Neither of those are necessarily problems with careful equipment selection, but you will have to research carefully...
You would probably need to find a combi with weather compensation, and then setup your control system such that when in hot water heating mode, it also spoofs the boiler into thinking its really cold outside!
The external temperature sensor with most weather compensated boilers is basically just a NTC thermistor in a box. So it would be trivial to have a relay switch a resistor in parallel with it, when a programmer calls for hot water heating, thus convincing the boiler its now really cold out and causing it to ramp up the flow temperature.
Yup, approaching 8kW loss rate on an old Victorian place is not hard to believe. However your smallest gas boiler is likely to be 15kW... and 24 would be quite common. That should be plenty for most space heating requirements.
Yes - very pleased I did on this old Victorian pile. It seems to maintain a constant temperature in the main area better than the old system with thermostat did.
Differing water temps are no problem with non-combis, with 2 separate stats. And they're very cheap off ebay. I wonder if you could roll your own combi with a tiny tank with big heat exchanger?
Not overly relevant. The old radiators are sized for a Delta T radator to room of 50+ C. Knock 10 degrees off that and the amount of heat they can transfer to the room is reduced. Also the heat transfer to temperature difference curve is non-linear as well, the hotter something is the faster it cools.
It's a common problem but I don't think there is an easy solution.
Our 38 kW struggles when it's down to -5 C and blowing a gale. Not uncommon weather conditions for here.
water
In broad terms yes, 10l/min with a 50 C rise requires 35 kW.
Radiator size needs to be done on a per room basis with some form of heatloss calculation for that room. Gawd knows what "rule of thumb" was used here, I suspect "single panel, non-finned, same width as window reveal". This means one small bedroom has 6' x 3' radiator and another about 3 times the size has just two 2' x 3' radiators, both have two solid external walls and roof above so considerable heatloss. The larger bedroom requires an extra 750 W or so all the time to keep it warm. The small one is toasty within 10 mins of the heating coming on...
We learnt the hardway that you never ever switch the heating off. If this place is allowed to cool down it's more like 4 days to get comfortable again.
Easier to not let the place cool too much. B-) The programable stat runs at 18.5 during the day rising to 20 C late afternoon to late evening when it drops back to 15 C overnight. It very very rarely gets down to 15 C, the high thermal mass see's to that and the heating can cope with the few degrees variation.
I wish... but this is a big, exposed, solid walled, some draughty windows in need of replacement, place. Sorting the druaghts would make a hell of a difference. It can be under 5 C outside and still and the boiler will do 15 mins every few hours, wind gets up and it'll be 15 mins every hour...
If you can fix any of the heat leaks, that reduces the size of boiler you need, and the resulting capex and opex.
The condensing boiler will be significantly more efficient than the one it replaces even if it never operates in condensing mode, although you obviously want to aim for condensing operation if possible.
It's worth considering your current system and how well it copes. i.e. can it get all the rooms up to required temperature? Can it heat the house up from cold in a reasonable time (if that's important to you). If there are areas of the house that struggle with either of these, it would be a good time to consider some new rads in those areas.
I did oversize rads when I installed central heating with a condensing boiler 14 years. It means I can run the boiler at 45/35 to maintain the house temperature when it's freezing outside (just under 12kW loss IIRC), and I can heat the house up from cold very quickly by running the boiler at max power output (25kW).
Low radiator temps are also a consideration where children or elderly are present, to minimise risk of burns.
Oh, I didn't know Myson have a current one. I use a beta java one they released ~15 years ago, but it was never fully released AFAIK. I have managed to get it working again recently, by loading it onto an ancient java version on unix, and running from a filesystem with caseinsensitivity turned on. It is rather unstable though. I accurately calculated the heatloss from each room when designing the system and got it pretty bang-on right. TRVs are installed but not needed - they're all just left on max and the rooms are perfectly balanced.
My post of 20/10/15 19:55 was in reply of a combi used to heat a hot water cylinder, as well as for CH. I'm therefore not sure what I misread where I said it was a show stopper that you couldn't have two different temperatures, one for the hot water cylinder and another for central heating.
Of course that is in addition to the combis own DHW.
Something I should have added perhaps in my other post... I considered using a combi for my system refit since while the cylinder was near all the bathrooms, it was a reasonable distance to the kitchen, and there was a long delay getting hot water there with the gravity fed hot water system I was replacing.
I also wanted multiple heating zones with their own stat as well as weather compensation, and integration with a unvented cylinder using a temperature sensor rather than a normal cylinder stat. It was all getting a bit complicated, but I did find a way to do all that "out of the box" using Vaillant's own controls - but only with their system boiler and not the combi, so that is what I went for.
It turned out that also mostly solved the dead leg problem to the kitchen since the hot water flow rate at 3.5bar is now so much faster anyway, its not so much of a wait!
You might consider spitting the system up and having several boilers (eg on e on each floor/decentralisation)) This is commonly done these days in large commercial heating systems, Especially viable when different zones have different occupation patterns. This will be far cheaper to run depending on the occupation pattern. There will also be reduced distribution losses.
The original boiler in this sort of place was likely coal fired. Gas however lends itself to decentralisation.
I expect it's steel pipework, you might have a problem finding a "plumber" who can deal with steel pipework. Most these days are totally clueless.
As and aside you might find one telling you that the steel/iron system is n aff and needs to be replaced. This is bollix,such systems can last over a h undred years. The truth being they don't have the expertise to deal with i t.
All depends on how long she anticipates staying there (to recover the outla y) and if she can afford the cost. It's not the sort of thing that adds hug ely to the house value as most people are too thick to understand the advan tages.
Just downsize the kitchen pipe-work to 10mm. Should save heat and reduce dead leg volume. Just a shame it's such a pain to change pipe-work.
My mother has a combi boiler in the next room to her kitchen but the pipe-work is tortuous and 22mm. Takes an age (comparatively) for the HW to come through. If it wasn't so inaccessible I'd seriously consider resizing the pipe work.
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