On Fri, 3 Mar 2006 21:38:13 +0000, mark
wrote:

In message , mark
writes
In message , Andy Hall
writes

One comment that I would make is that if you are going to go for a
large cylinder and 35kW boiler, do make sure that the cylinder is a
fast recovery type - not just a part L1 model. Otherwise you will
find that during HW recovery, the boiler may well modulate down to
half to 2/3 output if the heat isn't transferred fast enough. Most of
the major manufacturers have these - e.g. Albion cylinders. etc.


RMC Ultrasteel; I believe they are fast recovery.



OK so no opinions on the boiler then

More food for thought.
rcm say that the cylinder will reheat in 28 minutes given 21kw of input.
My calculations indicate I need about 14kw, sedbuk site says 22kw (!!!)

How did you do the sums for the heating? Did you take into account
air changes?

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.

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.

- Calculator or spreadsheey and do the sums by hand.



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.

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.





--

..andy