External frost stat is in action or not doing anything?

[Jurgenjay]

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."

Thank you again John!
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?

 

[John.g]

The internal frost protection will only operate if the boiler water temperature falls below 8C (that's my interpretation of "the temperature within the appliance") so if the boiler was programmed on until say 2200 hrs each night then the boiler water temperature may well be ~ 50/55C at shutdown and it will take it a long time to fall to 8C, it may take more time to do this than the interval between night shut down and next morning start time, if so, then the external stat will have operated as its measuring the garage air temperature and it has fallen to ~ 6/7C.
You could look at the boiler water temperature a few times (if available on the boiler menu) during the next cold night and compare it with the garage temperature.
Remember, on Jan21st, the outside temperature was -6.0C and the garage temperature was +6.6C so the external frost stat operated more or less which now know will happen with a setting of ~ 2C (due to the 4C offset) and there is no way IMO that the boiler water temperature would have fallen to 8C (from 50/55C) during that period.

For interest, was the HW very hot that morning as the HW zone valve is opened during external frost protection period(s), I would have expected it to reach the boiler water cut out temp of 70C or whatever you have it set to.

 

[Jurgenjay]

The internal frost protection will only operate if the boiler water temperature falls below 8C (that's my interpretation of "the temperature within the appliance") so if the boiler was programmed on until say 2200 hrs each night then the boiler water temperature may well be ~ 50/55C at shutdown and it will take it a long time to fall to 8C, it may take more time to do this than the interval between night shut down and next morning start time, if so, then the external stat will have operated as its measuring the garage air temperature and it has fallen to ~ 6/7C.
You could look at the boiler water temperature a few times (if available on the boiler menu) during the next cold night and compare it with the garage temperature.
Remember, on Jan21st, the outside temperature was -6.0C and the garage temperature was +6.6C so the external frost stat operated more or less which now know will happen with a setting of ~ 2C (due to the 4C offset) and there is no way IMO that the boiler water temperature would have fallen to 8C (from 50/55C) during that period.

For interest, was the HW very hot that morning as the HW zone valve is opened during external frost protection period(s), I would have expected it to reach the boiler water cut out temp of 70C or whatever you have it set to.

Thanks a lot again John!
Your analysis regarding how the built-in frost protection works and external frost stat works look very reasonable!
This can explains why I never seen internal frost protection worked as you said, the boiler water temperature will take a long time to drop below 8C.
For that coldest day--21st of Jan (I tracked the temperature those days) in the morning 8:35am, the boiler started to fire, should be because the garage temperature dropped at around 6.6C---this should trigger the external frost stat to demand boiler to fire. And until the garage air temperature reached 7.5C(I measured at the time) the boiler stopped firing and zone valve closed.

You are right again for the HW! I did not check that day(21st of Jan) morning HW temperature, but I did have a shower right after the HW programmer was on a long time, I could feel HW was very hot!

 

[SJB060685]

I haven’t read every above comment so forgive me if this has already been mentioned.
The idea of a frost stat is as you know to prevent water in the boiler/system from reaching freezing point. The problem is as John mentioned above the garage will take quite some time to satisfy the frost stat on cold days because the only energy heating that air space is standing heat losses from the boiler. Boilers have both a combustion efficiency and an appliance efficiency. If the standing heat losses from your boiler are low then it would take hours, maybe even never satisfy the frost stat on cold days due to the garage having high heat loss itself.
Now the frost stat should be wired in series with a clamp on low temperature pipe stat on the return to the boiler, which SHOULD open the CH zone valve to dump the heat. The return clamp on stat is usually set to about 20°c and when the return water reaches that temperature the stat contacts will open and cut power to the CH valve and in turn the boiler. Without this additional clamp on stat you will get constant cycling at the burner.
Remember the idea of this setup is to prevent the boiler from freezing, while using as little fuel as possible.

