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From what you're describing, 400w will cause you quite big heating issues.
I downsized from a 400 to a 250, because without proper vent in a cupboard, 400w is going to be too hot.

I added fluors instead to compensate.

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faithless Reviewed by faithless on . heat differences: 250w or 400w HPS? A friend of mine is starting a small closet grow in which the ventilation issues could be a problem. ventilation will be somewhat limited, so heat is a concern. He is about to purchase a light, but is unsure which one. He is sure of a HPS system, but wondered if 250 or 400 would be better. Its a small closet so he will still have plenty of lumens with a 250w, but 400 is obviously better. Our question is, how much of a heat difference will there be between the two bulbs? Is the 400 much Rating: 5