For decades now, growers have become so accustomed to dealing with high temperatures from HPS grow lights that when switching to LED the opposite environmental challenges posed will often make the grower feel like they're starting from the ground up again. Environment is absolutely critical to good yields and many unfair conclusions about LED have been drawn by growers who have simply swapped their HPS for an equivalent wattage LED without properly considering these factors, expecting better results to be a given but only getting sickly looking plants and poor growth in return.
To say nothing about air exchange and leaf temperatures, the speed in which crucial chemical and biological reactions that govern growth take place (up to a limit of around 32C+) is proportional to the temperatures inside the grow space. If we want to get the most out of our shiny new LED, we need to get temperatures up!
So what's the point of LED then?
Aside from the improved spectrum, reliability and luminaire longevity that LEDs posses over HPS, there are several advantages to using LEDs in terms of both power consumption and how that power is used:
For single fixture spaces (such as 1.5m2 tents and under), LEDs - and strip lights in particular - provide a much more uniform distribution of light across the area because the light source is spread across the fixture. This is often misleadingly framed as 'better canopy penetration'. Single HPS fixtures however, emit light from a single point, often producing too much light in the centre and not enough around the edges - stressing plants and affecting productivity. By more evenly lighting up the entire canopy with LEDs, optimum photon levels can be consistently provided to the plant, giving better and more consistent growth from the centre right out to the edges.
Underheating vs Overheating
While it may on the surface seem that underheating via LEDs is equally undesirable to overheating via HPS, we should look at these things in terms of energy efficiency. When HPS lights provide excess heat, not only is electricity wasted by this, but extra electricity is needed to remove this heat, creating further inefficiencies from the use of fans and air conditioning equipment. While it is true that during the very coldest months these will be switched off, the addition of them during any instance where the HPS alone provides too much heat amounts to wasted electricity.
In contrast, because LEDs provide so much more light per watt used, growers will often find the need to add heat during colder months. This may seem counterproductive to the aim of reducing electricity, but by adding a radiator with a thermostat, only the exact amount of electricity needed to reach the desired temperature will be used. During hotter months - when HPS growers are throwing extra electricity at their heat problems - your LED grow lights will be ticking along nicely, providing all the light your plants need but with a much reduced total wattage consumption. In either hot or cold weather situations, LED powered grow rooms will never consume more than their HPS equivalents and more often than not will be consuming considerably less while providing a more balanced intensity and spectrum of light over a larger area.
So LEDs are cooler, right?
Contrary to what is often said about LEDs being 'cool', LEDs like any power consuming device will produce heat within the grow space. Here in store, we often say to customers that 'a watt is a watt'. What we effectively mean by this is that regardless of the equipment you're using, the same amount of heat will be generated per watt consumed (google BTU). Believe it or not, a Lumatek Zeus Pro 600w will generate the same amount of total heat as a 600w HPS (assuming the power draw at the wall is identical). This may sound ridiculous, but it's true! The massively increased surface area of a large LED strip fixture (think of the finned heatsinks) spreads this heat over a much larger area when compared to a small HPS fixture, and the massive amounts of Infrared emitted by HPS will warm all surfaces in their line of sight. If you were to take a series of shaded temperature readings from within two identical grow areas, one with a 600w HPS and another with a 600w LED, your average temperature would be exactly the same.
Anyway, unless you were underlighting your area with a 600w HPS, you wouldn't replace this with a Lumatek Zeus 600w. Because LEDs provide much more light per watt consumed (this is referenced the micromole/umol rating you may have seen), you'd only need a Lumatek Zeus 465w to match this total light output (PPF); meaning that immediately you'll be cutting out 1/3 of the electricity consumed - and therefore 1/3 of the heat generated!
A quick note on Leaf Temperatures:
Leaf surface temperatures also play an important role in regulating a plant's metabolism and can be another contributing factor if growers are experiencing slower growth than HPS. Too cold and transpiration will be reduced, so heat is still needed within the canopy - something that HPS lights have been able to provide in abundance, due to an ability to heat surfaces with their high infrared output (far more than your plants can actually photosynthesise). Too hot and transpiration will be excessive, causing plants to uptake feed faster and risking overfertilisation. With this in mind, growers focusing on VPD should note that the difference between air temperatures and leaf surface temperatures will be greater under LED and should adjust their calculations accordingly. Additionally, the lower infrared output, air and surface temperatures experienced with LEDs can raise relative humidity levels, so this should be considered and monitored alongside your heat management techniques.
