Yes, ideally a smaller heat exchanger, parallel to the existing, can be used for just the bath and you can still use just the one pump and flow switch.
The monitoring is neat. When fully on, it shows the stratification and how wide the temperature difference is from top to bottom. The water at the bottom gives very good condensing boiler efficiencies. But in this installation the stratification temp difference could be even wider.
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There is a fault on the solar panel installation. The solar panel's return to the cylinder should be tapped into the cylinder at the halfway point or below,
not the top of the cylinder. It is clear from the real time data presented that the lower temperature water coming from the solar panel (it is December) is mixing with the higher water temperature at the top of the cylinder generated by the solid fuel stove, and unnecessarily cooling the top of the cylinder. The solar panel does give useful heat (21c in, 42C out on 6th Dec), but it must not compromise the efficient operation of the thermal store.
There is also an fault on the DHW to the plate heat X. Look at the temp above and below the plate. When no DHW draw offs and the DHW outlet is cool, the temperature on the cool side of the plate, before the pump, is high. This indicates no check valve after the DHW plate pump. This cause unnecessary circulation from the top of the store into the middle, through the plate, causing unnecessary mixing, partially destroying stratification.
I'm not sure if the boiler on the diagram is a condensing boiler or not. If not the blending valve in the return to the boiler is necessary to raise the return temperature above dew-point, to prevent condensing.
Having the valve will ensure a one-pass of the cylinder water through the boiler to heat up the cylinder - a very fast re-heat. A blending valve can be fitted using a condensing boiler. If the setpoint of the store is 70C, then find the delta T of the boilers heat X, the makers will tell you on the phone the max it can take - the MI always go on the short side.
So, setpoint of 70C, delta T of 25C, then set blending valve to 45C, boiler stat on maximum. This means 45C minimum, will be returned to the boiler until the temp in the cylinder rises above 45C. The return temp to the boiler will steadily rise as the cylinder heats up. But, it will only get above dew-point (approx 56C) just before setpoint is reached, ensuring a super fast reheat and condensing through approx 90% plus of re-heat. Only water at least 70C will enter the top of the cylinder and the cylinder is heated top down maintaining stratification. The system will never run out DHW as the when the store is exhausted it delivers what the boilers burner can provide (similar to an infinitely continuous combi).
There is a way using a blending valve as a diverter valve. Have this set to 70C and boiler set to 72 to 75C. Have the valve on the boilers flow, with the hot port of the valve (70C) running to the top of the cylinder. The other port of the valve tees into the boilers return to the cylinder, which will be the bottom. Then only water at 70C will enter the cylinder and any water less than 70C will be returned to the boiler to be heated up to 70C.
The solar/solid fuel: Have a high limit stat on the top of the cylinder, set to 95C. Have this also switch in the DHW pump to take heat from the top of the cylinder to the bottom. It just pumps harmlessly through the plate heat X. Gledhill do this.
