In Control: A Guide to Achieving the Perfect Indoor Climate

By Stephen Keen
Published: December 1, 2014 | Last updated: April 21, 2021 05:56:51
Key Takeaways

Does it seem like you’ll never achieve your ideal harvest? If you consider all the climatic factors, it might finally be within your reach.

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As every gardener knows, hydroponics and indoor farming can present a number of challenges. Cycle after cycle, it seems that there are always obstacles that have to be overcome. After each harvest, though, we challenge ourselves to do better next time—regardless of the hurdles, we keep at it and eventually we usually end up getting it right.


One of the common early mistakes that gardeners make is failing to consider how absolutely vital climate is to the health and wellbeing of their plants. Temperature, humidity and CO2 levels in the garden have every bit as big an effect on the success of a plant as nutrients or lighting.

And it’s not just the direct effect that climate variables have on your plants that must be considered, but also the indirect results—the more controlled the climate, the less inviting your garden will be to pests, pathogens, fungi and other harvest killers.


Deciding exactly how to control the climate in your garden can be intimidating and we often underestimate our needs; the goal of this article is to help you consider all of the factors that will affect the climate in your own garden.

Heating the Growroom

Rarely is the novice gardener prepared for the sauna that their first garden can become! This is usually an early hurdle for the small gardener and an enormous consideration for commercial farmers.

When you consider the size of your cooling system, remember that cooling anything is simply the removal of heat, which is measured in BTUs. You don’t addcoolnessto something—you take heat away from it.


So if you’re going to cool your garden, you have to consider how much heat is being produced in the garden and how many BTUs you will have to remove. Most cooling systems are measured in tons and one ton of heat is the equivalent of 12,000 BTUs.

Lighting is the biggest contributor to your garden’s heat load. A standard 1,000 watt HID bulb produces approximately 4,000 BTUs of heat. (Watt for watt, the heat production of an LED is similar to that of an HID lamp).


Air cooling the reflector is commonly the first measure for removing this heat, but if the ambient temperature outside is hotter than you need your garden to be you aren’t really addressing the problem. Under most circumstances, air cooling can remove between 10 and 40 per cent of the heat your lights produce.

There are custom-fitted heat shields available through major reflector manufacturers that can be utilized to boost the efficiency of your air cooling—these reduce the amount of heat that can escape from the reflector, which increases the amount of heat being removed by your fans.

There are some drawbacks to exhausting air outside of your garden as well—some of the circulated air will escape into the garden and it can carry pests and fungi with it. Also, if you’re using CO2, some of it will be drawn into your reflector airstream and out of the garden, increasing your need for CO2 production.

If you’re using a CO2 burner, this actually adds to your heat load. Venting your room or exhausting air from the garden out and bringing fresh outside air in will present similar obstacles on a much larger scale. In this circumstance, the outdoor temperature is even more important, as venting won’t help you when it’s 90°F outside.

We also often fail to consider the heat load created by the other equipment in our garden. Ballasts, for instance, are big contributors to overheating. Magnetic ballasts create approximately 3,500 BTU of heat and digital ballasts create about 2,500 BTU per 1,000 watts.

The most energy-efficient solution for ballasts is to keep them out of the garden entirely—while you don’t want them baking, it’s just not necessary to keep them at the comfortable temperature you’re probably shooting for in the garden, so there’s no need to expend the same amount of cooling energy as you do in the garden to offset their heat load.

If it’s not possible to get them out of the garden, you must account for their heat production when you size your cooling system. CO2 generators are also a huge contributor to the garden’s heat load. The BTU rating on your generator is a ‘per hour’ rating, so if it’s capable of 12,000 BTU per hour but is only running for 15 minutes an hour, it will produce 3,000 BTU of heat per hour.

Most manufacturers of CO2 generators will have a chart available that will help you determine how often your generator will run and therefore how much heat it will produce. Water cooled CO2 generators are also available; these can be used in a drain-to-waste capacity or on a chiller and recirculating pump.

In these units, most of the heat produced by the flame is removed by the water, which reduces the heat load on the climate control system dramatically. As mentioned, air cooling your reflectors or venting your garden will affect the run time and heat production of any CO2 generator.

