Get a space heater and ink bird or lux pro temperature timer so you don't have temperature swings of more than 10 degrees Fahrenheit between lights on and off. Fungus and mildew love huge fluctuations in temps and humidity. And your fine dimming your light to 400w with a 600w bulb. I wouldn't buy a 400. It's not going to have any effect on you. I work at my local grow shop and I promise you will be ok lol how were you cooling in the summer?Cool, the reason i am asking is because i am currently running 18/6 @600w and it costs me a bomb in electricity. now with the winter months approching i am having issues with cold so my solution would be to compromise with the lighting and run 24 hrs at 400w.