> By: The Cooling Company
Key Takeaways
- Use a room-by-room Manual J load calculation, not a square-foot rule.
- Select equipment using Manual S and verify ducts with Manual D for comfort and efficiency.
- Avoid oversizing: short-cycling wastes fuel and creates uneven temperatures.
Sizing HVAC for a grow room requires separate sensible and latent calculations and careful conversion of electrical loads to HVAC units. A dehumidification strategy sized for peak plant transpiration is essential. Start by listing all sensible heat sources (lights, people, equipment) and latent sources (plants, irrigation, outside air). This guide breaks the work into clear steps. It focuses on practical.
Why does HVAC sizing for grow rooms matter?
Correct HVAC sizing affects crop health, yield, and operating cost. Grow rooms need tight control of both temperature and relative humidity. Temperature control alone is not enough. Plants release large amounts of moisture during lights-on periods. A system sized only for sensible heat will fail when latent load rises.
Failures show as RH spikes, which invite pests and disease and stress plants. Equipment life and energy use also depend on correct sizing. Oversized units cycle frequently and do not remove moisture efficiently. Undersized units run continuously and still cannot meet setpoints. Both extremes raise maintenance and operating costs.
Right-sizing improves energy efficiency, stabilizes setpoints, and lowers utility bills. It also reduces risk. Properly sized systems give longer run times and better moisture removal per run. That improves crop consistency and lowers the chance of a costly failure.
How to size HVAC for a grow room: what loads to calculate
Load calculation is the core of how to size HVAC for a grow room. You must separate sensible and latent loads and sum each independently. Sensible loads determine temperature control. Latent loads determine moisture removal needs. Coils have a sensible heat ratio (SHR). Match coil performance to your room's sensible fraction so the system can meet both temp and RH targets.
Begin with a load sheet. Record canopy area, lighting watts, fixture room-to-fixture (RTF) or room-to-ballast losses, people counts, pump and fan watts, irrigation plan, and outdoor design conditions. Note desired indoor temperature and RH. Account for ventilation and infiltration. Tally sensible BTU/hr and latent lb/hr separately. Use these totals to size cooling tons and dehumidification capacity.
What are sensible and latent loads?
Sensible load is heat that raises air temperature. Common sensible sources are lighting, people, pumps, and hot water. You measure sensible heat in BTU per hour. Latent load is moisture added to the air. Main latent sources in a grow room are plant transpiration and media evaporation. Ventilation and infiltration add both sensible and latent components.
Tally both loads separately. Coils lose latent capacity when entering-air wet bulb rises. A coil that cools well may not remove enough moisture. If latent demand exceeds coil latent capacity at design conditions, you need supplemental dehumidification or a DOAS.
How to calculate lighting sensible load
Lights are usually the largest sensible source. Record total lighting power on the canopy in watts per square foot. Multiply watts by canopy area to get kW, then convert to BTU/hr using 1 kW = 3,412 BTU/hr. Include ballast or driver losses and wiring heat if those losses dump into the room. Also consider fixture RTF (room-to-fixture) numbers from the manufacturer.
HPS and MH transfer more radiant heat to plants and surfaces, while LEDs put more heat into the fixture and then the room via convection. Use manufacturer data and measured fixture temps to refine your sensible load numbers. Small errors per fixture add up rapidly in high-density arrays.
How do plants add latent load?
Plants transpire water through leaves and via media evaporation. Transpiration varies by species, VPD (vapor pressure deficit), light intensity, CO2, and growth stage. For planning, use conservative peak transpiration rates: 2-6 L/m2/day for low-transpiring crops, and up to 8 L/m2/day for high-light, dense canopies.
Convert liters per day to pounds or pints per day for dehumidifier sizing. Factor in irrigation schedule. Peaks often occur during lights-on and after watering. Size for peak moments rather than daily averages to avoid RH spikes that damage crops.
How to account for people and equipment
People add both sensible heat and a small moisture load. Use standard values (about 250 BTU/hr sensible per person for light activity) unless crews are large. Pumps, fans, and controllers add heat equal to their electrical input if that electricity ends up as room heat. List device nameplate watts and convert to BTU/hr.
Also account for on/off cycling and timers. Devices that run intermittently still matter for part-load estimates. Detailed equipment lists give accurate results when you prepare psychrometric checks and part-load performance testing.
How much ventilation is required?
Ventilation must meet code and crop requirements. Use ASHRAE 62.1 or local codes for minimum outdoor air. Ventilation introduces enthalpy. In hot-humid climates, outside air greatly increases dehumidification needs. Account for both dry- and wet-bulb effects of incoming air.
