I think I would look into the world of PEX myself. I know they are usually for large areas. However, I think their stuff could be worked into a great small space heating solution, with some repurposing ideas, over buying every PEX item, maybe just a few PEX fittings.
Like you said above, some type of circulatory system, heated with a immersion heater, or something on that order, with a small pump to cycle it while lights are off.
This is just one site a gave very quick look at a bit ago.
Good luck in your endeavor.
My plants seem to grow fine. I have the exhaust fan on only 15 mins per hour so there will not be a super lot of condensation.
Temps go down in the course of the day to 46.5 Fahrenheit which is 8 degree celsius.
When lights on the temps go up from 68 to 77 Fahrenheit, which is 20 to 25 degree celsius.
I am planning now to switch my double ended lights to led, to save some more electricity. But I am thinking that will cause the temps to drop when lights on. Because I guess the led will give off less heat and the exhaust fan is already running very low.
Yes your temps will drop, you need to get a temperature and humidity controller, that way the fan will only run when the temp or humidity reaches a certain level. I have the same problem if the fan is running constantly the temps are too low. My temp controller switches the fan on at 80f and back off at 75f, roughy every 30-40 minutes for about 3-5 minutes. You really need a heater for lights out to keep your temps around 70f otherwise your soil will get cold and growth will slow.
This in a nutshell^^^^^^^^
temps go down to 5 degree clesius now. It seems the plants use much less water now.
Check the medium to see how cold their roots are
had to google that as I think F but 41 F is cold and they will stay wet much long
and not in a good way 