Grow room cold how long can I leave my fan off?

I haven’t yet begun to grow, I am just setting up and testing to make sure everything will be okay. My problem is my grow tent is in an unfinished portion of my basement that has been staying steady at 59 degrees F. I am running a 400W HPS light in my 4’x4’x80" tent and without any ventilation it brings the temps up to 72 in the tent.

I have a 6" 309 CFM inline fan that I will use. I’m wondering how infrequently I can cycle it to provide enough fresh air/CO2 to the plants while trying to keep the temp up?

I cant answer the question about how much fresh air you need, but there are at least two options that will help with the basic problem.

  1. Insulate the grow tent. Adding some 1" thick foam ($20 at Home Depot) or 4" thick fiberglass insulation to the sides and especially the top, will help keep in a lot of the heat from that lamp. You may end up needing to remove some of it even.

  2. You could also add a small heater inside the tent. In fact, you may need to do that to keep night time temps from dropping too much when the lights are off.

I have a much smaller tent (7 sq ft), and my grow room never goes below about 62 at night, but I had to do both those things to keep night time temps up where I wanted them - mid 70’s F.

I have a 70CFM fan exhausting full time, plus an outside air source, to keep temps down when the lights are on,


Oh - I use an STC1000 temp controller to turn the heater on and off depending on the temp in the tent. It also functions as my temp monitor.

It also can control a fan for extra cooling if you need it - for example, you could have another exhaust fan on the light that only turns on f the tent temp goes too hi. You can program the ON/OFF temps for both with the STC1000.

This is one of the ones I use, but there are other options - like Sonoff switches.


Thanks for the reply. You have some good tips and I especially appreciate the link to the temp controller. I considered a heater in the tent but I am a bit nervous about that. I know it sounds stupid since I already have 400W HPS in there, but I am just nervous about a fire. I was hoping I could just run the exhaust fan sparinglyto keep the temps down.

Does anyone know a formula or something to calculate how much fresh air is needed to keep the plants healthy?


well your tent is ~104 cubic feet and your fan does almost 3x that much so you could probably run it 1/3-1/2 speed all the time if you got a variable speed controller. could run it bare minimum like 5-15 minutes an hour if you really want, or use a temp controller to run the fan when it gets too hot and run a heater when it gets too cold.

i didn’t even turn my exhaust fan on until flower started and it started stinking last time tbh. i have all vent ports open with three 6" fans above the canopy and a small oscillating tower fan under the canopy tho and dehuey controlling humidity. now i am running it constantly on low w/ a heater in the room. i need a inkbird.


400 watt hps in a 4x4 tent is way under lit… your gonna need a bigger light… that will help heat the area some…


What size is your tent? If I leave the vent ports open then the temps will drop. Probably to the room’s temp. I was hoping to keep everything else sealed and just run the fan intermitently.

I’m not utilizing the entire area for grow. I will only be growing one small plant and will, in fact, be actually trying to keep my yields low. It’s just for me and I don’t smoke everyday so I will easily out produce my needs. My mine concern is quality and stealth.

You’re going to have to heat that room when the lights are off, or you will have huge temp swings, and it will get too cold at night, which will stress the plants.

In my case, just draping a blanket over the top and sides of the tent plus adding a small “personal” heater was enough to keep temps up in the mid 70’s at night. You do need to think about where to place the heater so you dont melt the tent plastic, burn the plants etc, but its not that hard to do with one of those small personal heaters. I got mine at Target for like $19 or so. Its barely larger than a can of coke. I have a separate fan running full time inside the tent to circulate air, so that keeps the heater from causing hot spots.


my tent is 48x48x79 - you will need air intake around equal to your exhaust. i use passive intake and that is why i have all my vents open. the tent is inside a room 6x10x81" and i use a small 750w space heater outside the tent blowing towards the vent holes.

are you using active intake, are you maybe able to pull hot air from another part of the house to vent in the bottom of the tent?

i did get one of these small handy heaters to try only heating the tent instead of the room but it wasn’t powerful enough and would continue to run until it overheated and shut off during lights on, with the thermostat at 69



Simple suggestion: avoid heating the roots directly…

Good day, have fun!! :peace:

If you are concerned about stealth then have a fan connected to a carbon can that never stops running. If temp is an issue then night time is when you need to worry about temperature.

Smell is a 24/7 issue and does not care if your plants are too cold or the light is on or off. Make sure that smell is always dealt with, insulate your room to stop temperature swings, add a heater for night time on a thermostat.

If you are worried about fire that is good, it means that you will be less likely to have one!

So put all electrics above possible splashes, all vents clear of anything flappy, allow air to move around anything hot, don’t put anything too close to the output of a heater, fit an RCD to your electrical input. Don’t overload sockets either, keep track of how many amps you are drawing from each socket.


Larry - I know I am going to have to heat the tent at night. My problem though is that I will be pulling air from the cold room and exhausting all the heat in the tent. I intend to have a small oscillating fan in the tent.

legalcanada - I plan to use an active exhaust through a carbon filter to rid the odor. I do not have any practical way to pull air from a warmer part of the house.

