External frost stat is in action or not doing anything?

[Jurgenjay]

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.

Please see attached photo of stat on cylinder and cylinder, if they are good to judge?
Please refer to my last reply to John that how I set the cylinder temperature and boiler heating temperature.

 

[SJB060685]

The theory revolves around the first law of thermodynamics. Basically if the water leaving your boiler is max 60°c then the maximum temperature the water in the cylinder could ever be is 60°c. You have the cylinder stat at 65°c which in my opinion is 5°c too high because if you’re in a hard water area this temperature can cause lime scale to form. Set the cylinder stat to 60°c and leave the boiler at about 70°c. Any higher and you start to lose any condensing

 

[John.g]

The divisions on the cylinder stat are extraordinary at 6.66C/division just to create more of a challenge when setting it?.

Anyway, to test the stat, just turn it down and see if the HW cylinder zone valve closes while programmed on.

 

[Jurgenjay]

The theory revolves around the first law of thermodynamics. Basically if the water leaving your boiler is max 60°c then the maximum temperature the water in the cylinder could ever be is 60°c. You have the cylinder stat at 65°c which in my opinion is 5°c too high because if you’re in a hard water area this temperature can cause lime scale to form. Set the cylinder stat to 60°c and leave the boiler at about 70°c. Any higher and you start to lose any condensing

Thank you for letting me know this theory!
My cylinder stat is not set as 65°c, it is about 60-65°c. It is because between 65°c---45°c the gap is 20°c, there are 3 gaps, so i set it between 65°c and the next mark.
The reason I set on that position (60-65°c )is because I learned that the hot water should be set over 60°c in order to kill legionella bacteria. (as attached picture)
So for my cylinder stat temperature, I should/could turn the heating setting knob smaller, say between 4-5 to make it around 71°c ?(this attached photo2)

 

[John.g]

That sound correct Jurgen but to check correct wiring, you might do the test I suggested in post #33.

 

[Jurgenjay]

The divisions on the cylinder stat are extraordinary at 6.66C/division just to create more of a challenge when setting it?.

Anyway, to test the stat, just turn it down and see if the HW cylinder zone valve closes while programmed on.

Exactly! Not sure why they designed and produced the stat like that!

Your solution to test the stat proved right! A few months ago, the very first time when we found the boiler was still firing even the programmer was off, the zone valve for hot water was opened, by chance, we turned the stat of the cylinder smaller, and saw the zone valve closed and boiler stopped, so at the time we could see the stat temperature setting seems working.
(then next time when it happened again: zone valve for hot water opened & boiler fired outside of programmer setting period, even if we turned the stat of cylinder smaller, boiler still continued to fire, so second time maybe zone valve issue.)

 

[SJB060685]

Yes legionella bacteria is killed off at 60°c. Yes set boiler control stat to that but no higher. The dew point for natural gas boilers starts at around 57°c, so your return water temperature needs to be that or lower, the lower the return temperature the greater the condensing. You also get greater condensing when the boiler is at part load but I won’t go into that now. If I was designing your system I would of done things a lot differently but that’s irrelevant now. As John says run his test to see what happens to the HW valve.

 

[SJB060685]

Depends on how it’s wired. If the frost stat by passes the cylinder stat it will hold the valve open,

 

[John.g]

Re cylinder stat, with HW programmed on, turn stat down and note what its setting is, (check zone valve closing), turn stat back up slowly to reopen the zone valve, note the setting then turn it back down again to close the zone valve, gain note the setting, there should be ~ 5/8C differential.

Also see if the cylinder stat is located above the coil return from the cylinder.

 

[Jurgenjay]

Re cylinder stat, with HW programmed on, turn stat down and note what its setting is, (check zone valve closing), turn stat back up slowly to reopen the zone valve, note the setting then turn it back down again to close the zone valve, gain note the setting, there should be ~ 5/8C differential.

Also see if the cylinder stat is located above the coil return from the cylinder.

I got your point for how to turn and note the temperatures. That will have 3 temperatures?
i.e. 1st temperature: turn down when zone valve closes; 2nd temperature: turn up when zone valve opens; 3rd turn down again to close the zone valve.
So three temperature notes---to test what? Sorry I don't understand this?

Also see if the cylinder stat is located above the coil return from the cylinder.---is attached photo good to see? (if not, I can take another one)