"Boss Therm" is enigmatic but it may mean something to the experts...
I have seen theories (on here) about self-firing boilers when the timer is not calling for heat.
The frost stat - if there is one - will typically cut in at about 4 ºC and it's been cold recently.
Some strange faults with the switches in a diverter-valve control-head can send a power-up signal back to the boiler.
There may be other reasons - I don't know...
Peccavi, December 2009
