| Parameter | Value | Note |
|---|
Quantifying outdoor air's heating and cooling load impact
Outdoor air ventilation, while essential for indoor air quality, imposes a thermal load on the HVAC system separate from the building envelope load. Every L/s of fresh air must be heated in winter, or cooled and dehumidified in summer, from outdoor conditions to the indoor setpoint. This load is often underestimated in tight modern buildings, where envelope loads have shrunk but ventilation rate requirements have not.
The sensible load is Q = density × specific heat × airflow × temperature difference. The latent load is Q = density × airflow × humidity ratio difference × latent heat of vaporization. The tool counts the sensible and latent load in the direction of the selected season only: heating and humidifying in winter, cooling and dehumidifying in summer. The humidity ratio calculator shows the moisture content behind the latent term.
Winter: mostly a sensible load
Cold outdoor air carries almost no moisture, so the winter load is mainly sensible. At a −25 °C design temperature and 21 °C indoors, every 100 CFM of outdoor air needs about 1.08 × 100 × 83 = 8,960 BTU/hr (2.6 kW) before any envelope load. The latent part, humidification, counts only if you add moisture, so the tool includes it only when you tick the box. For the design temperature, the British Columbia Building Code explains that the January 2.5% design temperature is the value ordinarily used for heating system design. Confirm the airflow itself with the outdoor air calculator before sizing equipment.
Summer: sensible plus latent cooling load
In humid climates, outdoor air carries moisture that the cooling coil must remove, and the latent load can exceed the sensible load. For 45 L/s with outdoor air at 32 °C and 50% humidity and indoor air at 24 °C and 50%, the sensible load is about 0.43 kW and the latent load about 0.76 kW, a total of about 1.2 kW. That is where an ERV with latent recovery helps most.
The heat recovery impact
Set the recovery type to none, HRV or ERV to compare the load at your design conditions. In the summer example above, a 70% HRV cuts the total by about 25%, while an ERV with 55% latent recovery cuts it by about 60%. Real units vary: a Building America laboratory test of one membrane ERV measured latent effectiveness of 0.44 to 0.55 at balanced flow, so enter the latent efficiency from your unit's rating. For yearly dollar savings, use the ventilation energy recovery calculator.
What this load leaves out
The result is the load of the ventilation airstream only. It excludes envelope losses, fan heat and unplanned air leakage, which the infiltration load calculator estimates. It is also a peak design load, so it sizes equipment but does not give annual energy.
Frequently Asked Questions
Sensible load in watts is about 1.2 × airflow in L/s × the indoor minus outdoor temperature in °C, or 1.08 × CFM × the temperature difference in °F for BTU/hr. For 45 L/s (95 CFM) heated from −23 °C to 21 °C, that is about 2.39 kW or 8,160 BTU/hr. This is on top of the envelope load, and any humidification you add is extra. The calculator also adjusts air density for elevation.
A core with 70 percent sensible recovery efficiency removes 70 percent of the sensible load at its rating condition. For 5,000 CFM from −25 °C (−13 °F) to 70 °F, the sensible load is about 1.08 × 5,000 × 83 = 448,000 BTU/hr, and 70 percent recovery leaves about 134,000 BTU/hr, a cut of about 314,000 BTU/hr. Sensible recovery efficiency is rated at 0 °C and −25 °C, so use the colder rating for a cold design day.
Humid outdoor air carries a lot of moisture, and removing it takes energy even when the air is only a few degrees warmer than the room. For 45 L/s with outdoor air at 32 °C and 50 percent humidity and indoor air at 24 °C and 50 percent, the sensible load is about 0.43 kW and the latent load about 0.76 kW. A 70 percent HRV cuts the total by about 25 percent, while an ERV with 55 percent latent recovery cuts it by about 60 percent.
No. This tool gives a peak load at one set of design conditions, which sizes heating and cooling equipment. Annual energy depends on weather over the whole season, so use the ventilation energy recovery calculator for yearly savings. Check the outdoor airflow itself with the outdoor air calculator.