HVAC Sizing for Cannabis Grow Rooms

Where the heat in a grow room comes from

Almost none of the electricity you feed a sealed grow room leaves the building as anything but heat. Fixtures convert watts into light, the light lands on leaves, benches, walls, and floor, and it turns back into heat. Fans, pumps, ballasts, and drivers add theirs directly. The only meaningful exception is the small share of light energy a plant stores in sugars, and for HVAC purposes engineers ignore it. The conversion is fixed: 1 kilowatt-hour equals 3,412 Btu [eia-energy-conversion], so 1 kW of connected load running continuously is 3,412 Btu per hour that the mechanical system has to take back out.

Lighting is the biggest line. In the Northwest Power and Conservation Council's survey of licensed producers, lighting was 66 percent of total facility electricity, with cooling at 15 percent, ventilation 12 percent, dehumidification 4 percent, and heating 3 percent [nwpcc-cannabis]. An IMEG mechanical engineer writing for the trade puts grow room design density around 80 watts per square foot, with about half of that attributable to the HVACD equipment itself, and notes that horticultural fixtures account for the bulk of the sensible cooling load [phcppros-streit-hvacd]. So the lights set the load, and the equipment that handles the load is the second largest load. That circularity is why HVAC sizing decisions show up so plainly on the bill.

For the lighting side of the calculation, see LED vs. HPS grow lighting energy cost; this page picks up once you know your connected kW.

Converting lighting watts to cooling tons

A ton of refrigeration is 12,000 Btu per hour. Divide 3,412 by 12,000 and you get 0.284 tons per kW. That is the whole rule of thumb: roughly 0.28 tons of total heat removal for every kilowatt of connected load in the room, before margin. Some designers quote it the other way around, as one ton per 3.5 kW, which is the same number.

Two things about that figure trip people up.

First, it is total heat, sensible plus latent. The plants do not add heat; they move it. As plants evaporate water they absorb sensible heat from the room air and convert it into latent heat, which is why a heavily transpiring room runs cooler than the lighting math predicts and wetter than anyone expects [hpac-latent-2023]. The equipment still has to remove all 3,412 Btu per kWh; it just has to remove some of it as condensed water rather than as cooled air.

Second, it is a sizing floor, not a design. Engineers add margin for hot-weather derating of condensers, envelope gains through a roof in July, people, CO2 burners, and the fact that dehumidifiers themselves put heat back into the room. The margin is a judgment call for the engineer of record. A round 10 to 20 percent is common for planning; we have not seen a published cannabis-specific standard, so treat any single number as an input, not a fact.

Sensible load versus latent load

Sensible load is temperature. Latent load is moisture. Conventional direct-expansion or chilled-water equipment is built for buildings where the load is mostly sensible; it performs best at a sensible heat ratio of 80 percent or higher, meaning at least 80 percent of its capacity goes to cooling air rather than condensing water [hpac-latent-2023]. Grow rooms often run well below that, and during lights-off the ratio falls further because the sensible load disappears while the plants keep transpiring [hpac-latent-2023].

The practical failure is run time. If the sensible load is satisfied quickly, the run time required to properly dehumidify the air may not be met [hpac-latent-2023]. The thermostat is happy, the compressor stops, and the humidity climbs. Manufacturers and engineers answer this with dedicated dehumidification, reheat, or desiccant systems that dry independently of cooling; the tradeoffs are covered on dehumidification load for indoor cannabis. Denver's cannabis energy guide warns that variable refrigerant flow systems, efficient as they are, lack the latent capacities required for the amount of dehumidification a cultivation facility needs and do not allow direct humidity control [denver-bmp-energy-2019].

How much latent load is there? It comes from irrigation: water in, water out. Southern California Edison's market study reports indoor flowering irrigation of 0.096 to 0.16 gallons per square foot per day [sce-etp-2021], and an HPAC Engineering analysis estimates plants return about 70 percent of irrigation water to the air [hpac-latent-2023]. The worked example below turns that into tons.

A worked sizing example

Every number here is an input we chose so the arithmetic is easy to follow. Your engineer's numbers will differ, and they should.

Inputs (assumed):

Step 1: connected load. Lights: 5,000 × 35 = 175 kW. Plus 12 kW of other equipment. Total 187 kW.

Step 2: total heat. 187 kW × 3,412 = 638,000 Btu per hour ÷ 12,000 = 53 tons of total heat removal at lights-on, before margin.

Step 3: latent share. Irrigation: 5,000 × 0.13 = 650 gallons per day. Transpired: 650 × 0.75 = 488 gallons, about 4,060 pounds of water. At 1,000 Btu per pound that is 4.06 million Btu per day, or 169,000 Btu per hour averaged over 24 hours, about 14 tons of latent load. Transpiration is heavier under lights, so the lights-on latent load is higher than the average and the lights-off latent load is lower, but the water still has to come out every day.

