most manufacturers say no to this (3 I know of) wb, vaillant and baxi.
MI's are to be adhered to in every aspect at all times!
Just pulled up vaillant MIs of random model.
WHAT'S this
The discharge pipe from the boiler condensate drain
must have a continuous fall (45mm per metre) and preferably
be installed and terminated within the building to
prevent freezing. The condensate discharge pipe must
terminate in a suitable position, e.g.:
a)preferably the discharge pipe should be run and terminate
internally to the house soil and vent stack (at
least 450mm above the invert of the stack). A trap
giving a water seal of at least 75mm (3) should be
incorporated into the pipe run.