The condensing boiler frozen condensate pipe problem is popular at
this time of year so here is my question:

The boiler is a Vaillant eco TEC plus and yes the insufficiently large
diameter un-lagged external condensate pipe does freeze in very cold
weather and I will sort it out in due course but that is not the
question. I have noticed that the condensate never comes out in a gush
as the syphonic description implies that it should. What I observe is
a slow trickle of condensate when there is a plume from the exhaust
which stops when the boiler shuts down.

Browsing other peoples reported condensate problems suggest that this
is a common occurrence and there is even one report that someone
tested a brand new syphonic trap only to find that it also fails to
syphon and merely trickles.

So the question is has anyone observed a Vaillant eco TEC plus
condensate trap working 'syphonically' or do they all just trickle?

Brian