 

[Jurgenjay]

I haven’t read every above comment so forgive me if this has already been mentioned.
The idea of a frost stat is as you know to prevent water in the boiler/system from reaching freezing point. The problem is as John mentioned above the garage will take quite some time to satisfy the frost stat on cold days because the only energy heating that air space is standing heat losses from the boiler. Boilers have both a combustion efficiency and an appliance efficiency. If the standing heat losses from your boiler are low then it would take hours, maybe even never satisfy the frost stat on cold days due to the garage having high heat loss itself.
Now the frost stat should be wired in series with a clamp on low temperature pipe stat on the return to the boiler, which SHOULD open the CH zone valve to dump the heat. The return clamp on stat is usually set to about 20°c and when the return water reaches that temperature the stat contacts will open and cut power to the CH valve and in turn the boiler. Without this additional clamp on stat you will get constant cycling at the burner.
Remember the idea of this setup is to prevent the boiler from freezing, while using as little fuel as possible.

Thanks for your input!
The first part I now understand it & it explains on 21st of Jan(coldest day of those minus degree days) morning why my boiler stated to fire outside of programmer setting period and lasted nearly 4 hours!

The second part, there maybe another issue from my newly installed system: it seems my external frost stat is wired in series with HW instead of CH! i.e. on the morning of 21st of Jan, when the boiler fired, the zone valve for HW opened NOT zone valve for CH!

The pipe stat is like attached photos?

Also, now, another issue I don't know whether it is same for zone valve for HW: even if when HW programmer is on for a long time, say a few hours, the zone valve for HW never close! and the blue light always on! The burner light (green light) is off, then on, then off for every a couple of minutes.

 

[SJB060685]

A clamp on stat is more like the image furthest to the left.
The HW zone valve should close when the cylinder stat is satisfied. If the motorised valve isn’t closing it’s still getting a feed from somewhere. It could be a faulty cylinder stat or the cylinder stat isn’t connecting properly with the cylinder if it’s an open vented HW system?

 

[Jurgenjay]

A clamp on stat is more like the image furthest to the left.
The HW zone valve should close when the cylinder stat is satisfied. If the motorised valve isn’t closing it’s still getting a feed from somewhere. It could be a faulty cylinder stat or the cylinder stat isn’t connecting properly with the cylinder if it’s an open vented HW system?

Yes, I also learned that the zone valve for HW and zone valve for CH should both close if the cylinder stat and house setting temperature are satisfied, but only zone valve for CH closed when it reaches house setting temperature, but zone valve for HW NEVER close during programmer "on" period (when programmer setting time is off, then the zone valve for HW does close---so it this looks zone valve is no problem, right?)
My system is unvented cylinder system.

 

[John.g]

Does the HW zone valve close when HW programmed off?. if so maybe something as simple as the boiler temperature not set 5 to 10C higher than the cylinder stat, ie if cylinder stat set to 60C, boiler should be set to 65/70C.

 

[SJB060685]

It could be wired wrong, it could be a dodgy stat or not sitting on cylinder properly. A photo of the cylinder and stat would help.
John raises a good point as well. If the cylinder stat is set to 60°c then the boiler flow should be higher.

 

[Jurgenjay]

Does the HW zone valve close when HW programmed off?. if so maybe something as simple as the boiler temperature not set 5 to 10C higher than the cylinder stat, ie if cylinder stat set to 60C, boiler should be set to 65/70C.

HW zone valve indeed close when HW programmer is off.
But your comment does draw my attention: "the boiler temperature not set 5 to 10C higher than the cylinder stat, ie if cylinder stat set to 60C, boiler should be set to 65/70C."
Is this a must? Could you explain this theory?
I just checked my temperature settings:
Please see photo1 which is my boiler photo, that is central heating setting, right? according to photo 2 "setting temperature for your heating", I turned the knob to the position around 5 which is 74C;
My cylinder stat is set to close to 65C, maybe 60C, or 61C, or 62C, please see photo 3.
So based on your theory, my central heating temperature(74C) and cylinder stat temperature(60,1,2C) are correct?