Egzoset - I don’t intend to heat the roots directly. Although if I can;t keep my water temps up I may throw an aquarium heater in the main res to keep the temps at an acceptable level.

MicroDoser - Stealth is very important to me and I will definitely have a carbon filter connected to my active exhaust. I was assuming that with the tent all sealed up that the smell would be contained within the tent while my fan was off. Am I wrong on that?

I can certainly add a heater in the tent but won’t that be pretty much negated if I constantly run my fan and pull in that cold air outside the tent ?

I will certainly be diligent in safety and fire/shock prevention.

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Thats where the temp controller comes in - STC1000 or Sonoff, etc. Most all of them can turn OFF the exhaust fan when the heater is on, then turn it on again when the heater goes off.

If you insulate the tent, the heater wont have to run very long to heat up, then the heater shuts off and the fan turns on to bring in fresh air. As soon as the temp drops to the pre-set level, the fan turns off and the heater turns on again.

I have mine set to a temp range of about 6 deg or so IIRC. When the temp gets down to 72, the heater comes on and the fan goes off. Then when the temp gets up to 78, the heater turns OFF and the fan comes on.

It will cycle back and forth all the time during lights off.

During the ‘day’, the fan will stay on as long as the temps are up to 74 or higher.

If the tent is insulated, it will heat up fast and cool down slowly - other than the fan bringing in fresh, cold air.

So, yes, you will be heating the room, but I dont see any way around that. Insulating the tent will help, but you’re still going to be heating the basement no matter what.

The way the STC1000 works is you set a target temperature - say 75 deg. Then you set an offset value. I think Im using 3 deg. That means when the temp is below 75-3 = 72 deg, the heater comes on and it stays on until the temp reaches 75 deg and it shuts off. The “cool cycle” wont start unless the temp hits 75 + 3 - 78 deg. The fan will run until the temp drops back to 75 deg, but the heater wont come on until the temp drops another 3 deg to 72.

I have my temp sensor placed so that it is a ways away from the heater so there is some lag between its reading and when the heater shuts off. The temp always keeps climbing for a while after the heater shuts off as the air circulates inside the tent.

The short version - you set your desired temp - 75 in this case - and the STC1000 will allow the temp to fluctuate based on the offset valve, from 72 to 78 deg.

You can set any offset you want.

The better insulated the tent is, the less cycling you will have.

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Larry3215 - Thanks. I now understand the way the controller works. I just think that when the fan does run it’s just going to cool my grow space too low. I may just have a HVAC contractor come out see why my temps are so low on that side of my basement.

Remember, the temp controller will shut the fan OFF when the desired temp is reached - no over cooling.

IF the tent is insulated, and you use a controller like I described, conected to both the heater and the fan, then the temps inside the tent will always stay within the range you desire. The fan will cycle ON/OFF only when needed and the heater will do the same.

so are you planning on using passive intake? because if you seal the tent up completely as u suggested your exhaust is gonna suck the walls of that tent in and overwork the motor. so u can either have passive intake or an active intake. if u had 2 fans you could run a line of ducting to somewhere warmer to pull your intake. if you’re using passive intake the exhaust fan will be sucking in the cold air from the room. you’ll probably need to heat the room. where are you planning on exhausting your air to? outside? or back in to the room the tent is in?

idk if you don’t even have any plants started i don’t think you need to worry that much, you’ll have lots of room in your tent to safely run a small space heater and for tiny seedlings you don’t really need to be exhausting the air constantly and it won’t stink for a while. i’d just have the exhaust on the same timer as the lights. by the time u really need to worry about it it’ll be warming up outside and you’ll prob need AC already.

i didn’t even hook up my fan until 4 weeks in and didn’t connect my carbon filter until the 3rd week of flower or something

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Yes and no, it will help to a degree but depending on the strain you are growing they can ignore tent walls, house walls and the smell permeates everywhere.

Also your nose can become used to the smell so you do not notice just how much it carries.

I always fall back to the safest choice and have a fan and can running 24/7. If stealth is important then it is important that you always do what you can to be stealthy.

My plants are about 15ft from the road and after I fitted a webcam to my kitchen window I saw that roughly 200-300 people pass my kitchen window every day. I have two fans and cans, one to extract from the tent and then another to pull air from the main room to inside my home where it is dehumidified. I find that one can does not cope well enough with the smell and the double filter works best for me.

That was too many words. The short answer is do not rely on a half measure if you have a full measure available. I would advise (if you have funds) getting a variac to turn down your fan and have your fan and can running 24/7 and then use a heater if things get too cold.

59f is about 15c, could you not just put a heater with a thermostat in your basement to bring the whole room up a few degrees then not worry about temps?

EDIT : Of course, as has been said above, you do not need a can until they start to smell and you do not need a fan until they start to make the tent too humid and are using up the CO2.

I keep overlooking that fact. I guess I should get a small heater with a controller like you linked and run a trial and see how it actually works for me.

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My apologies. I wasn’t clear. I will be using a passive intake via 6" duct along with an active exhaust via 6" duct. I meant sealed except for that.