Yes, I remember my grandad using such phrases :) I am actually English myself, just live in Cyprus.
unfortunately you just can't get the proffessional assistance there.
My guess is the system circulation thermostat on the Kazis controls the temperature of the water, but if it dips below...
I really hope you guys can help me :( They built my system and even they don't know how it works.
I will try to give as much info as I can.
Boiler is a Kazis SUN (25-81 kW) running on heating diesel, Link here...