Ed,

I know what you're saying, when we first bought the house I was
sceptical about having only a combi boiler as all our previous houses
had traditional gravity fed systems with hot water cylinders. In fact,
in one of the three similar houses that were being built the elderly
couple that were buying it paid extra to have a standard heating system
fitted (hot water cylinder + header tanks in loft).

I guess the advantage of the combi / sealed system is that we don't
need to worry about header tanks (which would be difficult now that the
loft has been converted into a bedroom & bathroom) and we don't loose
space for housing a large cylinder.

The down side, as I have found too ofter, it that when the boiler packs
in that means no central heating and no hot water until it is repaired.
Unfortunately, I think the builder (or the plumber) cut costs by
installing a Saunier Duval boiler and I certainly wouldn't choose one
again given the problems we've had with it.

Having said that, when it is working we have never had a cold house or
run out of hot water except when filling a large bath - we have to wait
5 minutes half way through. I guess this is due to the small hot water
cylinder in the boiler being replenished. Most of the time we have
showers and the pressure from all three is good, even the shower in the
loft is acceptable. I'm not sure we would get the same pressure from a
gravity fed system.

So, I'm quite happy to stick with a good quality combi boiler and would
prefer something more powerful (and more efficient) than the Saunier
Duval boiler as I always get the impression it is being over stretched.
It's also a little noisy which is unfortunate as it is located in the
garage below our bedroom so it tends to wake us, especially when it's
not well!

Paul.