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How to choose the size of an air conditioner for a grow room

Air conditioners come in different sizes with a wide range of cooling capacity. So, it’s really important to choose the right one that works for you. Many split units where the air blower and condensers are separate provide the most efficiency. Portable air conditioners, however, conveniently do fewer jobs. So, the truth is, though many Growers underestimate the size of an AC unit, they need to consider it more seriously.

If not calculated before buying one, then you’ve got the hassle of uninstalling it, packing it up, driving all the way back to the hydro store in a terrible mood, and trying to convince them that it’s defected. When all along, it’s just the wrong size.

So, let me show you a brain-dead simple way of choosing the right AC unit for your grow room in the first place. So, you don’t end up being that guy that everyone loves to hate.

As I said, let’s keep it simple. I don’t care what kind of grow lights you are using. HP’s metal halides, light-emitting ceramics plasma, T5 fluorescent CFLs, LEDs, they’re all eaters for your air conditioner. LEDs produce heat too and plenty of it despite what you may have read. So, for this exercise, just add up every watt of power in your grow room. So, shoot for the maximum power for your AC unit.

Okay, you know the juice you’re using with all your grow lights switched on but forget to include your pumps, dehumidifiers, oscillating fans, and anything else that consumes power. So, what’s your total? If you have multiple rooms, then calculate the separate totals for each room. If you have ten light grow, that’s 10×1000 watt. HPS lights with a dehumidifier, some oscillating fans, and pumps may be pulling upwards of 11000 Watts. Whatever your number, make a note of it as this is valuable and the key to working out your air conditioning needs, which we’ll do in just a second.

Let’s pause for a moment to have a sober reflection. Think about what we’re actually doing here or planning to do. You’re not just cooling a bedroom in your house on a balmy summer evening for a few hours before turning in. You are air conditioning a grow room, a room full of plants underneath powerful heaters.

Remember that grow lights are heaters switched on for 12 to 18 hours a day. Air-conditioned Grow rooms tend to be sealed just like a padded cell. Sealed grow rooms do not rely on ventilation as the primary means of cooling, dehumidification, and carbon dioxide replenishment. Instead, your indoor garden’s hot air is sucked into a heat exchanger. A lot of heat is removed, and that very same air is blown back into your garden, only cooler. Growers using this sealed room method have to supplement carbon dioxide levels using a tank or generator during the day so that their plans can photosynthesize efficiently. Just before lights out, the extra CO2 Supply is shut off. Then the room’s air is dumb to the outside world, and regular fresh air is pulled in for your plant’s respiration phase during the night cycle.

Now think again about how hard your grow room air conditioner is going to work. Your goal is to keep the daytime temperatures in the range of 60 to 70-degree Fahrenheit. If you live in a hot climate like Arizona, you probably need an air conditioner to reach this target without any grow lights switched on. Include 11 thousand watts of energy into this equation, and hopefully, you’re beginning to see the picture.

Now air conditioners are rated in BTUs or British thermal units. According to the amount of thermal energy they can remove from space, the common sizes are 12,000 BTU, 24,000 BTU, 36,000 BTU, and 48,000 BTU. You may also hear folks refer to their AC units in tons; one ton, two tons, three tons, or four tons corresponds with 12, 24 36 and 48 thousand BTUs.

Now take your Grow rooms total Watts from earlier and multiply this number by 4. This number is the minimum. Let me repeat, the minimum number of BTUs you’re going to need. The actual number is 3.42, but I prefer to multiply by four because the math is a lot easier. Okay, but over specking means far less wear and tear on your AC unit and prolonging its working life. So, if my total is eleven thousand Watts, I’m going to need at least 44,000 BTUs of air conditioning.

Should I choose the 48,000 BTU unit? Maybe, but a smarter choice is to buy two 24,000 BTU units. That way, all my eggs aren’t in one basket. If your AC unit decides to give up three weeks before harvest on Labor Day weekend, at least you have the option of running half of your lights on the other AC power while you track down a friendly AC engineer.

If your target BTU Falls halfway between unit sizes, always go up to the next bracket. As I said, it’s infinitely preferable for your air conditioners not to be running full throttle. Okay, that’s it. I hope I deliberately made this super simple. The outside temperature in your building plays a huge part in determining your cooling needs. So, if you have doubts, consult a qualified air conditioning professional for advice. That’s all for now.

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