I just did an early morning calculation, so could be wrong.

If this heat store had a mass of 5,000 kg, was 40 K above ambient, it would store 45 kWh.

That this 45 kWh of waste heat you are trying to recover.
You may be better of having a rethink about your heating/cooking if it is throwing out that much energy as waste.

In reality, only the first few millimetres of the store (closest to the bit that get hottest) will actually store energy.
Also, the few people that I know have tried this have had little success with it. It may feel warm to the touch, but is not actually transferring much energy to the air.

Thanks ST.
The cooker/range heats a space of 150cubic meters as well as doing the cooking. It only emits about 2KW to the room, but will be fired up day and night for, I guess, 6 months of the year, during the day for perhaps 3 months and not at all for around 3 months. If we could store some of the heat the ratio of daytime only burning would be extended so saving fuel and perhaps allow the avoidance of morning central heating use.

I,ve recalculated the volume this morning which ends up being less than I first thought at O.8 cubic meters (when the chimney is excluded, with a mass of 1600kg - so I guess that would store around 15KWh which emitted over an 8 hour period would make a serious contribution to overnight heating.

Or am I calculating this all wrong?

Thanks

Ian

The first few millimetres line is nonsense, I use all my masonry to store heat and a few meters of ground under the house too.

Mow are the first few mm disconnected from the next few through a wall?

The masonry stoves cited in the link that Richard has given are constructed with internal flue ways that allow the transfer of heat from the fire and gasses from that fire into the firebricks that form the construction of the stoves. These stoves have to be run flat out otherwise tar deposits can build up in them and this would involve, after a time, taking them apart and rebuilding them to properly clean them. (They have cleaning ports but these are only good for removing fly ash). The other problem that can occur if they are run closed down is that combustible gasses can go into the flue ways and when the stove door is opened the inrush of air can cause a small explosion which can lift the top of the stove.

The point being that a chimney going straight through a 0.8 m3 mass is unlikely to gather much heat without the addition of internal flue ways.

Another problem to consider is that the stoves mentioned are designed to store heat from the fire within the stove, that is, the only place the heat can go is into the stove or up the chimney. Your cooking range will emit heat to the room so you will be losing heat before the gasses get to the thermal store, if you then extract too much heat from the gasses in the thermal store then you may cool the flue and gasses too much for the chimney to work properly. (If you have that much waste heat going up the chimney then your range is not very efficient).
Rather than put in a thermal mass above the range I would suggest a brick chimney that the range would warm without the need for internal flue ways and without the danger of over cooling the chimney and perhaps retain the idea of the cupboard which would then be a warm cupboard which could be used for drying herbs or proving yeast baking etc.

And then the other responses appeared .........

Good point about cooling the chimney - not something I want to do.

My last house had a central chimney with a large brick chimney breast which backed onto the adjacent hall.

The woodburner in the fireplace heated this structure and the house would still feel warm for a few days after it had gone out - indeed we used no central heating to speak of and let this fire tick away 24/7 for long periods.

Same fire in similar sized opening and chimney breast in another room - but with a cupboard, had no appreciable heating effect the following day if left to go out in the evening.

The heat entering the block work would be from the range underneath, via the concrete slab rather than through the chimney, which is insulated - I'm not sure that the chimney would really "see" the blockwork other than when first lit.

As for wood - time will tell if I'm going to have enough - I have a five acre mature wood to go at. but I may need more as time goes on - I'll let you know! The house renovation seems to have been successful to the extent that the structure, even with a few windows and doors missing (boarded up) is warmer and drier than our temporary accommodation - a 1650's cottage with lots of cement and dampness. Fingers crossed!

Thanks for all the input - food for thought.

Ian

Wouldn't this work best if the heat store was fully within the thermal envelope? Presumably there a direct conduction path from the inside to the outside through the bricks.

our multifuel stove in our lounge is surrounded by a large brick built chimney. The chimney is concrete rings surrounded by brickwork. the chimney is not double skin. the chiney goes up through the first floor such that you can walk around it at first floor level. it is in another lounge area. The chimney still noticeably emits heat radiation the next morning when I walk around it with the fire down stairs having been banked down all night. This morning the outside air temp was 2 degC but it was 19 degC in the upstairs lounge and 18 in the down stairs lounge.

just to butt in with another thought...

There might be another approach to this question, namely:

Rule-of-thumb-wise, if this fireplace/chimney are on an internal wall, there is the option of "just forgetting" the heat-recovery thing, period... (because any "recoverable lost heat" can be considered as going in to thermal mass, within the heated envelope).

Whereas if the assembly is on an outside wall (gable etc.), then OK, going for a "heat-recovery" approach might be considered "thoughtworthy", because the potential for reducing lost heat will be (presumably) greater...

gg

Posted By: gyrogearRule-of-thumb-wise, if this fireplace/chimney are on an internal wall, there is the option of "just forgetting" the heat-recovery thing, period... (because any "recoverable lost heat" can be considered as going in to thermal mass, within the heated envelope).

Not sure how the physics works here, but all chimneys pierce the thermal envelope. So even if it's in the middle of the house, it rises through the insulation layer in the loft or roof and so heat can conduct up the bricks to outside before it makes a contribution to stabilising the temperature inside over the longer term.

Would be interested at how fast either effect would happen. Seems to be related to general "thermal pathways" - lengthening when trying to tie IWI to EWI for example and minimising bridging.