In article , mike.james
writes
I've read most of the condensing boiler discussions in this group and as a
result decided to fit a standard boiler - mainly because of not being able
to upgrade old radiators to work with the lower return temperature.
Fair enough, but for most of the year, the reduced output from your rads
would be enough to heat your home with the boiler set to say 60 or 70
degree flow (50 deg return) and you would have had the benefit of
condensing action. Then, in the coldest part of the year you could jack the
flow temp up to 80 degrees (with the boiler stat) and get the same heat
from the rads as you would with a conventional boiler. For this period
though you would lose the benefit of condensing action, but hopefully only
for the coldest month or two of the year.

I was helping a friend select a new boiler and told them the same story but
then realised I might be missing something.
They have a heated swimming pool via a heat exchanger plumbed to the CH.

What if they put the heat exchanger on the return flow?
That would drop the return temperature even if the old radiators didn't.
Am I being too simple minded?
mikej
Sadly, I think there is a flaw here and it revolves around the boiler being
optimised (by design) to run at (say) a 20 degree temperature difference,
which I don't think is optimal for your system.

I assume you want to keep an 80 degree flow to use existing rads, but
then want a 50 degree return to take advantage of condensing boiler
action. That's a 30 degree system drop which I don't think the boiler heat
exchanger will be capable of overcoming on the heating cycle; as it will be
designed for 20. If you slow the flow to promote a 30 degree rise I think you
will get a low flow alarm from the boiler.

If you instead run with a 70 degree flow, with 10 degree drop on rads and
then a 10 degree drop on the heat exchanger you will have the requisite 20
degree drop, but you will only have a 65 degree mean in the rads. This is
better than the full condensing case, but worse than the 75 degree mean of
an 80/70 system (13% worse).

So, nice try, but I think if you want to keep your rads, you would be better
going with the idea at the top and making do with the non condensing
action for part of the year. It's what I've done (on a new system) to keep a
reasonable compromise on rad sizes.
--
fred