8 minute read
Grow Room Ventilation 2.0 How LED Grow Lights Have Moved The Goal Posts
Growers moving from HPS to LED grow lights need to adapt their ventilation strategies to maintain yields and maximise the benefits. But that’s not all that needs to change, writes veteran indoor grower Everest Fernandez.
Growers have historically ventilated their grow rooms for three reasons:
1. To remove hot air from their growing space and replace it with cooler air
2. To reduce humidity levels
3. To replenish carbon dioxide levels
There it is—your humble extraction fans killing three birds with one stone—three problems, one solution.
Ventilation (aka air exchange) remains the most energy-efficient method to maintain a healthy indoor growing environment. Out with the old, hot, moist, carbon dioxide-depleted air, and in with the new, cool, moderately humid, fresh air. Of course, due to geographical location, some growers have never really enjoyed the option of ventilating their grow rooms. Cultivators living in Florida or Phoenix are obliged to go the sealed room route, relying on an energy-hogging mix of air conditioners, dehumidifiers, and supplemental carbon dioxide rather than ventilation simply because the ambient air available is too hot, too dry, or too humid to draw into their grow rooms. However, ventilation offers the most energy-efficient climate control for indoor growers living in cooler climes (i.e. Portland, Seattle, Alaska, British Columbia, Michigan, New England, etc.).
LED Grow Lights Have Changed the “Priority Order” of Grow Room Environment Challenges
Plants have not changed since the advent of LED lighting technology, and neither have the above three reasons for ventilating our gardens. What LED grow lights have changed is the order or relative magnitude of these challenges.
High Air Temperatures Aren’t Such a Big Issue
Back in the High Pressure Sodium (HPS) and Metal Halide (MH) days, the main bogeyman in practically every indoor garden was high daytime air temperatures. Heat production was wrapped up with light production. Not only do HPS and MH lamps generate plenty of convection heat (heating up the air), they produce plenty of infrared radiation too. Now, it’s worth pointing out that infrared is not an intrinsically bad thing. Over half of the incident solar energy reaching our planet from the Sun falls into the infrared band. It’s the main reason we feel the warmth from the Sun. And plants, especially those native to higher altitudes in the sub-tropics, use this infrared radiation to warm up their plant tissue, not unlike a cold-blooded reptile basking in the Sun.
Traditionally, indoor growers would aim to keep air temperatures around 75 – 77 °F (24 – 25 °C) for most warm season herbaceous annual plants to offset the warming effect of the significant infrared generated directly from their HPS and MH lamps. However, modern LED grow light fixtures emit virtually no infrared directly from their diodes. (Although a small amount of infrared output is now being incorporated into some more recent LED fixtures.) To compensate for this comparative lack of infrared radiation, indoor growers using LED grow lights run their rooms warmer; they elevate the daytime air temperature in their grow rooms to the region of 82 – 85 °F (27 – 29 °C). In these markedly warmer conditions, plant tissue doesn’t miss the warming effect of infrared so much. It can reach sufficient warmth via convection heat transfer with the air to drive metabolic processes.
Not only do HPS and MH lamps generate plenty of convection heat (heating up the air), they produce plenty of infrared radiation too.
Massively Elevated EC / TDS with LEDs
It’s also worth noting that LED growers run their nutrient solutions at significantly higher EC than growers running legacy lighting. In fact, commercial growers cultivating in zero-CEC media, such as Rockwool, will start their rooted cuttings at 3.0 mS (instead of the typical 1.2 mS) and go up to 7.0 or 8.0 mS in flower. Yes, you read that right—that’s 3500 or 4000 PPM! That’s three or four times the typical nutrient concentration with legacy lighting. Admittedly, these are seasoned commercial growers who aren’t using your standard GROW A and B and BLOOM A and B formulas. They’re mixing up their primary and secondary nutrients using specific salts, not blended stock solutions, keeping their nitrate levels low, and elevating magnesium levels significantly. (A decent target ratio of calcium to magnesium ions is 2:1.) The higher osmotic pressure created by these strong nutrient solutions helps to offset the lower transpiration rates generated by LEDs.
