I installed my version of one of these when building my greenhouse.

It got a good test last night when the temps outside went down to 22F.

By 7am the next morning, the inside GH temp was 38F and the air coming out of the SHCS fan was measuring 43F.

I didn't use an air heater that night but did have a 1500 watt water heater on in the main 250 gallon fish tank from 9pm until 3am. The water temp in the fish tank was 66F when I checked it in the morning. The water circulates thru five 50 gallon grow beds full of expanded clay which makes a rather large heat sink.

My design is different than Mr Hobbit's in that I could only dig down 16 inches doing only only one level and only had enough room for (4) 4" perforated ADS lines that run from one end of the dome to the the other. Each line is about 20 feet long and all join in a plenum at one end.

I could post a few pics if anyone is interested.