"ChrisJ" wrote
| I've also been told that if your system leaks a combi boiler
| will make it worse because of the pressure. (Remember its needed
| toping up already!)

If by 'combi boiler' you actually mean sealed pressurised radiator (primary)
loop, then the *advantage* of this is that if there is a leak, the amount of
water in the system is limited. With a header tank (fed by mains from a
ballcock) the amount of water is unlimited.

| ATM my understanding is our DHW comes from the hot water tank in the
| airing cupboard downstairs (there's also a pressure vessel in there
| too, not sure what for though).

But from where does the cold water come from for the hot. From the "header
tank" or is the hot water tank (cylinder) actually a mains pressure hot
water system?

| Does the above sound feasible? (IE are there any glaringly obvious
| mistakes I've made or things I've overlooked.?)

It sounds like you don't want a combi, so you don't need a new boiler. You
need a pressurised primary system and a mains pressure hot water system.

| Are there issues with microbore pipes for CH?
| Can you mix micro bore and standard piping in one CH system?

No and yes.

| We will then have the possibility of using 2 showers at the same time
| (one a floor up from the boiler and one a floor down from the boiler if
| than makes any difference). I'm presuming some sort of HW storage tank
| (as opposed to a combi) will be required to allow both to be used
| together. Is this the case (most "Combi yes/no?" threads so far have
| only concerned one shower being used. Could a combi and mini tank be
| used or would the best idea be to go for a mains pressure storage tank
| solution?

Mains pressure, I think. Remember you do have the option of having some
taps/shower on the 'hot water' part of the combi, and a conventional hot
water cylinder powered by the 'radiator' part of the combi.

| As the back half of our garage has been converted to a store room there
| is not a space issue with putting a hot water tank in there with the
| boiler but I'd prefer it to be compact as possible.

As you will be taking a 2 storey to a 3 storey building you may be subject
to additional fire precautions with your loft conversion. I say "may"
because with a middle-floor ground level you may be able to plead exemption.

You can avoid the hot water taking ages to reach the taps by taking a loop
back from the far end taps to the cylinder and putting in a circulation
pump. With insulated pipes and the pump on a timer for peak use periods
only, there should not be significant energy loss.

Thermostatic blending valves below all the hot taps except the kitchen may
save accidents.

With 3 storeys you might want to put some cabling in for a home intercom
system.

Owain