I suggest your capability to read is sadly impaired, I have never described a balanced flue, my only comment was that most boilers are of the balanced flue type today.
Forums seem to be full of members of various institutions answering posts they they thought they read, it appears there is another
I gave my opinion of boiler contracts to the OP and certain pitfalls on some contractors, not my problem if you have difficulty reading or deciphering from your high horse my PC is slightly to the right of Genghis
2012 HSE SD4 In Orkney Grey now gone, best car ever.

although Dartman and Joe 's posts slightly off original topic still very appropriate in bringing it to my and others mind that gas safety should not be taken lightly.

you can't afford to let a cowboy mess about with your boiler.

thanks guys

Iain