Is my Cylinder Stat in the wrong place?

[LeeB]

Hello buildhub community, 

 

I've searched endlessly for my own answers but can't find it... Is my cylinder Stat on my solar system in the wrong place?

 

Recently had a new solar thermal system installed with a indirect twin coil 300l unvented cylinder. Connections from bottom to top - Solar feed through the lower coil, then cold feed, then cylinder stat to boiler (about 1/3 way up, 2nd pocket up of 4), Immersion to iBoost, boiler feed through upper coil (about 2/3 way up, 3rd pocket empty).  There are also 2 sensors (S2 in 1st pocket,S3 in 4th pocket) for the solar thermal system, but this is a separate system entirely. 

 

Obviously I want to ensure I'm not heating the whole tank unnecessary and what to make the most of the solar power.  I plan, as recommended, to top up the temp in the top 1/3 of tank during evenings via a short, timer period from the boiler; relying on the the cylinder stat to stop heating when at 60C. I understand that the boiler will use the upper coil to provide 70C coming indirectly to heat the top 1/3 of cylinder by convention; But also understand the principal of stratification means it won't efficiently (or at all) heat the standing water below the upper coil. So how will the cylinder stat positioned only 1/3 way up ever know when the top 1/3 is the right temp? The installer said it will heat below the coil and there is a temp differential of about 20C between the cylinder stat position and the upper coil,  so with experimentation if you set your cylinder stat to about 40-45C, this will be a proxy for 60C at the upper coils.  Is this true? Or will the thermocline form and hold just below the upper coil keeping the heat where it is? The last thing I want is to fire KW of gas overheating the top 1/3 unnecessarily and the cylinder stat not used effectively. I'm testing the Installers theory each night when I top up.

 

Should the cylinder stat be higher up the tank? I have a spare pocket which the instruction manual refers for the 'Dual control/overheat stat pocket (boiler)' which I think is really where the cylinder stat should be, in-line with the upper coils and best placed to control the Tmax. With all the knowledge in the world, its experience I lack...

 

Any help improving my understanding and maximising efficiency would be welcome.

 

Lee

[LeeB]

Picture - Stat is in position 6, I'm asking should it not be in position 9.

[tonyshouse]

Yes, to control HW produced by you boiler otherwise it will needlessly cycle heating the house and wasting energy 

[Bramco]

Agree - at 6 it won't 'see' what the boiler is doing which is all happening further up the tank.   Also on the key, it's pretty clear - 9 is labelled boiler and 6 is labelled solar

 

Simon

[LeeB]

Tonyshouse and Bramco - Thank you for your confirmation. I have since also had confirmation from the cylinder manufacturer that it should be in 9. 

 

Appreciate you taking the time to comment back. L

[John Carroll]

I wonder where the solar store temperature (control) probe was inserted when the cylinder stat probe was in 6?, it should allways have been  be in position 6, just above the solar coil inlet, if its in 12 then this might actually result in recooling of the water just above the coil inlet and result in less than optimum performance of the solar system.