Search the Community

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

Excerpt from a previous thread. A member asked..... I am looking at an HTG thermal store for a DHW & Heating upgrade, I will be using a gas boiler heating the TS only, together with PV and immersion heater. The TS comes with a solar thermal coil fitted as standard. The gas boiler would normally be connected directly to the TS but I was wondering if I could use this solar coil for the system boiler to obtain some fluid separation ? The size of our home with # of radiators/bathrooms etc. would suggest I need a TS of around 300 litres but if I view the store as a battery should I install a larger store say 400 / 500 ltrs to help with the gaps in PV availability ? I guess there is a risk of oversizing as the volume should be within the capabilities of the immersion / gas boiler to recharge it ~ any thoughts or advice would be welcome. The TS is presently required for 12 radiators, 2 bathrooms & DHW, I am hoping to take advantage of my PV power hence the TS otherwise a fair size combo probably would have been the way to go. I am also building extra ground floor space so I also have been considering UFH but from your previous advice I am going to need more insulation to even consider going down this road. I have a beam & block floor with only 40mm poly then wyroc so not good, I am able to put 75mm underneath the floor between the beams which would help ~ but still not sufficient although if I am using this space underneath the ground floor as a store should I be too concerned. I have had experience with TS excessive heat in a flat some years ago, it was located in the centre of the flat off the hallway and I did ask engineers how to reduce the heat output but they looked blank at me. The small gas boiler was set at max heat output for efficiency as per the operating instructions but after not getting any useful help from heating guys, I had to turn the boiler down to half heat which totally made our lives much better, so JSH I appreciate this problem..... I presently have a traditional copper tank upstairs about 110 ltrs and is generally heated by the PV system and overall meets our requirements as there is only two of us. I am considering putting the TS in the utility room and using the heat to make a drying room etc. The TS is stainless steel and comes pre plumbed with a high efficient external heat exchanger (46lts per min, we have high water pressure) I could also add to this TS a similar external heat exchanger for UFH so it would give me scope for going the UFH direction. The additional copper header tank can be installed above the TS , is there any requirement for this to be higher than the radiators in the bedrooms or will it be ok ?