Rebecca wrote:

Very useful information Andy. Thanks.

I wonder if a bit more info will narrow it down then...

Our house has a mixture of new and old steel rads. The new ones are
fine (not as hot as they could be but they heat up more or less
evenly), but the steel rads are heating very badly (only the top 2
inches)- despite us removing them last year and cleaning them
thoroughly (and cleaning the system with Fernox etc).

So, we were recommended to buy new rads to replace these old ones,
which we have done - I must admit the plumber who told us the sizes to
buy did guestimate. Looking at the B&G catalogue, the output of these
4 new rads ranges from 4500 to 6200. 3 of them are currently on 8mm,
and the smallest is on 6mm.

From your figures Andy if the general rule of thumb applies, it looks
like we will have to replace most of the pipes (as 8mm generally deals
with up to 4500w). Have I interpreted this correct?

The plumber that recommended changing all the pipes to 15mm (but did
not recommend changing the rads) was the only one which was
recommended to us.

Your opinion with this extra bit of info would be much appreciated!

Changing tac slightly 0 another plumber told us that we would have to
change the pipes coming form the boiler (some sort of Potterton) from
22mm to 28mm and that is why the circulation is bad. This sounds dodgy
to me...we had about 6 plumbers round and no-one mentioned this. What
do you think?

Thanks
Rebecca

I think that unless the boiler is rated at 35kW or more then the 22mm
pipe will be fine.

Over all it sound as if they system needs rebalancing and the 6mm pipe
upgrading (becasue the slightest debris would have a really big effect
on it, I guess it is perhaps only 1/3 the area of the 8mm pipe
_internally_.

The guy that wants to do all 15mm is likely being over cautious - the
system nearly works.

I'd probably choose to upgrade the 6mm pipe to 8 or 15 (depending on the
layout), leave everything else as is and then spend time rebalancing the
system to get the 'old' rads to heat up (the water is taking the easy
route thru the 15mm new stuff).