Cannabis and greenhouse growing facility HVAC

Indoor growing facilities, whether licensed cannabis production sites or vegetable and flower greenhouses, push HVAC systems in ways few other buildings do. Plants transpire most of the water they receive into the air, grow lights add intense heat, and growers want tight control of temperature, humidity and CO2 around the clock. Since cannabis was legalized in Canada in 2018, many facilities have learned that undersized or poorly specified HVAC leads directly to crop loss from mould and mildew.

This guide covers the core engineering issues for both sealed indoor grow rooms and greenhouses.

The moisture load

The defining load in a grow room is latent: moisture released by plants. Nearly all of the irrigation water delivered to plants, apart from a small fraction retained in plant tissue and any runoff, is transpired into the room air. In a densely planted flowering room, that can amount to many litres of water per hour.

This moisture must be removed continuously. When lights are on, plants transpire heavily and the room also needs a lot of sensible cooling. When lights turn off, transpiration slows but continues, and sensible load drops sharply. Without dedicated dehumidification, the room temperature falls, relative humidity climbs, and conditions favour mould such as botrytis (bud rot) and powdery mildew.

See sensible and latent heat for background on how these loads are calculated.

Vapour pressure deficit

Growers often specify conditions as vapour pressure deficit (VPD) rather than relative humidity alone. VPD is the difference between the water vapour pressure the air could hold when saturated at leaf temperature and the vapour pressure actually present. It drives transpiration.

These are illustrative ranges from common horticultural practice; each grower sets targets for their cultivars and methods. The HVAC engineer's job is to design a system that can hold the grower's targets through both light and dark periods.

Indoor grow room systems

Sealed indoor rooms use little or no outdoor air, both to retain enriched CO2 and to maintain biosecurity. Common system approaches:

Whatever the system, it must handle two very different conditions:

1. Lights on: high sensible cooling plus high latent removal.
2. Lights off: low or no sensible cooling but continued latent removal, which requires reheat or a dehumidification process that does not overcool the room.

Reheat using heat recovered from the refrigeration cycle, such as hot gas reheat, is far more efficient than electric or boiler reheat. Air distribution must reach all levels of the canopy, especially in multi-tier vertical racks, to prevent stagnant humid pockets. Horizontal airflow fans inside the canopy are common.

Lighting loads

Lighting is the largest sensible load. LED fixtures have largely replaced high-pressure sodium lamps in new facilities. LEDs produce less heat per unit of useful light, which reduces cooling but can also increase the dehumidification and heating demand during lights-on periods. Load calculations should use the actual fixture schedule and photoperiod.

Greenhouses

Greenhouses use sunlight and are far less sealed, so their HVAC is quite different:

• Heating in Canadian winters is a major cost. Hot water heating via pipe rails or under-bench systems, fed from boilers, is common. Some large greenhouses use combined heat and power, capturing flue gas CO2 for enrichment. See combined heat and power.
• Thermal and shade screens retain heat at night and reduce solar gain in summer.
• Ventilation through roof vents and fans controls temperature and humidity much of the year.
• Dehumidification in winter often uses venting combined with heating, which is energy intensive. Semi-closed greenhouses with mechanical dehumidification and heat recovery are increasingly used.
• Supplemental lighting extends growing seasons in Canada's short winter days and adds heat.

CO2 enrichment and safety

Growers commonly enrich rooms to roughly 800 to 1,500 ppm CO2 during lights-on periods to increase growth. CO2 is supplied from bulk liquid tanks, burners or CHP flue gas.

CO2 is an asphyxiant at high concentrations, and burners also produce CO. Facilities need:

• CO2 monitoring with alarms in every enriched room and in storage areas.
• Automatic shutoff of CO2 supply on high concentration or when doors are opened for extended work.
• Purge ventilation capability to clear rooms before workers enter if levels are high.
• CO monitoring where combustion-based CO2 generators are used.

Provincial occupational exposure limits for CO2 apply to workers.

Odour control and Canadian regulation

Health Canada's Cannabis Regulations require licensed production facilities to have an air filtration system that prevents the escape of odours, and municipal bylaws often add odour requirements. In practice, this means:

• Keeping grow and processing areas under negative pressure relative to outdoors or controlled corridors.
• Passing all exhaust through odour control, most commonly activated carbon filters, sometimes supplemented by other technologies.
• Locating exhaust discharges to minimize impact on neighbours. See odour control and exhaust re-entrainment.

Facilities must also meet good production practices for sanitation, which affects filtration choices and how easily HVAC equipment can be cleaned. Higher-efficiency particulate filtration helps reduce mould spore and pest introduction. See filtration, MERV ratings and HEPA.

Energy

Grow facilities are energy intensive per square metre, driven by lighting, cooling and dehumidification. Opportunities include efficient LED lighting, heat recovery from dehumidification, waterside or airside economizers in cold weather (carefully managed to maintain CO2 and humidity), and condensate recovery for irrigation. Energy modeling of hourly lights-on and lights-off loads helps size equipment and compare options.

Key takeaways

• Plant transpiration makes latent load the defining design challenge in grow rooms.
• The system must dehumidify effectively when lights are off and sensible load collapses.
• Growers often specify vapour pressure deficit, and HVAC must hold targets through every stage.
• Greenhouses rely on heating, screens and ventilation, with growing use of closed or semi-closed designs.
• CO2 enrichment requires monitoring and safety interlocks to protect workers.
• Health Canada requires licensed cannabis facilities to filter air to prevent odour escape.