Thermal store sizing / advice

[Nickfromwales]

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 ? 

[Nickfromwales]

The first reply was.....

What sort of heat exchanger does the thermal store have for DHW? Ours has an external heat exchanger (that needs a pump) rather than an internal coil in the tank. We loose a lot of heat from the extra pipework/pump despite attempts to insulate.

Not sure about connecting via the solar coil. Is that coil at the right height? If so it might stop the boiler stirring the store. You would also need to check the solar coil power is large enough for the boiler or the return temperature to the boiler could be too high to keep it in condensing mode. Overall I think a direct connection might work better

Go with a store that has two stats at different height so you can set up hysteresis. Some only have one which doesn't allow quite the same degree of control.

If the store is set up right (stratified) what matters is the time it takes to recharge the bit you have just used not the whole store.