If you use a DOAS, size it to condition outdoor air to supply conditions before it mixes with room air. Active dehumidification on the outdoor air stream is often more efficient than relying on the main cooling coil in humid climates. Poorly sized ventilation systems are a common hidden cause of RH problems.
Which lights produce more heat?
HPS and metal halide lamps produce more radiant heat and often show higher room sensible load per watt. LEDs are more efficient electrically, but most of their input power still becomes heat in the room. The difference is where the heat is released and how it affects canopy microclimate.
Use manufacturer RTF numbers, fixture ambient losses, and measured fixture temps to refine numbers. In high-density arrays, small differences per fixture add up and affect coil selection and airflow strategy.
How do you convert loads to equipment capacity?
Translating sensible and latent totals into equipment capacity requires careful use of manufacturer curves and basic psychrometrics. Total sensible BTU/hr divided by 12,000 gives an initial tonnage estimate. But coils perform differently at various entering-air conditions, so confirm capacity with coil data for your specific wet- and dry-bulb temperatures.
Convert moisture loads into dehumidifier ratings. Dehumidifier output is often given in pints per day or lb/hr. Match peak latent load and add a safety margin for irrigation spikes and infiltration. Consider elevation, supply-air temperature, and part-load behavior when selecting units.
How to translate BTU/hr into tons?
Divide total sensible BTU/hr by 12,000 to get nominal cooling tons. Round up to the next model size and then check part-load behavior. A 12,000 BTU/hr load becomes 1 ton. But real loads fluctuate. Prefer modulation, variable-speed compressors, or staged units rather than fixed oversize.
Also check the indoor unit sensible heat ratio (SHR) at your entering conditions. If the coil SHR is too high relative to the room's sensible fraction, the system will fail to meet RH targets even if it hits temperature setpoints.
What size dehumidifier for latent load?
Convert moisture mass to volume per day. For example, 1 lb water equals about 1.36 pints. Select a dehumidifier rated above peak daily removal and add a safety margin of 10-25% based on infiltration and humidity spikes. Mechanical dehumidifiers lose capacity as supply-air temperature drops.
Consider desiccant dehumidifiers in spaces with low supply-air temps or for climates with high enthalpy. Desiccants maintain latent capacity at lower temperatures but have different energy profiles. Compare lifecycle costs, not just first cost.
When is a dedicated DOAS required?
Use a DOAS when ventilation needs or outdoor enthalpy drive moisture loads the main cooling coil cannot handle. If you must supply significant outdoor air continuously or if outdoor humidity is high during design conditions, a DOAS often reduces overall energy and improves RH control.
A DOAS treats outdoor air separately and offloads moisture control from the main coil. That leads to tighter RH control and often lower operating cost in humid climates. In drier climates, DOAS may still help by controlling intake temperature and dewpoint for better VPD management.
Which system strategies work best?
System choice depends on facility size, climate, and redundancy needs. Small single rooms often use a split system with a dedicated dehumidifier. Larger facilities benefit from a DOAS plus dedicated dehumidifiers and a central chilled-water or packaged rooftop system for redundancy. Prioritize systems that let you size sensible and latent capacity independently.
Plan for staging, modulation, and backup. These features prevent crop stress during equipment failure. They also let you avoid oversizing while meeting peak loads. Check part-load curves and control options before you select any major equipment.
Are packaged rooftop units suitable?
Packaged rooftop units (RTUs) work well for larger single-zone grow rooms. They integrate air handling and make maintenance easier. Confirm that the RTU can handle required latent loads and offers modulating capacity and outside-air control.
RTUs often require supplemental dehumidification or a DOAS in very humid climates. Specify coils and controls that permit good dehumidification at part load and confirm available coil selections with the manufacturer.
Do separate dehumidifiers improve control?
Dedicated dehumidifiers let you size latent capacity independently of cooling capacity. They remove moisture without overcooling the room. This is useful during lights-on when transpiration is high. Separate units simplify control strategies and reduce uncertain interactions between temperature and humidity control.
Running dehumidifiers during peak transpiration while keeping main cooling modulation for temperature improves both energy use and crop stability. This strategy is common in commercial controlled environment agriculture.
When to choose DOAS plus dedicated dehumidifier?
Choose DOAS + dehumidifier when ventilation rates are high or outdoor enthalpy is significant. The DOAS conditions outdoor air while the dehumidifier handles peak internal latent events. This pairing reduces strain on the main cooling coil and stabilizes supply conditions.
