The 29-34kW range is normally Installed in Mid-Sized Properties. In most cases The kW output of your new boiler should be chosen depending on the household's hot water needs and not the number of bedrooms or size of the property.
29kW boiler's normally produces around 12 litres per minuite. This is more than enough for a hot shower and could with a little tweeking just about provide for the kitchen sink at the same time. 34kW boiler's will give that slight bit extra around 14 litres per minuite. This size boiler requires good water pressure and a little tweeking will just about provide enough hotwater for 2 showers at the same time.
"There is no point in getting a higher sized boiler if the incoming cold water delivers less that the hot water output. "