How to make the most of your heat
Because people have been growing with HPS for so long, heat management techniques have become rather static and embedded. When it comes to growing with LEDs, we need to take a fresh look at how all equipment can be best utilised in order to get the most out of available heat.
Because of the excessive heat associated with HPS lighting, it's been standard practice for growers to situate their extraction at the highest available point in their grow area; after all, heat rises, so this is an effective method of ensuring that the hottest air in the space is immediately drawn out. When using LED lighting however, it can be advantageous to situate your exhaust outtake at the lowest available point, drawing heat generated by the fixture down through the canopy. If possible, why not lie your carbon filter down on the floor and extract straight out of a side duct?
Fresh air exchange is crucial to healthy development, but the rate at which this needs to happen is often exaggerated, understandably because this has been coupled with the need to manage heat from HPS. By turning your extraction down, you'll help heat build up inside the grow area before it's taken out. However, caution should be taken and extra circulation fans used. Care should be taken to ensure that humidity doesn't increase too much, otherwise you'll be risking mould and mildew.
Circulation Fan Position
The previously explained technique of moving heat from the top of your tent to the bottom can be aided further by placing a circulation fan above your light pointing downwards, ensuring that any rising hot air is thrown back down through the canopy to then be extracted once it's been utilised at the plant level. Not only will you be stopping hot air from uselessly sitting above your plants, but you'll also be helping to cool your grow light even further by increasing airflow across the heatsink surface area, boosting convective heat transfer and increasing diode lifespan.
As many LED users will be aware, the drivers (ballasts) that power your LED grow light can get quite warm. While it's not possible to dismount drivers from LED lights with integrated drivers, some models such as the PAR+ Pro series come with a remotely mounted driver as standard, while many other models such as the Lumatek Zeus strip light range can accommodate remote driver mounting via an additional cable (sold separately). By strategically placing these sources of heat where they can be better utilised (such as in front of a circulation fan or on the side of a raised bed), you'll be able to get a bit more heat where it's needed.
It should go without saying that temperature of the air coming into your grow space hugely influences your internal air temperatures. If you use an active duct intake you may be able to better situate this closer to a heat source such as a radiator connected to your central heating system.
Grow tents in particular are poorly insulated and growers should consider what can be done to reduce the heat being transferred through the lining: from draping an old duvet over the top/sides to fully boxing it up with Kingspan, many additional materials can be used to reduce heat loss in the grow area.
An underutilised yet extremely efficient way of getting heat to the rootzone is through the use of heat mats, placed under pots. These low wattage heat sources and can be attached to a thermostatic controller with a probe to go in your medium, ensuring maximum efficiency and effectiveness. If you want to really save on electricity, consider setting other heat sources (such as radiators) to synchronise with your lights via a timer, so that only your heat mats are used during lights off. Again, just be mindful of humidity spikes and ensure that the air always stays above its dew point, otherwise you'll be risking mould.
How can I reduce heat?
If you're situated in a warmer climate and your grow room temperatures are excessively high, there are a few things you can do to combat this too.
Lights on/off Cycle
If you haven't already, try moving your lighting schedule so that your lights are on during the coldest part of the day (night time) and are off during the hottest parts (daytime).
Improving LED efficiency
As was briefly explained earlier, the wattage consumed by your lighting fixture is directly proportional to the heat generated. With this said, the more efficient your LED grow light is at converting energy into photons (micromole/umol per joule), the less wattage you'll need to use. The most efficient grow lights on the market right now (such as the Lumatek Zeus 2.9, PAR+ Ultra and HLG Diablo ranges) sit at around 2.9umol/J; set your sights on them - although be aware of cheap Chinese lights with inflated specifications. In the case of LED technology, the less power you run through a diode, the more efficient it will operate. By oversizing your LED grow light and running it at a reduced power, you'll be able to get the most light vs power consumed possible out of it: upwards of 3umol/J in most cases.
Many of the principles mentioned above in the 'How to make the most from your LED heat' section can be inverted to help reduce heat in your grow area. Things like the strategic placement of LED grow light drivers (this time outside of your area) and intaking air from a colder source (such as a basement or north facing location) will all help.
While many of the suggestions provided in this blog may be unsuitable for your specific situation or grow style, hopefully you'll be able to take a few of these ideas to improve your environment and lower electricity consumption when using LED. The temperature of your grow space should never be taken in isolation; consideration of humidity, air flow and light intensity should always be taken alongside this to ensure that your plants are in the best environment available to thrive. After all, there's no point in trying to save electricity at the cost of quality or quantity!