Step 4: split. Of the 53 tons at lights-on, roughly 14 is latent and 39 is sensible. That is a sensible heat ratio around 74 percent, below the 80 percent where packaged equipment is comfortable [hpac-latent-2023]. During lights-off the sensible load falls to whatever the fans, pumps, and dehumidifiers themselves produce, while several tons of latent load continue. Equipment selected only for the lights-on number will hold temperature and lose humidity at night.

Step 5: margin. 53 × 1.15 = 61 tons of installed capacity, split between air conditioning and dehumidification in a way the engineer chooses. The answer is not "a 61-ton rooftop unit."

Step 6: what it does to the bill. Assume the installed equipment draws about 1 kW per ton at full load, a planning figure for mid-efficiency commercial equipment, not a measurement. At lights-on the room's HVAC could add up to 61 kW of demand on top of 187 kW of lights and equipment, for a coincident peak near 250 kW. At an assumed delivery demand charge of 12 dollars per kW-month, the HVAC share alone is about 730 dollars a month, before energy. At Illinois' June 2026 average commercial price of 14.53 cents per kWh [eia-epm-5-6-a], HVAC running an average of 40 kW around the clock uses 28,800 kWh a month, about 4,200 dollars. Multiply by the number of flower rooms.

Oversizing versus undersizing

Growers who have lost a crop to an August heat wave oversize on purpose, and it is easy to see why. But oversizing has a bill of its own. ENERGY STAR's sizing guidance is blunt: oversized cooling equipment costs more to buy, cycles on and off frequently once installed, shortens equipment life, lowers efficiency, increases power bills, and can lead to moisture problems [energystar-sizing-guidelines]. That guidance was written for homes, but the mechanism is the same in a grow room and the moisture problem is worse, because a flower room makes its own humidity.

Oversizing also has a utility-side cost that the mechanical contractor never sees. Installed HVAC capacity is connected load, and connected load drives the service size, the transformer, and sometimes the rate class the utility assigns. A facility with 300 tons on the roof and 150 tons of real load can end up in a demand-metered class with a higher per-kW rate than its actual peak justifies. See utility rate classes explained.

Undersizing fails the other way. Equipment that struggles to manage humidity and maintain control through changing conditions is the failure mode the IMEG engineer flags [phcppros-streit-hvacd]: compressors run flat out, room temperature drifts up through the afternoon, and the crop stresses. Undersized systems also run at 100 percent for hours, which is expensive in kWh even if the demand peak is lower.

The right answer is the engineer's load calculation with the latent side done honestly, then staged equipment: several smaller units or a modulating system rather than one big one, so capacity tracks the load through the day and the night. Streit's design practice of 20 to 40 air turns per hour [phcppros-streit-hvacd] tells you the air handling side is also substantial and needs its own fan energy in the calculation.

HVAC and demand charges

The HVAC decision that matters most to the bill is not tons, it is timing. Utilities bill demand on the highest 15- or 30-minute interval of the month, and a grow room has a predictable worst interval: the one right after lights-on, when every compressor starts because the room just gained 600,000 Btu per hour of load. Demand charges explained covers the mechanics; the HVAC-specific points are these.

• Stage the start. A controls sequence that brings cooling on in steps over the first 20 to 30 minutes after lights-on keeps the compressors' inrush and full-load draw out of the same interval as the lighting step. Energy use is unchanged; the billed peak falls.
• Stagger rooms. Denver's guide notes that Xcel bills Denver cultivators on total kWh and peak kW, and that staggered room schedules can significantly reduce energy costs and impacts on the grid [denver-bmp-energy-2019]. Two flower rooms on opposite photoperiods halve the lighting peak and halve the HVAC peak that follows it.
• Run rooms warmer at night. Denver's guide recommends keeping rooms warmer at night to manage latent load [denver-bmp-energy-2019]. Warmer air holds more water, so the same transpiration produces a lower relative humidity and the dehumidifiers work less during the hours when nothing else is running.
• Watch your interval data. The peak is not always lights-on. A condenser fan bank, a chiller restart after a power blip, or all the dehumidifiers cycling together after a heavy irrigation can set the month's peak at 3 a.m. Interval data from the utility's AMI portal shows you which; see interval data and AMI meters.
• Envelope first. Every Btu that comes through the roof in July is cooling capacity you bought and demand you pay for. Weatherization and building envelope is cheaper than tons.

What to do before you commit to equipment

1. Fix the lighting spec first, in connected kW, including drivers or ballasts. Everything downstream scales from it.
2. Hand the engineer the irrigation plan in gallons per day per room. Latent load starts there, not at the thermostat.
3. Ask for staged or modulating capacity and a written lights-on sequence of operations.
4. Check the utility's rate classes against the resulting connected load before the service is sized; a build-out phased over two years should not pay the year-two demand class in year one.
5. Model the demand line. The demand charge estimator accepts lighting kW and an HVAC ratio and shows the monthly number.

The physics of 3,412 Btu per kWh will not change. What you control is how much of that heat you create at the same minute, and how much equipment sits idle the rest of the month waiting for it.