rooms where there is a particularly large heat loss because of
exterior walls and large window areas then you will notice more
quickly.
Does this mean I could go for a smaller boiler?
I could use a 622e as this pumps out 22.4 kw or is this too small a
margin?
It doesn't matter for the cylinder. It would be OK for the house if
it really is 14kW.
Both boilers will modulate, although curiously, the 635E goes from
10.5 to 35kW, whereas the 622E is from 13.4 to 22.4kW at 80/60
operation.
More sums then
There are boilers on the market which will go to a lower output. 7kW
is fairly common now and you can get down to 3-4kW but these tend to
be more expensive.
The advantage is that they will cycle less frequently and that does
save some energy.
If you are doing this renewal exercise, you could also consider
running the system at 70degrees/50 degrees rather than the
conventional 82/70. This can be done anyway in spring and autumn
by turning down the boiler thermostat. You will enable the boiler to
operate at a better place on the efficiency curve. Basically
efficiency improves with reducing return temperature and below the dew
point (about 54 degrees) condensing takes place and the rate of
improvement of efficiency with reducing return temperature increases.
There isn't a magic step change when condensing happens as some people
think.
If you want to improve efficiency in general, you could look at
designing radiator outputs to match the heat losses at 70/50 rather
than 82/70. Radiator manufacturers publish a table of correction
factors which are applied to get true output for a given mean water
temperature (i.e. (flow + return) /2) and air temperatures. The
nominal figures are based on an EN standard testing method using 90
degrees. The practical effect is that for a room at 20 degrees and
82/70 operation, the factor to be applied is 0.9. For 70/50 it's
about 0.6. In other words, if you drop the temperature, you need the
radiators to have about a third more output.
I did this exercise when I upgraded my system. I checked each room
and its radiator and found that several were oversized and could be
left alone, some were moved to different places, and three were
replaced with double panel where they hadn't been before. It was a
reasonably painless exercise when they were all removed for flushing
anyway.
The effect is that in spring and autumn, my system will drop to around
40 degrees flow and in winter, seldom exceeds 60 degrees.
Obviously YMMV depending on whether the radiators were oversized in
the first place.
--
..andy