Sounds like a fun setup to work with!

Advanced Nutrients "scientists", claim the ideal tempurature fluctuation is - 15° F during the lights-off cycle (both during veg @18/4, 16/8 or during Flowering @ 12/12).

Green light will not affect the plants during the dark cycle so maybe a filter of some sort can help, maybe - double check that.

I would definately put the floros on the sides, there are no reasons not to!

Harvesting the heat... ... ... Solar water heaters? Your grow room would look like something out of a Bond film.

How cold will it get during your flowering cycle and how well is the shed insulated?

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