Replacement combi boiler

look them up and see

they don't have good reliability now.

for that to be true your oldie would need to have a cast iron heat exchanger

why would it?

They're retail salesmen. Do they ever give anything but a stream of silly bull?

NT

Depends on your definition of "much"... they will likely be somewhat more efficient when compared with non condensing versions, but a 20 year old boiler is still likely relatively modern with fanned flue, premix burner, and electronic controls etc, so the jump will be nothing like as dramatic as it would be from a cast iron high water content lump...

Without knowing what you have that's hard to quantify.

Unlikely I would have thought. Comparing to an old balanced flue floor standing conventional boiler with pilot light etc, you might be able to demonstrate 30% - but against something a bit more modern you won't.

Look up your existing model in the SEDBUK database:

Even if you DIY you would still need to add things like a condensate drain - which depending on the location may vary from trivial to reasonably challenging.

Yeah right...