CMH heat exhuast?


Happy New Year everyone! :slight_smile:

I’ve noticed many of the CMH fixtures don’t have a proper sealed inline exhaust in the fixtures. Any ideas why? Am I missing something?

I found one that does (glass cover w/6" ducts), another similar but without the glass (no seal, no vacuum).

I realize a 315w CMH will put out less heat than say a 400w HPS, but will it not need direct inline venting? What if two lights are used? That would have to have some exhaust to keep temps in range. Would a general pull through a carbon filter to exhaust provide enough temperature reduction?



I think they are generally sold that way to keep down on cost and a cooled hood is considered an upgrade just like with hps. I also believe that there is separate bulbs designed for cooled hoods.

There’s a lot that goes into cooled vs open, @Donaldj is probably your guy for this info.


I have xxxl 8" ducted sealed hoods that will accept any light bulb or ballast… i.e 1000 watt , 600 watt , 400 watt
Hps , mh , cmh… maybe you need to by the hoods that you want separately instead of trying to find a package deal… it might cost a few extra bucks but at least you will get exactly what you want… :wink:
I even have the option of putting two bulbs in one Hood if I wanted to do that as well… :wink:
:v: :sunglasses:


Actually it is about the UV they emit which is major part of their high PAR and effectiveness they already are double jacketed bulbs they run very cool compared to most HID’s. Adding a pane of glass much like any light costs 5-10% of light intensity and PAR the double jacket on bulb is because they much like quartz headlight bulbs would fail from contact with dust dirt oil or moisture this also means multiple gasses are in bulb for spectrum adjusting.
I wouldn’t use vented hood since they are costly bulbs and airflow can trigger rapid temp shock (shatter bulb) most manufacturers advise against it main reason they aren’t packaged or sold with vented hoods :wink:


I really don’t see that light loss as necessarily accurate. It certainly isn’t with cinema and photography. Even so, with ambient external room temps @ 75º can two 315 W in a 25sf tent run 12 hours without exhaust?

The air being drawn through the hood would be the same (ideal) temp as inside the tent. Do HPS bulbs routinely shatter when exhausted?

Maybe I need to contact a light manufacturer. Thank you.


Different waves of light refract differently what we see is less than the actual light waves which are there we see mainly in spectrums which plants reflect and use least. UV in particular is very easily refracted :wink: I am sure you could use with vented shade But I would steer away from the glass as every write up I have read on CMH LEC advised against it.
I use 3 all in open shades but my temps are far cooler but I expect that simply pulling heat out of shade without glass would work fine too. I do use exhaust in my spaces but it doesn’t connect to shades simply draws hot air out of top of space which is about 24sqft easily enough mind you running single LEC 315 on lightrail 3.5 with 22" track. 2 LEC’s in my foot print I may actually have to turn my exhaust past 1/3rd speed and add active intake rather than passive? I should find that out this spring when I do side by side against cobs?


HPS is a ceramic rod inside thicker glass but MH are known to literally pop and blow I actually posted a picture last year of one which went on me and deformed the outer glass :slight_smile: Can’t find it or would share picture


Ill ad this to the discussion
I recently purchased a 600w cmh set up
And as suggested by @Donaldj i went with a non vented hood
I installed it into my 4x4x7 tent
My grow room is 60- 65 f with lights off and and tent is tbe same
With lights on my tent has maintained no more than 70- 75 f
So with proper ventilation and air movement
Heat does not seem to be a issue
In my case the 600w chm set up is putting out around the same amount of heat that my leds where generating
Since i had to run 5 fixtures


Cool. 60-65 in the winter equates to a ten degree increase in temp. What happens when it is late spring/summer, and the temps w/lights off are 75º?

How are you venting now?
Pulling straight through a ceiling mounted filter, and out?


btw - at what height are you measuring temps?