Outdoor climate will be a consideration for you no matter what your circumstances. When it’s really hot outside, your garden will need a certain amount of air conditioning regardless of whether you have equipment running or not.

Most manufacturers of cooling equipment will provide reference materials to assist you with determining the amount of heat that must be removed from the garden even when no equipment is running (the ambient heat load).

In cooler climates where the temperature stays moderate all year this is often not an issue, but in warmer climates seasonal outdoor temperatures are a big consideration. Your garden’s ambient heat load number is going to be arrived at by calculating a combination of square footage, insulation quality and the average high outdoor temperature.

Consider also that the hotter it is outside the less efficiently most air conditioners will run—the same is true of chillers in water cooled applications, but to a much lesser extent.

Humidity in the Growroom

Humidity is also a big factor in the health and success of your crop. Humidity levels that are too high will give certain fungi a perfect breeding environment, which will in turn result in a less-healthy plant and a smaller—and usually lower quality—harvest. Humidity levels that are too low will often result in rapid transpiration of moisture by your plants, which will usually result in reduced nutrient uptake.

Gardens by their nature are generally humid environments, so the addition of humidity is usually only necessary in exceptionally dry environments—high humidity is a much more common problem.

When venting or air cooling, the humidity outside is going to have a direct effect on the humidity inside the garden. If you’re striving to maintain a humidity level of 50 per cent and you introduce air that’s 90 per cent humidity, you have to offset that somehow. Gardeners in moderate or humid environments nearly always have to use a dehumidifier in the garden.

Dehumidifiers are also a source of heat (usually anywhere from 1,000 BTU to 5,000 BTU per hour, depending on the size of the dehumidifier and the amount of time it’s running), so this will need to be considered when the cooling system is being sized. Liquid cooled dehumidifiers are available as well.

All air conditioning and chiller systems will dehumidify to some extent, but if you’re able to maintain the humidity where you want it with lights on using only the a/c or chiller, during the lights off cycle the humidity will build quickly, because the cooling system doesn’t need to operate as frequently and less dehumidification will occur as a result.

For gardeners who prefer not to use a separate dehumidifier, some air conditioning systems are available with a 24 hour dehumidification option, which utilizes a heater in conjunction with the a/c system to achieve dehumidification without adding cooling. These same options are also usually available in chiller systems, as well as an ‘extended’ dehumidification option which utilizes a thermostat in conjunction with a humidistat for tighter control.

Nutrients in the Growroom

In hydroponic applications maintaining the correct nutrient temperature is absolutely vital. A certain amount of dissolved oxygen is necessary to maintain the health of your plant’s roots and its ability to uptake nutrients—if the nutrient temperatures get too warm, the level of dissolved oxygen in the nutrient solution goes down, which results in slower nutrient uptake by the plant.

Compounding this problem is the fact that the warmer the nutrient water temperature, the more dissolved oxygen the plants need for proper nutrient uptake—so they require more dissolved oxygen in this circumstance, but they’re actually getting less. If the problem continues, the dreaded pathogen pythium is the usual result.

If the garden is warmer than the recommended nutrient solution temperatures for your application, a nutrient chiller will need to be employed. If you’re using a standard a/c, the chiller is a separate piece of equipment that must be accounted for as part of the heat load when sizing the a/c system—unless you’re able to keep it outside the garden—because all of the heat being removed from your nutrient reservoir is eventually exhausting out the back of your chiller.

Complete water cooling systems are available that will allow you to utilize a single chiller to control both the climate in the garden (or gardens) and maintain the proper nutrient temperature as well. If you are utilizing a water cooled CO2 generator, this can be incorporated into the single chiller system too. Chiller systems are extremely energy-efficient alternatives for climate control.

What you’re doing when you cultivate a garden indoors is trying to create the perfect environment for your plants to thrive. In order to be successful—regardless of what you grow—each gardener’s specific environmental challenges must be considered and overcome. As long as you take into account all the relevant factors, you can eventually achieve that perfect climate. And if the climate is perfect for your plants, that perfect harvest will finally be within your reach.


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Written by Stephen Keen

Profile Picture of Stephen Keen

Stephen Keen has been an indoor gardening hobbyist for more than 10 years. His personal successes with his garden led him to want to bring new ideas, mainly water-cooling, to the mainstream.

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