|In this case, the temperature inside the Sunbox was|
more than 30 degrees hotter than the ambient air!
Here is a photo of the Sunbox controls, showing temperatures on a sunny but wintry Saturday.
Morning temperatures were below 9º and rose during the day thanks to clear sunshine.
At 3pm, while the air temp was about 14-15º, the Sunbox internal temperature was over 45º, and the liquid going down to ground was 21.4ºC. It's coming back up at about 15º, so is leaving plenty of energy down there.
When the heating comes back on in the evening, it will find the earth at the temperature that has been going down today, so it gets its 'diurnal' boost, because the small ring of soil immediately around the pipes has had the extra quantity of heat.