In temperate areas the growing season I garden greenhouses can be expanded by storing sun heat I the greenhouse floor.
My greenhouse is 152 Square Feet, it has a 160 Feet black corrugated one inch pipe sun collector in top and 60 Feet one inch pex about two Feet in the ground. The pipes are filled with water which is circulated by a small aquarium pump, controlled by an Arduino and software written in C+. Temperature is measured by five DS18B20 sensors.
This is a rather simple system both to construct and run.
The greenhouse is situated near the town Kalundborg (55.680128, 11.090762) in Denmark.

Storing heat

Glass is a very bad insulator, so in cloudy weather the temperature inside a greenhouse is about the same as outside. On a sunny day the temperature will rise quickly and some of the energy can be stored.

In Denmark strawberries and potatoes can normally be harvested around June 20th – in my greenhouse I harvest potatoes April 24th and strawberries May 1st . Runner beans are usually harvested at the end of July – in the greenhouse I start harvesting in the beginning of June.

What is the temperature in the floor?

On a cloudy day
October 20th was one of several cloudy days with outside temperature around 53F/12C. At 7:45 PM the temperature in one foot debt was 49.4F/9.69C outside the greenhouse and 51.20F/10.69C inside.
Next day at 8:40 PM the temperatures were 49.2F/9.56C outside and 50.30F/10.19C inside the greenhouse. The pump does not start under these conditions.

On a sunny day
October 25th was a day with sun and clouds. At 9:23 AM the soil temperature outside was 50.56F/10.31C and inside it was 51.59F/10.94C, and the pump started. At 4:55 PM the temperatures were 50.79F/10.44C and 56.54F/13.69C so the temperature of the floor to a depth of two feet’s (the depth of the plex) had increased by 4.95F/2.75C.
October 26th was a wonderful, sunny day. At 9:26 AM the soil temperature outside was 48.30F/9:13C and 50.79F/9.13C. The air temperature outside was 48.43F/9.13C and inside the greenhouse 43.16F/6.20C due to radiation to the clear sky during the night. At 16:15 PM the soil temperature outside was 49.10F/9.50C and inside the greenhouse it was 56.75F/13.75C.

Conclusively

Autumn and spring the soil temperature is only slightly higher inside the greenhouse on cloudy days – not surprisingly.

On sunny days the temperature in the ground can rise 5.4F/3C and in a period with sunny days there is a little left over from day to day.

The benefits are greatest in spring where soil temperature can exceeds 68F/20C.

If the air temperature is too low for a plant to grow it will still grow if the root temperature is adequate, and that is the secret of storing heat in the greenhouse floor.

Nu er alle slanger lagt på plads.
The pex pipes in the greenhouse floor.

Sun collecting pipes in the greenhouse.