Heat Store under south facing conservatory

[Pile-o-stone]

What you are doing sounds similar to some ideas I've seen with heating passive greenhouses. The video below is an example, but there are a lot on the internet. They normally just vent hot air from the top of the greenhouse into the soil underneath, which is then released at night and maintains a more constant temperature. The video is a slight diversion from that as they are using an old oil tank burried underneath to store more heat.

https://www.youtube. com/watch?v=LgsniiYCwFQ

(to access the link, delete the space between 'youtube.' and 'com'.

[Phil.Chaddah-Duke]

I think ideally it would not drop below 10-13 degrees that's the minimum that plants like Orchids can handle.  Just to add its not strictly a conservatory, the roof is probably 50% insulated with roof lights, 70% of the front south facing wall is buried in the ground as we are on a hill, I attached a quick plan.

[Phil.Chaddah-Duke]

Looking into the Heat Pumps more it seems there would be no way to duct air from either outside or in, they need to pull air in from large area so could be inside and have a thermstat so as soon as the air drops below a given temperature it would switch off but I could't use the same unit pulling air from outside to heat the space, none of them seem to come ducted, they all seem to be design to be sited outside which is a shame.

[brackwell]

On the back wall besides the concrete one could face it with phase changing cement additive. Various temps are available. Above the temp chosen heat is absorbed and below it is automatically given up.

You clearly know what you are doing and wonder if you are not over thinking it and some oil filled rad pipes running on TOU leccy tariff would not be a sensible way forward.

[Countrypaul]

Having seen your sketch and realised that most of the area I assumed would be open to sunlight isn't, I suspect Ken is right and that you may be much better off with a simpler solution.

IIRC underground caves etc. in this county tend to be quite stable temperature wise at around 10C so using the ground to cover over half you south facing wall effectively gives you significant insulation on that part. The conservatory is also about half the volume I used in my calculations, so a 4000L thermal store would only rise about 1C per hour if the conservatory abosorbed enough sunlight which now looks doubtful to me. I think an the ASHP we discussed would be overkill and expensive as a solution.

You may want to look at an air to air heat pump which could certainly keep the temperature up in the shoulder months, possibly in winter - I'm not sure others better informed would have to comment. It might have the option to cool the house during summer too , if the predicted climate moves that way.

You could also consider insulating the roof lights during winter as the heat loss through them could well be higher than the heat gain.

Last resort could be electric heating either of the thermal store or direct through oil filled rads.