The better boilers prove to be more reliable, few fitted now will last as long as long as the old beast you have! Fanned flue is the norm now, the flue 's are about 4" diameter and use a fan to expel the waste gases. (Some making good of the wall where the existing flue exits may be required.)
Unless you have exceptional reasons why not, then a condensing (High Efficiency) boiler is required, these extract more heat from the gas used compared with the older types.
Unless you have exceptional reasons why not, then a condensing (High Efficiency) boiler is required, these extract more heat from the gas used compared with the older types.