Condensing Combi on a 3 Storey House - Good Idea?

Hi
Our old Potterton Netaheat 16-22 is still pressing on, even though it's around 20 years old, but I'm thinking it may be time to replace it for all the obvious reasons.
The cost of gas isn't going to go down much and the efficiency of these new boilers is hopefully higher than the Netaheat, so the running costs should be lower. I'm guessing a replacement is going to be around ?2000 mark fitted, so I need to work out how long it would take to pay off that cost in terms of savings in running costs.
I wondered if anyone has any suggestions for where I can get some numbers to fill out all these vague feelings!
Finally, does anyone has any experience of running one the new condensing combis in a fairly large 3 storey house. ISTR the old combis weren't thought to be too good in such circumstances, but I'm sure things have changed now. Anyone have any boiler recommendations? I know some boilers are easier to work on than others and some are more reliable, etc.
All comments gratefully received
Steve
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NewsOnline wrote:

FWIW my local council (Fife) has been busy installing the Worcester Greenstar 30Si in all its 3 bed council houses during the last month or so.
From an engineering point of view, these are apparently thought pretty highly of, so my local heating engineer says.
http://www.worcester-bosch.co.uk/homeowner/products/gas-fired-boilers-and-systems/greenstar-30si http://tinyurl.com/5hvr5z
--
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We replace an elderly gas boiler the year before last with a 1755 Ideal Mexico HE36 in our, not 3 story, but largish house. The main problem was that the new condensing boiler could not be vented up the chimney the old one used so had to be relocated onto the other side of the kitchen so as to vent through the external wall. Even so the whole job came to 2875 which included running not only new heating pipes under the kitchen floor but also a new incoming gas pipe and re plumbing the whole system to run with one pump instead of the previous two pumps.
On the whole we are well pleased. The gas bills have gone down by very little, but as gas has increased in price so much since the change I am happy. The only downside is this http://share.ovi.com/media/Muddymike.Housechanges/Muddymike.10568?sort=5 So be careful where the vent comes out.
Mike
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