400 vs 600 w hid

I usually use a 400 w mh bulb during vegging and then go to a 600w hps bulb for flowering, but I tend to suffer heat stress on the plants, with canooing and drier leaves near the tops.
Would using a 400 w hps during flower decrease the yield a lot but help with reducing the heat stress effects?

Is your fixture air cooled? What temps are you seeing in the grow area?

I think it would help with the heat yes.

Rosebud said:

I think it would help with the heat yes.

Click to expand...

Would that gain be worth losing lumens over?

Thanks. My main concern is how much the yield is reduced between a 400 and a 600 w hps light during the flowering stage.
Could the yield be maintained by keeping the light lower and still result in less heat stress, everything else being equal?

I don't know for sure, it depends on so much like air circulation. I just hate fighting the heat indoors. Makes me crazy. I got a small airconditioner and that seems crazy and wasteful but if you want the dank you have to keep the temps down. Truth be told I couldn't keep them down with the air conditioner either. After 85-90 for extended periods the plants show it. Other than having a grow room designed and built for growing, i don't know what to say.
Maybe you will lose yield, maybe you would lose yield because of heat issues. I used to grow with 400 when i added the 600 it was WAY too hot. Maybe someone can come in and tell you for sure... Good luck..

Tell us more about your space and how hot it gets. Are you sure it is heat stress and not something else? How is your ventilation set up? Do you run your flowering lights at night?

I feel like your yield will suffer a whole lot--you are cutting your lumens almost in half when you go from 600w to 400W. IMO, there is no way that the yield can be maintained. There is no doubt though that the heat will be less, but it will most likely cost you substantial yield. Let's see if we can figure out a way to deal with the heat.

I go from 400w MH during veg, and 600w HPS for grow. I did this for 1st time last grow and although I have no hard data I am sure my yields were significantly higher. I was just using the 400w previously.
I use an air cooled ref. with no glass ( keeps the room warm enough ) and 6" in line fan bringing air in and venting it back out. I keep a remote themo at the plant height and keep the temp around 82*, seems to have worked well before and I am doing it now. Just have to keep the lights high enough to balance light and heat. I try to keep light levels at the plant to a min of 35K lux

if I could chime I for a sec I am running a 400watt x2 cool tube and when I am running just one light my temps are 85-90 in a 3x3 tent when I run both temps are above 95. what I notice about my tent is that . my tent seams to mimic my room temp. if my room temp is 80 then my temp in my tent is around 84 or so. so if you can keep your room temps down you can keep you tent temps down also. are you could look into a bigger tent. a bigger tent cold help the temps also

An interesting question,


25 years ago I went into 'The Gas Pipe' head shop and asked about 'a grow light'. The guy, as it turned out, was exactly right when he told me all about hid lights. He said if I smoked a lot and lived alone, then a 400w would be about right. But that I would be smoking out of 'my one hitter' by the time the next crop was ready. If I had a chick... a 600w was recommended but for one man he recommended the 400w.

He mentioned 'heat' and how the 400w would be best for that reason too. They didn't have glass covered lights with 4" cooling ducts back then and as I think back to the views of the guy at the 'Gas Pipe' he was very knowledgeable. The old 400w heats my closet up to 80 degrees but no more. The thermometer is right behind the light but lower on the wall and the heat never goes above 80 as long as the A/C/ is on or the windows are open like at night. The compressor went out the other day and the temps got up to 85 {in august} but no problems with the plants. {had the A/C fixed in two days}

I went to Wallyworld and bought a kiddie swimming pool with no drain that is 3' across, perfect for a 400w hid. It will hold 8, 3 gallon pots. I bought a brand new 2"x4" and cut it to fit across the closet from one top shelf to the other and the light is secured and fully supported. I don't worry about it falling.

Also a fluorescent and a 150w for vegging along with my new 'Root Riot' tray/dome and plugs and I am set up!

It all started when I realized about cloning, that one can clone a female and raise nothing but females. Cloning enables me to reproduce strains at will and with the Root Riot tray/dome and plugs they all live!

I forgot... it also really started to get off the ground when one could get store bought seeds!
My first 'Satori' seeds from Mandala never hermie.
I love the strain, 'Satori' always has a place in my dope box.
And the old 400w will always be my light of choice.


Thank you...


The Poet...♪

Ah....heat....my nemesis.

My two rooms are running in the high 80's to low 90's during this summer....and am also seeing it have a negative effect on my plants.

Yes, a 600 watt light is hot and a 400 is relatively cooler, but I wouldn't hold either in my hand while they're turned on. They're both hot.

I've been focusing on ventilation instead of backing down on my wattage as I feel that I have the proper lighting for my 4 x 8 grow spaces. I've got one of those portable A/C units in one that I've attempted to keep both rooms cooler and while it helps, it's only a few degrees cooler.

Next summer I'm going to get another portable A/C unit and each room will benefit much (I hope).

How would the yield be affected if I used the 400 MH for veg and a 400 HPS for the first half of flowering and then increase it to a 600 HPS for the last 4 or 5 weeks?
This would help reduce the heat stress and hopefully give it the extra light at the time that it is developing the weight and bulk in the colas.
I don't really have too much heat when I grow in the winter but the last grow ended at the start of June and I was having a lot of 90 degree+ days.