I plan to plumb the condensate pipe from my new boiler into the waste
pipe that currently serves the kitchen sink, the washer and the
dishwasher.

Having looked at old topics on this group, it seems I may need a trap -
no problem.

What I am worried about is drain water travelling under pressure up the
condensate pipe to the boiler when the washer and dishwasher are
pumping dirty water out at the same time. Goodness knows what this
would do to the boiler. I guess negative pressures could be a problem
as well.

Does anyone know if this is this a realistic concern, and is there
anything that can be done? Maybe some sort of mini air admitance valve
like the ones you get for soil pipes?

The boiler is only about 2 foot higher than the top of the washer and
the appliences are sealed to the waste with clips. The waste joins a
ventilated soil stack outside.

Thanks.