For greenhouses or retrofit projects, this combo often yields the best balance of control and energy efficiency. It also simplifies maintenance and failure isolation.
How to troubleshoot humidity and cycling
Troubleshooting uses methodical checks and trend data. Start with supply-air temperatures, coil entering and leaving conditions, and measured airflow. Inspect coils and filters for fouling and check dehumidifier operation and drain function. Verify controls and sensor calibration.
Log temperature, RH, and equipment runtimes. Trend data helps you see short cycles, spike events, and gradual capacity loss. Fixing control logic often gives big improvements without replacing equipment. When in doubt, follow the load sheet and test under peak conditions.
Why is RH rising during lights-on?
RH rises during lights-on because plants transpire more under high light. Lights also raise canopy and air temperature. If latent capacity is insufficient or airflow is low, moisture accumulates and RH spikes. Irrigation events timed during lights-on also cause spikes.
Address this by increasing latent capacity, shifting irrigation to off-peak periods, improving airflow over the canopy, and staging dehumidification to operate during peak transpiration.
What causes short-cycling in compressors?
Short-cycling comes from oversizing, incorrect controls, low refrigerant charge, or poor thermostat placement. It reduces run time per cycle and lowers moisture removal per run. Short cycles also increase wear and raise failure risk.
Add minimum run timers, choose variable-speed compressors, check refrigerant charge, and ensure thermostat sampling points measure representative air, not cold discharge locations. These fixes improve moisture removal and prolong equipment life.
How to diagnose air distribution problems?
Measure supply and return temperatures and CFM at several locations. Look for dead zones where air is stagnant or stratified. Poor mixing yields hot and humid pockets near canopy that stress plants. Adjust diffusers, add circulation fans, and rebalance ducts to improve mixing.
Ensure supply air reaches canopy level without blowing directly on plants and causing extreme VPD swings. Good mixing improves coil performance and reduces local humidity problems.
When to rebalance supply and return?
Rebalance when measured CFM differs from design or after any remodel that alters openings. Also rebalance after adding equipment or changing lighting density. Balanced airflow improves coil performance, reduces stratification, and stabilizes temperature and humidity across the room.
Regular rebalancing is a low-cost step that often resolves persistent hot or humid pockets and reduces complaints from growers.
Call to action
If you need an on-site load survey, design help, or equipment quotation, contact NATE-certified technicians who know controlled environment agriculture. On-site surveys verify breaker and service limits, confirm canopy maps, and let technicians measure actual lighting and equipment loads. This reduces surprises during installation and speeds commissioning.
If you are local to Las Vegas, Henderson, or North Las Vegas, call The Cooling Company at 17029308411 for an on-site load survey and equipment plan. Our techs size systems for grow rooms and install DOAS and dehumidifiers. Outside our area? Ask for NATE-certified technicians and a CEA-savvy HVAC engineer. Bring canopy maps, lighting schedules, and utility rates to your consultation.
When to consult a pro?
Call a professional before you order major equipment or upgrade electrical service. A pro verifies load sheets, checks breaker and service limits, and runs psychrometric checks. Engineers can provide redundancy plans and energy modeling to support payback analysis and ensure your design meets code.
If you are local, call The Cooling Company at 17029308411 to schedule a consultation and get a tailored HVAC and dehumidification quote. Proper documentation and a verified load sheet prevent costly mistakes and help your installer deliver predictable performance.
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This aspect deserves careful consideration as you evaluate your options. Understanding the details helps you make more informed decisions. Consulting with qualified professionals provides insights specific to your situation. Well-documented designs and accurate load data reduce long-term costs and equipment churn.
Take time to assemble accurate load data before buying equipment. Bring canopy maps, lighting schedules, irrigation timing, and utility rates. These documents speed design and reduce surprises during installation.
Sources
Understanding sources is essential for homeowners and facility managers looking to make informed HVAC decisions. This section lists key references and industry resources you can consult for standards and deeper technical guidance. Use these sources for design standards and best practices.
- HVAC resources from the Better Buildings Solution Center
- "hvac technician" resource from The Cooling Company
- ASHRAE Technical Resources
About The Cooling Company
The Cooling Company has been serving the Las Vegas valley with professional HVAC services for over a decade. Our team of licensed, NATE-certified technicians specializes in air conditioning repair, heating system maintenance, and complete HVAC installations. We're committed to providing honest, reliable service with upfront pricing and a 100% satisfaction guarantee on all work performed.
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