Thomarse wrote:

I did look at the diagrams on the Honeywell site, but I cuoldnt really
make too much sense of them, having not much background knowledge on
the subject...

And yet knowing nothing about it, and without attempting to inform
yourself, you decided that you were competent to drain the system and to
determine that it was OK to run the boiler after draining the system.

shakes head

I suppose you run your car after draining the coolant on the grounds
that you need the heater to work.