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LED Test: Hot or Not?

LED Test: Hot or Not?

Alex Grady

In summer, heat can really dent your yield. Plants set differently...growth is slow. And that's after you've dimmed or removed a light.

Imagine how much light and yield you're losing. 

Sound familiar? What if you could beat the heat and keep your wattage high? Swapping to multiple LED lights could be the answer.

We decided to find out how they compare to CDM and HPS lights.

Overall, we found that:

  • LED grow lights are 3oC - 4oC cooler per light
  • LEDs increase your humidity to help cope with heat.
  • Your temperature only increases by 1oC when you add an extra LED
LEDs Telos ExplainedFREE download

The Aim

We decided to see what each of these lights did to the temperature, humidity & leaf temperature in a Large BudBox:

Just remember, if swapping a 600W light for a 240W LED or 315W CDM, your wattage drops dramatically...of course your temperature will be lower. So will your light output.

So, to be realistic, we also tested:

  • 2 x 240W Telos 0008 (480W total - much closer to a 600W HPS)
The aim

What We Did

Normally, we run side by side tests. Not this time. To be as fair as possible, we decided to see how each light performed in the same tent, with the same plants.

Step 1

We kitted out a Large BudBox, with a:

Our base readings were:

  • Ambient air temperature was 20oC
  • Airflow was 198m3/h

We tested each light, one after the other, in this tent.

Step 2

Before starting each test, we waited for the air temperature to return to 20oC.

For each light, we measured the:

  • Air temperature
  • Leaf temperature
  • Relative humidity
Tent and LED

We took two sets of readings for each light - 40 minutes and 80 minutes after lights went on.

Overall, 3 different temperature probes were used for accuracy. We also took the leaf temperature from the same leaf every time.


As expected, LED lights were by far the coolest to run.

What did surprise us, is that adding a 2nd LED didn't really add much heat. Air temperature was still 3 - 4oC lower using 2 x LED's than it was using a single 600W HPS.

Based on these results, you could go to 3 x 240W LEDs (720W total!) and your tent would still be cooler than when running a 600W HPS.

As a bonus, the humidity was also higher when using LED lights! This helps plants retain water as they don't transpire as much.

CDM lights weren't too much warmer than LED lights - also a great shout in summer.

40 mins in
Air Temp Leaf Surface Temp Humidity
600W BAY6 HPS (magnetic ballast) 25.5oC 23.5oC 39% RH
315W Maxibright CDM 23.9oC 22.5oC 42% RH
240W Telos 0008 22oC 19.5oC 44% RH
2 x 240W Telos 0008 22.6oC 20.5oC 43% RH
1hr 20 mins in
Air Temp Leaf Surface Temp Humidity
600W BAY6 HPS (magnetic ballast) 25.8oC 24.5oC 38% RH
315W Maxibright Focus CDM 23.9oC 21.5% 41% RH
240W Telos 0008 21.9oC 19.7oC 42% RH
2 x 240W Telos 0008 22.9oC 20.7oC 44% RH

The Verdict

Really, it's a no brainer - if heat's a problem, swap to LED lights. Failing that, move to CDMs.

No more dimming or removing lights - by swapping in summer you can keep your wattage roughly the same (as long as you use enough lights).

Just make sure you've got a decent LED light - cheap ones that are badly made won't have the right light spectrum and may harm growth. Switching to them can do more harm than good.

Don't forget to switch back to your HPS lights in winter - you'll really need the extra heat then. You can still use any spare CDM lights to supplement your spectrum.

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