Well, I don't know if anyone is going to see this resurrected thread, but after getting conflicting reports from plumbers, I decided to stick with like-for-like and keep my HW cylinder and get a bigger CW storage cistern to provide for all the DHW demand, along with a Trevi venturi shower for decent washing pressure. Until, of course, the plumber waited until buying the shower and the new CW tank before telling me the tank wouldn't fit in the loft space available!
Were they mates of Fred Dibnah? I can see keeping the tanks, etc and going Trevi Boost showers (not cheap at all), but when you doing major works and new boiler is needed a high flow combi is the best solution for you. Some of these guys are in another world, doing what is best for them rather than the customer.
A friend is in the similar situation. I suggested two Trevi Boosts, but he said the boiler needs a new fan and is 16 years old. I suggested a high flow combi and cheaper mixers and no tanks or cylinders.
However, the one bright idea that popped out of this was putting a storage combi in the present airing cupboard space, so I'm back to the idea of a Vaillant 937 or Alpha CD50, and in the VERY near future as all this rubbish is happening in the middle of a refurb. So, again, any comments on the merits of either in terms of best HW flowrate (mixed or otherwise) would be very gratefully received!
Both are good. The Alpha is better as it stores more hot water. However Vaillant has a good reputation for reliablity.
Look at the Ethos 54C combi:
http://www.ethosboilers.co.uk/products_54c.php
Wall mounted and small. Delivers 23 litres/min @ 35C temperature rise for ever - it doesn't drop off as there is no stored water. It is small (a 937 is washing machine sized) and very well specced and uses a Gianonni heat exchanger, Grundfos pump, Honeywell 3-way vale, Dungs gas valve, with integral outside weather compensator. Best use an OpenTherm room temp controller like a Honeywell Chronotherm, to modulate the burner. They also have a secondary circulation kit to ensure instant DHW at the taps. It also has a keep warm DHW heat exchanger to prevent any combi lag. It is basically a downsized commercial boiler. Designed in the UK made in Holland.
Price? About £1200 to £1300 inc delivery plus VAT. Compare that to a system boiler, cylinder, controls, zone valves, etc. And compare it to a 937 and an Alpha too. You need an external expansion vessel and sealed system kit, which can go in the cupboard it is located.
It is a quality product.