I’m researching hvac with mass dehumidification and I’ve come across a lot of different info regarding tonnage for systems. The latest bid I’ve received quoted me 16 tons of AC for a 400sqft grow room with 20 1000w de hps lights. Wanting to find out what others are working with? The bids for
These pieces of equipment are outrageous especially when dealing with so much tonnage. Any info or advice or expertise or experience is welcome. Thanks
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Hey @vinny, how hot does it get where you are during the hottest months of the summer? Also 20 lights seems a bit excessive for 400 square feet. I may have some solutions in mind. We are all here to help you out!
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1,000 w light + ballast = ~5,000-6,500 BTU’s
20 x 5,000 = 100,000 BTU
1 ton A/C ~ 12,000 BTU’s
100,000 / 12,000 = 8.3 tons
Add this to the heat load of the building/climate without the lights and other heat-generating equipment plus a mild over-spec is usually a good idea.
Here’s a helpful calculator:
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Per the table layout I’m most familiar with (1 light per 5’ x 5’ with 16-20 sites/light), a 20’ x 20’ space would yield a maximum of 12 lights and 210 plants, leaving a walkway between and around the end of tables.
On that same plan, 16 lights should work for a bed the full size of the room.
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Vinny, the 16 ton estimate on 20 kW of lights feels a little high but is probably closer than you’d think at first. Joe Grow is absolutely right about how to calculate for sensible heat loads (the amount of heat produced by bulbs in the space)—essentially you convert kW of power to 3.413 BTU per watt and you have a sensible load. That doesn’t account for envelope loads though, which are usually 20-30 BTU per SF depending on where you’re located and how well your facility is insulated. The other complication is derating of the equipment for the environment—meaning, the hotter it is outside, the less efficiently the equipment outside will run. Most output of cooling equipment is based on nominal performance, then there’s a layer of derating based on a typical 95 degree ambient temperature. If you’re in the desert or somewhere that it routinely gets above 95 degrees, you’ll need to oversize the equipment further to accommodate the loss of output in the hot months.
Where it gets really complicated is on the dehumidification load (latent heat). All mechanical cooling equipment dehumidifies as a byproduct of cooling (and vice versa). When water condenses from gas to liquid, it releases BTU’s. Those BTU’s are absorbed into the cooling system, meaning not all of the capacity of the cooling system translates to cooling of the space; rather, it translates to dehumidification of the space. Most manufacturers of cooling equipment publish their total output, but somewhere in the manufacturer’s literature you’ll find sensible and latent broken out. Typically, a 5 ton air conditioner is only giving you 3.5-4 tons of sensible cooling, and that’s before derating for conditions. That latent load is there even if you have stand alone dehumidifiers in place, and even if you don’t have a specific dehumidification function on the air conditioning system. The amount of dehumidification you get out of that latent capacity is a whole new can of worms. Generally speaking, in a 20 kW room, you’ll need something like 10-15 tons of total air conditioning and at least 300 pints of stand alone dehumidification if you don’t have an integrated dehumidification function in the air conditioning system. That will vary depending on region, target parameters, and watering rates. Feel free to give us a call at 303-993-5271 if you’d like some help with it.
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90s for the most part. Central ok. More worried about humidity. I do have a solid building made of cinder block that’s going to be fully insulated as well
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I’ve been thinking the same. Thanks for your input
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Really good info thanks a lot
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How much would anticipate watering each plant in that scenario? I’d like to see what type of dehumidification load
That would require
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16-20 sites per light (25 sq ft) will accommodate 6" x 6" rock wool cubes or 1 gallon pots. Most hydro growers like to have a 30% runoff in order to avoid salt build-up and the resultant nutrient lockout as well as minimizing flushing (no feeding/reduced growth when you’re flushing).
That said, with the expense of waste and disposal under the new legal requirements I see more growers tending toward minimizing the nutrient dosage. Sounds like yields may be following suit.
The systems I work with are recirculating, which means that you feed the plants until the runoff tells you they are done, about 10 minutes. Perfect dosage every time and waste is reduced by 80% compared to a 30% runoff.
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Hi Vinny,
It really depends on whether the HVAC is meant to reduce humidity as well or if there is a different solution. Though the main purpose of an HVAC system is to control the temperature, A by-product of the cooling is dehumidification, but not very efficiently, meaning designing an HVAC to treat humidity requires a significantly higher capacity.
We recommend separating the two functions. It should let you to maximize the efficiency of both.
Using DryGair for dehumidification and HVAC for temperature control lets you to achieve the same conditions with a smaller capacity HVAC and lower operational costs.
Please feel free to contact us: at +1-604-385-4411, or Hadar- hadar@drygair.com
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Vinny.
The space being only 400 sq. ft. leads me to believe an Environmental Chamber may be less expensive and easier to control. I have installed many of these for plant research and they really work good.
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There is another option. A single piece of equipment that does the dehumidification and temperature control. We refer to this as a Dedicated Indoor Ag System (DIAS). The DIAS unit handles the complete dehumidification load and then uses the hot gas as recovered energy to temper the air back to the condition needed to maintain room temperature. In most applications, no additional supplemental heat is required. Feel free to call me at 210-867-3500. If I do not answer, it is probably because your call will look like a robo call. Leave a message and I will call you back.
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Thank you Dick. ill give you a call this afternoon.
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