It’s also worth noting that LED growers run their nutrient solutions at significantly higher EC than growers running legacy lighting.
But wait… even this seemingly small change can have significant knock-on effects for your grow room’s environment and the plants within it.
1. Now, your thermostatically controlled extraction fan isn’t working as hard
To compensate for this comparative lack of infrared radiation, indoor growers using LED grow lights run their rooms warmer; they elevate the daytime air temperature in their grow rooms to the region of 82 – 85 °F (27 – 29 °C)
Okay, so you’ve moved the set-point of your thermostatic fan speed controller up from 75 °F to 79 °F so that your grow room stays warmer. The set-point is the “trigger temperature” where your inline fans will go to max speed to try and stay within your desired temperature range. Now that your set-point is higher, your fans won’t kick into max power as much, meaning the overall amount of air exchange in your grow room could be massively reduced. This inevitably leads to higher humidity levels.
2. Your nutrient solution will probably be warmer
If your nutrient solution reservoir is located within the same room as your grow space, it will be subject to higher air temperatures. Depending on the volume of your reservoir, growing style (recirculating or run-towaste) and rate of usage, it may be necessary to keep a closer eye on nutrient solution temperatures. Warmer nutrient solutions equate to lower dissolved oxygen levels which can lower plant metabolic rates.
3. Wet/dry cycles may be quicker
Warmer nutrient solutions and overall warmer plant tissue will draw more moisture through the root zone as a whole, meaning plants could seem “thirstier”, leading to more wet/dry cycles in the growing media. This can increase the risk of salt build-in up your soilless growing media, especially when running more concentrated nutrient solutions.
So what do growers need to do to get the most from their LED grow lights?
If you can’t decide between two duct sizes, go for the larger option
Don’t under-specify your inline fan and carbon filter
If you can’t decide between two duct sizes, go for the larger option. Not only will it run more quietly for the same amount of air movement, but it’s always a good idea to have some power in reserve. Calculate the cubic feet of your active growing area (length x width x height) and multiply this total by 2.5. This gives you the CFM rating you should be looking for in your fan—taking into account carbon filter pressure drop and a reasonable amount of ducting. Use our fan size calculator to check what size fan and filter you need for your indoor garden.
Use humidity and temperature to guide your fan speed
Consider upgrading your environmental controller so that your extraction and intake fans kick into max speed whenever your relative humidity rises above a set-point, not just temperature. Sure, many LED growers are running more humid grow rooms these days to counter the higher temperatures and achieve a desirable vapour pressure deficit (VPD). Just remember how critical proper air exchange is for your grow tent or grow room. Some growers run dedicated dehumidifiers for their grow rooms—especially during the winter months and during the nighttime period. Consider upgrading your environmental controller so that your extraction and intake fans kick into max speed whenever your relative humidity rises above a set-point, not just temperature
3. Grow more plants, prune harder, flip earlier
LED grow lights are an incredible technological upgrade in some respects, and there is a wide variety of form factors available. Most home growers use multi-array “spider” style LED fixtures, and these certainly seem best suited to illuminate smaller plants rather than the huge, gangly beasts of some old-school HPS rooms. Higher numbers of smaller plants are the way to go. Use topping and pruning techniques to increase the number of flowering sites. Very short vegetative periods of just a handful of days with brutal pruning and removal of lower growth tips are the way to go.
BIO Everest Fernandez is a well-respected industry educator, veteran hydroponic grower, and grow light enthusiast based in France. He works primarily as a marketing and cultivation consultant and was the founding editor of Urban Garden Magazine in the UK, US, and Canada. He also writes and researches for the popular hobby horticulturist YouTube channel, Just4Growers.