@Countryboyjvd1971 is an HVAC tech by trade and as I said I barely turn my exhaust up also grow in a shed with no central air or ac in temps ranging from -30f winter to 110f during summer recycling exhaust in winter and sending it outside during summer :wink:
I don’t run AC during summer I simply pull air faster and yes it hit high 80’s the odd day but they can handle it. If you are growing indoors in a space with central air exhaust can be directed outside and fresh air still pulled in to keep temps quite close to ambient even though the watts are equal the temps they produce are easily 10-20% cooler than conventional 600w HPS.
Being that I know and have seen CB’s grows I am pretty sure he takes temps at canopy like most experienced growers


@latewood this may interest you


@Donaldj is definitely correct about lower wavelengths not penetrating glass as well, and that goes for all sources of light.

Outside of knowing that’s Phillips has different bulbs specifically rated for use with and without enclosed hoods, I have very little else to offer towards this conversation. But I’m following along for sure.


They Philips; produce the best CMH bulbs going Green power but we have to keep in mind that these fixtures came out primarily for mall and hall lighting with superior CRI than their cousins HPS?MH it just happened to have an odd result high UV which led to a different market. Leave it to gardeners to go lets see if my plants like it after that the research and gas tweaks helped spectrum and viola a brighter grow light to compete with all those evil LED’s stealing our share of the market :wink:


You betcha! When I worked in manufacturing our quality department used them for inspection lights. They were smaller than the bulbs anyone is using for horticulture, 70 watts maybe. I wasn’t real involved with them until they had a bulb explode. Which coincidentally is what got me started into the led world. Safety guy said they couldn’t use them anymore and we needed an alternative.

I’ve only seen one in use as a grow light, and can fairly confidently say that canopy temps are between more traditional hid options and leds. Everything else I would have to go a spec sheet for. I can also say with some confidence that if I wasn’t where I was with the leds, I’d probably grow with them over the other options.


That seems odd that a cmh bulb would be more likely to blow up do to temperature changes , because of the fact that it’s double jacketed …that seems pretty strange to me… will have to investigate CMH a little more… although I only run HPS I have to have a ducted hood do to the fact of high temps during the summer… :wink:

:v: :sunglasses:


they are double jacketed yes and weren’t initially designed for growing applications the double jacket is to reduce cooling shock to inner bulbs but the biggest factor once again is the lost UV. What I can say is no company sells CMH growlights with glass or vented hood unlike MH/HPS I would produce research on them but the links all have seedbank sponsors and the like. I do know that I can put these lights as close to my ladies as a vented 400-600 hps with less radiant heat the PAR they produce is too much at how close I can get them bleaching my plants before canopy every hits 80’s with simple air exchange


I think I know what your talking about… they have a cmh set up at my local hydro shop… it’s vented but the air never moves over the bulb … it just vents the hood and not the bulb… I’m pretty sure I researched this when I seen the hood… it was weird to me and when I asked the guy at the hydro shop about the weird design , he told me that the light has to be at a certain temp for it to work properly… :wink:
So what your saying makes sense to me… :wink:
Still going to do more research on it… :wink:
Thks @Donaldj


i’ve noticed growers online seem to keep their CMH fixtures fairly high (bleaching?).

As far as glass reducing UV - it doesn’t in picture frames. Even “UV resistant glass” won’t keep fugitive dyes from fading in art work.

I agree with PF on more research


You’re being to literal with your thinking. I don’t believe anyone was suggesting glass would block all of the uv produced from a cmh bulb. But it does reduce it, as it does with other wavelengths. The shorter wavelengths are just more prone to be affected by glass is all.

The effects aren’t limited to cmh bulbs either. There is reduced photon count when putting lenses in front of led and hps lighting as well. But you don’t get the uvb production out of them that you get out of cmh. Horticulture lighting is a numbers game, with jumps in efficacy prices start going up substantially. So you look at the reasons you’re considering spending extra on a cmh setup, and then decide if putting a piece of glass in front of it is still a good idea.