I am not gas engineer but presume that the burner will take some time to get to full burn so maybe that why their is a delay.
I have Powermax 185 boiler; it has omitted whistling noise since I moved into house (14months). Usually when first starting central heating (hot water only produced no whistling) and stopping after 30mins to an hour later; you could hear noise outside of house.
Last week the central heating ceased to work' the engineer said today that the pump had ceased but alos mentioned foreign object in pump. I got local gas repair company in and claimed it would be cheaper to get new boiler rather than fix whistiling. After reading other articles (showed success with Baxi engineers). I called HeatTeam of baxi website http://www.baxi.co.uk/aftersales/repairs.htm
I was qouted £299 for fixed cost repair including one year of cover on boiler; payable over 6 months interest free. They came today; fixed central heating (was programmer unit and pump) and also stopped whislting noise. Whistling was apparanetly issue with water pump and foreign object in pump. Was fixed in 90mins.
Was worth £300 just to get rid of whislting noise; never mind fixing central heating.
Tim
November 2008