Hi,
I made the same mistake as you are proposing.
I had a Vaillant and I thought all the pipes and flue hole being there things would be easier.
Well I got another Vaillant combi and that was my biggest mistake. I now regret it.
I find them unreliable. From day one they have caused me pain.
And their customer service is rubbish. Even though I have a break-down cover with them.
Make a long term decision. Get the best boiler for your house. Avoid Vaillant.
JM
September 2006