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If the ambient temperatures are too cold there's a couple of thing you can do.
In my case night temperatures get as cold as 32F (0°C) and in the grow space I can keep it at 77F (25°C), just with 60 watts of lights.
What I do is decrease the exhaust fan so the heat inside isn't vented out too fast. Use a smaller fan and / or connect it to a variable control (I use a PC fan and have it connected to a variable voltage adapter).
How much CFM? In the design sub forum in this very site there's a formula to estimate how much (or how little) you need to maintain a certain temperature.
It's something like this: CFM needed = 3.16 * watts / Difference in F
Watts been the electrical power that dissipates inside (in my case only the lights)
Difference in F is the allowed difference in temperature that you want between the inside of the plastic tot and the outside; in this case since the difference is quite high, you'll need very little exhaust power.