Thank you again John!You said.....
Below is the manual of my boiler regarding Frost Protection:
"If the temperature within the boiler falls below 8°C the pump will run to circulate water and prevent the system freezing. If the temperature within the boiler falls below 5°C the boiler will fire periodically, bringing the boiler temperature up to 12°C to avoid the possibility of the system freezing. This process will be repeated until such time that the boiler temperature does not drop below 5°C."
This is the full explanation of the above.
If the temperature within the appliance falls below 8°C the pump will run to circulate water and prevent the system freezing. – If the temperature does not rise to 9°C within 30 minutes of the pump operating, the appliance will fire up. ▶ If the temperature within the appliance falls below 5°C the appliance will fire immediately, bringing the appliance temperature up to 12°C to avoid the possibility of the appliance freezing. ▶ This process will be repeated until such time that the appliance temperature does not drop below 8°C."
i am a bit curious that the way of the built-in frost protection function seems never in action...
In middle of Jan, especially on 21st of Jan (outside -6°C ) which were cold and minus 0°C, but I never noticed the pump circulating or boiler started fire. I measured on 21st of Jan the garage temperature was 6.6 °C---maybe the boiler water temperature was still higher than 8 °C thus frost protection still not in action?
As mentioned earlier, on 21st of January in the morning at 8:35am outside temperature was -6°C, and the garage (where my boiler is locates) temperature was 6.6°C, at the time, my boiler started to fire (outside of programmer setting period), zone valve for hot water opened, this lasted nearly 4 hours till 12:15pm.
All this could be caused by the external frost stat triggered the boiler to start the frost protection function, right?
So it seems external frost stat worked(not sure yet)? But built-in frost protection not?