i use my air intake and exhaust and i connect the two of them together slightly, and i put a small electric heater set to 20 degrees. this is to stop it from getting down to around 15-16 degrees i think thats too cold for mid flower. if you are in a really cold room you could get autoflowers and keep the lights on 24/7. save your heat, and recerculate your heat as long as possible. now that i mention it, my little heater has bright blue lights on it, and they are probly bliding my plants in their sleep , i am going to cover it with duct tape now!