Thanks for your repy - and also to ShaunCorbs, Stu-B, and king of pipes - your views are appreciated.
However, you are all viewing the heating/hot water problem from a whole house review perspective, there are practical as well as financial difficulties with this.
The d/s combi is working fine and less than 10 years old. The d/s c/h is separate from the u/s and has always been so from when the heating was put in so to re-design the heating/hot water for the whole house as one would mean removing the boiler, joining the two systems (if this was posible or practical), with the probable disturbance to the small-bore pipework and consequent leaks under the floorboards - the disruption, time and expense of this let alone the trouble of removing whole rooms's worth of contents to take up floorboards is simply not worth it. The pipework for the d/s and u/s was put in with the original c/h and boilers nearly 50 years ago and is micro-bore (10mm) as that was the way it was done, (much more malleable and easy to sweep bends so eliminating need for most t's and elbows - amongst other reasons I'm sure) and we have had no problems with the pipework. Throughout the house where we have replaced rads we have replace pipework with 15mm but he majority is still 10mm. I have spoken to other heating engineers about this option some 3-4 years ago, The (current) installer, (he does call himself a 'heating engineer' - am I denigrating his job by callling him an 'installer'? I have avoided calling him just a 'plumber' for this reason), has recommended that he does not power-flush the system because his certainty that the pipework will not take it. As for being decent - he is courteous, friendly - and polite!
I am not "oversizing a boiler because the house is leaking hot air and is poorly insulated", the house is (almost) as well insulated as it can be. All windows and external doors are double glazed, all external walls where we have re-decorated the rooms (most of them) are internally insulated, the loft is going to be insulated (floor/walls/roof) shortly (local authority grant) even before we convert it, and there are no draughts.
This is an old Victorian house with many internal plasterwork features so internal insulation not always complete unless we cover these up completely and remove all character of the room but we have done as much as is possible. With solid walls, it is theoretically possible to fit external wall insulation but is extremely expensive and may not be allowed as it would reduce the alley width between houses on either side (I'm certain the neighbours would object) - and completely ruin the front bay windows - even if the local authority approved.
As for excessive gas consumption this is a false assumption: our gas consumption is not likely to rise. The u/s will be used to the same extent as it has always been used by the same number of people for the same frequency. When, and if, the attic is fully converted we may have a couple more persons up there using the heating/hot water and this may or may not be for extended periods. If this results in much bigger consumption then yes we'll have to find a way of meeting this, or re-evaluating the usage, but I am simple trying to give ourselves the best option without having to end up with a converted unusable space which cannot be used at a later date.
Anyone used this gas volume calculation (see earlier) when considering meter capacity?
