On 05/12/2011 21:50, Lieutenant Scott wrote:

But getting hot isn't enough with condensing boilers, the rads have to
drop
the return temp to about 50C so the boilers condense while still
providing
enough heat to do their job.
This can be somewhat difficult if a lot more water is going through
one rad
than another.

Then don't use a condensing boiler.

These days you don't get much choice.

Worst case scenario it doesn't condense, so you get less efficiency?

For it to condense first get a condensing boiler.

With a natural gas fired boiler the difference can be considerable - up
to 10%.

And why does all the water going to one radiator cause a problem? The
water would still cool by 15C or whatever it is. I assume a condensing
boiler can handle only one radiator being switched on, which would have
the same effect.

You seem to be making a lot of very strange assumptions. The heat loss
in any radiator is a function of the volume of water flowing through the
temperature drop across the radiator. The bigger the radiator the more
scope there is to give out heat and the narrower the pipework the less
water you can force through it.

If you actually want to get a 15C drop across the radiator you have to
slow down the throughput and sacrifice some of the heat output you would
have got had you the means to push water through the radiator so fast
you only get a very small temperature drop.

With condensing boilers you need bigger radiators so you can get the
return temperature to below IIRC 53C to get the condensing process
started but you need the return temperature a good way below that in
order to get a 10% advantage over a non condensing boiler.

Except for certain very limited
exceptions condensing boilers are mandatory both as new installations
and as replacements.

Mandatory energy savings - whatever next. It's MY money.

You want to replace your window you have to have double glazing using
special (ie) expensive glass. You want to improve the insulation of your
has you can't take any half measures, you must do it to new build
standards. Big brother is alive and well and living in your wallet.

I have what he called a balanced flu.

So do most people these days. You could well have a condensing boiler
without knowing it.

I think you need to read up on the basics of central heating design
which should include inter alia boiler types, heat output, circuit
design, pipe sizing, etc.

Then there is the more sophisticated stuff - programmable thermostats,
zoning, weather compensation, heat recovery ...

Ever heard of the acronym KISS?

Well if you want to be considered stupid I will oblige in future and not
bother you with inconvenient facts.

--
Roger Chapman