On 05/12/2011 21:50, Lieutenant Scott wrote:
On Mon, 05 Dec 2011 15:52:00 -0000, Roger Chapman
wrote:

On 05/12/2011 15:14, 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?

No, the condensing boilers have larger more effective heat exchangers,
so even if not running at optimum temperatures will still out perform a
normal one. There is a rise in the rate of improvement of efficiency
when the return temperature drops below about 54 degrees.

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.

It would be a problem if you wanted other rads to get hot and they were
being starved of flow.

Modern boilers will load sense and modulate - so as the differential
between flow and return falls (which in a correctly working and balanced
system indicates that the house temperature is rising), the boiler will
reduce its output to better match the rate of heat loss into the house.

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.

Perhaps, but its not your choice alas. Unless you have the skills to
install and commission your own boiler, then you are restricted by what
your installer is prepared to use and commission. They in turn are
restricted by building regs and Gas Safe etc as to what they can legally
install.


I have what he called a balanced flu.

You and everyone else. There are not many open flue boilers left out in
the wild these days, fortunately.

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?

Yes, but its not an option with any boiler you can buy today. They must
exceed 86% efficiency, and big lumps of cast iron with permanent pilot
lights don't.

--
Cheers,

John.

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