mine never overheats if I keep the sun out in summer and the sun heats it in winter. (3g low iron glass air tight, high mass, )

I doubt the sun would heat tony's house usefully in winter if it was lightweight. You need a time constant of multiple days or weeks to pull that off.

The fact that it would overheat after a few days or weeks of sunshine in the summer is neither a surprise nor a reason for criticism. Thermal mass can only extend the period over which the effects of outdoor temperature and, most importantly, insolation are averaged. With a 10:1 1:2 or more difference between available insolation in winter and summer, but combined with only maybe a 2 or 3 to 1 variation in indoor to outdoor temperature difference, it should be obvious that you'd need to tune the amount of insolation accepted to keep the temperature in reasonable bounds.

Tony, something I'm not clear about with your house: do you use any “deliberate measures” to move extra heat into the ground? E.g., dump excess heat from the solar thermal, blow air down there or anything?

If it takes 5 months to go up 0.5°C, that is just not doing anything. I suspect that it is your insulation levels and airtighness that is allowing the temperature to stay where it is. What else do you have in the basement that may be adding energy?

As an example, I have my back door open, the sun is now not on the back windows (I hate NE facing) and the room temperature is 21°C. It is an overcast day.

So you are adding somewhere around 50 to 250W of thermal energy directly to the room.
Here is my kitchen temperatures for yesterday and today. It flicks a degree either side of 20°C, I have a fridge, kettle, computer (on where I use it) and me. I make no attempts to stabilise the temperature (the window is always open a bit).
I just can't see what all this mass you have is adding. I am sure if my house was as well insulated and as airtight (you have MVHR don't you), as yours, then it would be a bit warmer and a tiny bit more stable (you have to allow for a degree of inaccuracy with temp measurements.

Thanks tony; yes, your site is clear about pumping solar thermal water through the ground. I must have forgotten that bit.

Posted By: SteamyTeaIf it takes 5 months to go up 0.5°C, that is just not doing anything.

Yes, but at this time of year you wouldn't expect the thermal store to contribute anything. Actually, if anything it'll be a hindrance to warming at this time. What it might well have done is help stop the temperature going below 19.5 °C over the previous few months.

Whether good insulation under the basement would have done better is a different question. I've not seen a good example of using “open” soil to successfully store significant amounts of heat in the UK and have severe doubts about its practicality so I wouldn't read too much into tony's experience. His house clearly “works” a lot better than almost all the others in country but it doesn't say much either way about interseasonal amounts of thermal storage within the insulated envelope. It is a good example, though, of heat storage on a greater-than-diurnal basis.

Posted By: Ed DaviesIs that Edinburgh research available publicly?

Is it the one from Historic Scotland, I think Joe Little referenced it in 'breaking the mould' or whatever it was called.

Good reference, RJ, thanks. The relevant bit is chapter 2 which can make a reasonable stand-alone read. Short summary: measured U-values of rubble-filled stone walls seem to be about 2/3rds of those assumed for various purposes such as RdSAP. Still not that insulative, of course.