I’m planning ahead as we transition from summer to fall, and trying to get ready for cold weather. I will be growing in my detached garage, in a room 3’w x 10’l x 10’h. I’m insulating the room with standard fiberglass insulation. I’m curious if 3 600w HPS would create adequate heating this winter. I live in NE TN, and we have fairly mild winters with nightly avgs in 20-30s, but some short periods of stupid cold. I plan on having supplemental heating on hand, but curious if anyone else has dealt with a similar situation and can tell me their experience on this, and possibly what to expect.
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I would think that as long as you are not running your ventilation through your light the heat should be enough most nights. I would keep a small electric heater on hand just to be safe however.
run a few test nights in the grow room and compare inside lows versus outside lows with lights on and lights off…should tell you the difference the lights make.
then take overnight lows at test time and compare them to expected lows over the winter…
not an exact science as the heat should be cumulative to some point as long as the door remains closed and the light is on,…but severe temp drops overnight can also have a bigger affect on room lows too… but it should get you in the ball park
You can expect a 10 degree differential with two 600 w HPS during lights on and the darkness stage (statistically you will need to find the ratio for 3 600 w HPS-lol maybe 15 degree differential). So no you are going to have a problem during the dark period. You really need to keep the temperature in their favorite range which is between 70-85 ° during light period and if I remember correctly slightly cooler than 70° during lights out. Even if you insulate the temperature will be approximately 55° degrees during the mild winters (night). I have a double insulated barn but the horses omit a lot of heat and tack room and it never gets above 55° and I live close to your growing area. I would keep the temp. between 65 at night and around 75-80° during the day. Use the supplemental heating as long as it is safe. You know away from paint cans or any other flammable items. Prepare your grow area so that it is sparkling clean and safe. It will save you problems and your plant or plants will thrive in a clean and safe environment with the above temperature recommendations. If you grow a maximum of 4 plants under these conditions, I think you will have a higher yield! Purple haze and other strains tolerate different conditions. So it really depends on the strain. Investigate via google and make a plan. Like a plan you would make when you travel. You would never just jump in the car and say…okay lets go…you would need a plan.
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If the lighting is adequate for heat, I’m going to grow only autoflowers, and start new plants every 2 weeks. Will be able to keep lights 24/7. I borrowed this idea from @garrigan62. Its a long, slim space, so I plan to actually mix the plants around to best use the space. I had planned to switch to hydroponic, but this option gives me freedom to move around all plants to best utilize space, light, temps. I’ve been hesitant about autos, but I started 4 WW autos from here in mid August, and have punished them with topping, cropping, excessive nutrients (inspired by @Careg), and light (have burned the shit outta some fan leaves with side lighting), had my carbon scrubber fall and crush one plant, taped it up, rubbed dirt on it) and they have proven far hardier than the info I’ve read about them. No harvest to speak of, but I’ve tried to push these plants as a test if autos are as delicate ad I’ve read, and they are doing great to spite my efforts to test them. So this may actually work
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I actually do this myself for the winter. I have 2 600w HID fixtures, and with the heat from them I don’t need to vent the lights or anything. It really helps keep the house warm too, since heat rises it goes from the basement to the attic, warming the house on the way 
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Thx! I had thought of buying some LEDs, but Then I’ll be heating this thing all winter, so its HID for now being I already have one, then by May next year I might look into a LED system to fight AC costs, led market will have improved greatly by then I imagine, the top shelf stuff now will be markdowns. Lol
It’s funny because that’s my thinking exactly! Hoping that in spring I can pick up some good LEDs and use the HIDs when needed. The heat this summer just made my buds airy, and it really bothered me.