Thinking of trying LED from HPS. Any info helps.

Hey there peeps. I have a little grow going on in the room under the stairs. Currently using a 400w HPS and growing some Blueberry and Skunk #1 from Seedsman. I started them on September 3rd.

I have an oscillating fan in there and a 4” exhaust fan sucking out. But those temperature get so damn hot in there. Going from low 80’s when the door is open to sometimes soaring up to 100 when the door is closed. (Which has to be closed most of the time) I know it is terrible for them so I have to try something different. I know the circulation in there is garbage and I’m guessing that light isn’t helping with that heat.

Now if I were to try switching over to a 400w LED I’m guessing that would help a lot with the heat because I hear they aren’t as hot but would that be bad for the plants changing over light sources like that? Just trying to get some info before I get too much further. Any help is greatly appreciated. Thank you in advance.

Hey Jax! If your plants are in flower you should be really careful introducing a new light. Specially if you are switching from 400w hps to 400w Led (high end led like Cob's and quantumboard/chilLed tech.)
These high end led will produce more light usable for your plants and will bleach plants easily if you put the lights to close. If they bleach there is no getting color back, and your plants will be stressed from it.

If the plants are in veg you got some wiggleroom to dial in the light and avoid bleaching, or if you happen to bleach them you can always bounce them back before you flip the switch.

Now this being Said. Whats the size of the room and height? This also plays a role. You might not even need 400w of high end LED to begin with.

My last suggestion is for you to search growmau5 up on YouTube. He's a great teacher, and will probably answer any questions you might have through his videos.

I think 75-80 is a good canopy temp, so if your 400s HPS is pushing temps much past 80 at the canopy, same wattage LED can be helpful. Since wattage is similar the total heat in the system will be similar along with AC cost, but the canopy temp can drop 5 or more degrees because there's less IR hitting the plants. There will also be a lot more PAR hitting the plants. I think 400w SE bulbs are just over 30% efficient so PAR watts will be around 130. You can achieve up to 60% efficiency with cobs, possible to hit 250 par watts with 400w output. 50% efficiency providing 200 par watts. 40% providing 160, etc.

It is possible to lower ambient temps with lower wattage and maintain the same light level. Even possible to both lower ambient temps and increase the light level. A consideration is that high temps may be an indicator of poor ventilation in which case simply lowering the wattage will not fully correct the problem.

As @Unagi mentions your space is a critical factor along with what your expectation of a good intensity is.

If your lighting strategy includes a specific light intensity then you will have a PPF goal which can be translated into a PAR watt goal, then it's just a matter of finding or building lights that will satisfy that goal at whatever efficiency level you are comfortable with. PPFD is the intensity of light in a space rather than a total amount of light. People generally reference PPFD, for flowering anywhere between 600 and 1500, 700-1100 being typical values while some push closer to 1500. Hot spots can often be well over 1500 so I think 1200-1300 is a reasonable high end intensity goal, similar to putting a 1000w SE bulb in a 4x4. Whether someone needs the power concentrated that much is a different matter. 800 PPFD can provide respectable g/sqft numbers with good technique and it's worth noting that spreading out the available wattage is generally better than concentrating it irrespective of space concerns.

A general reference (estimated + - 7% accurate)

5 PAR watts per sq foot - 250 PPFD
10 PAR watts per sq foot - 500 PPFD
15 PAR watts per sq foot - 750 PPFD
20 PAR watts per sq foot - 1000 PPFD
25 PAR watts per sq foot - 1250 PPFD
30 PAR watts per sq foot - 1500 PPFD

So while PAR watts alone isn't as accurate as doing PPF calculations it does represent a range based on typical spectral efficiency of 4.5 - 5 micromoles per par watt.

If you're getting temps in the 100s under 400w of HPS, you'll be getting pretty much the same from 400w of LED. You don't need 400w of LED to match a 400w HPS. Get the right lights, and I imagine 250w would more than suffice. Start reading some of the threads here, and you'll quickly learn more than you ever thought you wanted to know about LED.

Yeah my space under the stairs is probably a foot at it’s lowest to about 6 feet at its highest by 6 feet long. So it’s a tight space and the air flow is pretty restricted. The plants are also just in the Veg state.

Here I was thinking just swapping out the bulb would be able to fix the situation but it seems it’s a lot more than that. That whole ventilation issue is going to have to be addressed for sure. But I still feel I might want to give maybe a 250w LED a try.

Thank you Unagi, Rahz and Humple for all the info. You all gave me some good info to keep my wheels turning and for going back to the drawing board. I appreciate all your help guys. I’ll keep pecking away at the posts in this forum. Thanks.