DIY thermal store / heat bank

[esquilax]

There isn't anything  in particular that isn't likely to work  but  there are  a  probably two  things that are potential problems.  

The stove will heat from the top down if you have a suitable loading valve but it won't heat the  bottom of the cylinder at all  until the  heat gradually works its way down.  

Indeed. Maybe it'd clarify the requirements a bit if I worked on the assumption that both the stove and the oil-fired boiler input heat to the store with the aid of a circulation pump only, and that in the event of a power cut, the stove would be unable to heat the store and its output is dumped to the radiator circuit by gravity. How exactly that happens can be resolved later.

I get the impression that for most people thermal stores are all about the "storage" aspect / mains pressure hot water and the "neutral point" is an added bonus. For me it's the other way around pretty much.

The house is an old farmhouse - metre-thick stone walls, damp patches, single-glazed windows etc. The only thing that can bring it up to a temperature that a normal person would consider acceptable is the oil boiler. The stove can't because the pipes would start banging long before that happens under the current plumbing arrangements.

The reason I mention this is that although I'm talking about making a "thermal store", it is not really my expectation that this thing is going to be able to store enough heat to keep radiators warm for more than a few minutes. As far as I'm concerned, DHW is the priority. If the radiators are to get warm, either the stove needs to be well fired, or the boiler needs to be on.

As I envisaged it, the stove would be connected across the very top to very bottom of the store, as would the oil boiler and the PHE. I thought the radiators might be better connected middle to bottom as one of the features of the current system is that if the radiators are on for any length of time, there will be lukewarm DHW. I would really like to crack that one for the new system.

The other potential problem  is the  opposite  effect.  If the heat exchanger  returns water to the bottom of the tank  then  it will  mix the temperatures in the tank  quite quickly so it probably  will need to return  near the top to avoid this happening.       You may then find there isn't enough heat at the bottom of the store to transfer to radiators unless the oil boiler feeds that area or the store is big enough to carry over heat from a period when the heat demand was lower than the stove output.    This is why the tank size is impo

My store, 860 litres   (1 kWh per degree differential)  transfers heat to a separate DHW cylinder.   The supply to the DHW coil is  from  the top but it returns to the bottom.

Just the kind of practical information I am looking for, thanks. I don't think I could go for such an arrangement but as a matter of interest, what controls the transfer from the store to the DHW cylinder?

    When water is drawn off for one shower in the morning  transfer to the DHW cylinder quite quickly takes 7C  from the top of the store temperature and adds  1-2C to the bottom.     The next shower  is just about able to make the store all one temperature.   The DHW  cylinder,  250 litres,  does  however retain  some hot water at the top.    

That's worrying to me because 860L is a big store, and two showers is not a huge demand. However I'm guessing you mean this is what happens purely with the residual heat of the store, without being fully topped up the night before or while water is being drawn off.

[dhaslam]

The transfer between the  store and the DHW  is controlled by a difference controller, in normal use the controller is a  solar controller.   It uses sensors on the  top of the store and one  at the bottom of the  coil in the DHW cylinder and only transfers  heat when the  store is about six degrees hotter.   At present the  store is heated in the evening by the stove and  that does get the top of the store hot enough to transfer a small amount of heat Overnight the heat pump  gets the store temperature into the 40-50 C range and the  DHW cylinder reaches much the same temperature  but boosted at the top by the immersion.   One shower takes about 4 kWh  of heat and because it is taken from the top of the store  and  the water  below that area of the store is much colder due to underfloor heating needs the  hot water is replaced by cooler water from below.      I am making a few changes, one is to put an immersion in the top of the store to boost the temperature to 60C  using off peak electricity, this should avoid using the other immersion during the day.     The other change I am making is to have the bottom  of the store fed from the seasonal store as well as the  heat pump so  in very cold weather  the temperature won't drop as much as with the heat pump only.      I am also changing the stove although that is partly a fuel saving exercise  by having a smaller but much more efficient stove.