Latent Load by Source
| Source | Latent BTU/hr | Latent W | % of Total |
|---|
Understanding Latent Heat Loads in HVAC
Latent heat is the energy required to change the phase of water — specifically, to evaporate liquid water into vapor or condense vapor back to liquid. In HVAC cooling, the latent load represents all the moisture that must be removed from the air to maintain the target indoor relative humidity. This is distinct from the sensible load (removing heat to lower temperature).
Major Sources of Latent Load
Occupants are the most significant internal latent source. A sedentary person generates approximately 200 BTU/hr of latent heat through respiration and perspiration. An active gym user generates over 580 BTU/hr. High-occupancy spaces like restaurants and gyms have extremely high latent loads that require careful equipment selection.
Ventilation and infiltration bring in humid outdoor air that must be dehumidified. In humid summer climates, ventilation latent load can exceed the sensible ventilation load. An ERV (Energy Recovery Ventilator) pre-conditions incoming air and can reduce the ventilation latent load by 55–75%.
Internal sources — cooking, showers, and plants — add significant moisture, particularly in residential kitchens and bathrooms. Proper exhaust ventilation at these sources is more efficient than trying to dehumidify the whole space afterward.
SHR and Equipment Selection
The Sensible Heat Ratio (SHR) of your space determines what equipment SHR is needed. Standard AC equipment has SHR of 0.75–0.80. If your space SHR is below 0.70, you need either equipment with a lower rated SHR or a dedicated dehumidifier. See the cooling load calculator for full analysis. The psychrometric calculator provides detailed moisture analysis.
Frequently Asked Questions
The latent heat of vaporization of water is approximately 1,060 BTU/lb (2,430 kJ/kg) at typical HVAC conditions. This means removing one pound of water vapor from the air requires 1,060 BTU of cooling energy. In a space with 4 sedentary occupants generating 200 BTU/hr latent each, that's 800 BTU/hr — approximately removing 0.75 lb/hr of moisture from the air.
Compare your space SHR to the equipment's rated SHR (from the manufacturer's data at ARI test conditions). If the space SHR is lower than the equipment SHR, the AC will satisfy the temperature setpoint before removing enough moisture, leaving the space too humid. The solution is: lower SHR equipment, oversized equipment (runs longer removing more moisture), or a dedicated dehumidifier. Use this calculator to determine your space SHR, then compare to equipment specs. The SHR calculator covers this in detail.
In most Canadian climates, an HRV (Heat Recovery Ventilator) is appropriate. HRVs recover sensible heat only and do not transfer moisture — this is actually desirable in winter to keep indoor humidity from becoming excessive. ERVs (Energy Recovery Ventilators) transfer both heat and moisture, which reduces summer latent ventilation loads significantly but can add humidity in winter. ERVs are best suited for hot, humid climates where summer dehumidification is the dominant concern. In Canada, HRVs are typically preferred. See the ventilation rate calculator and payback calculator.