RzB wrote:

I am about to replace our 1960's Thorn M gas boiler
with a Condensing boiler..

Evidently condensing boilers run more efficiently
than non-condensing boilers. But my understanding
is that they rely on a return temperature of 55/60C ish
(I'm not sure what a typical flow temp for a
condensing boiler is - same as non condensing
boiler ?)

"Rely" is too strong a word in this circumstance. A modern boiler will
be significantly more efficient than an older counterpart by virtue of
more sophisticated control systems, and better design anyway. A
condensing boiler even moreso as a result of its larger heat exchanger
and its ability to recover what would otherwise be lost heat.

I believe that my system runs with a flow 82C of
and a return of about 71C. Not checked but that
would be typical.

Probably about right...

So what sort of efficiency am I going to get out
of my brad new Condensing boiler? Surely not
much better than my old Thorn M?

I would expect it to be significantly better - since the old boiler will
be fairly poor by modern standards even if comparing against a non
condenser.

Is there anything I can do to my system to help the
situation. I had a plumber come in recently who

The boiler will do much of the work for you, since it will use its
ability to modulate to actually match the heat it provides to the load.
It can also use this capability to maximise the amount of time it spends
condensing. In many cases it will run the system cooler, but for longer
(which is preferable to bursts of full output, which result in
overshooting the desired temperature, followed by gaps of zero output)

told me that I should be fitting thermostatic
controls to all radiators. Is that a regs requirement?

Yes, the controls on the whole system will need to be up to spec. So
TRVs on all reads except the one in the room with the room stat (which
provides the interlock to turn the whole lot off when the demand is
reacted (can also do it with all TRVs and a flow sensor)). Fast recovery
cylinder, fully pumped, and zoned if it is a big house.

Isn't that going to make the situation worse?

Nope they will help. As the TRVs close down, the rate of flow through
the rads falls, giving bigger temperature differentials across the rads
and lower return temps - this improves boiler efficiency still further.
As the system as a whole comes up to temp, there will be a time when the
boiler sees a rise in the overall return temp (since the heat being
dumped to the house is falling) this will allow the boiler to modulate
down further.



--
Cheers,

John.

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