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Quantifying outdoor air's heating and cooling load impact
Outdoor air ventilation, while essential for indoor air quality, imposes a direct thermal load on the HVAC system separate from the building envelope load. Every CFM of fresh air brought in must be heated (winter) or cooled and dehumidified (summer) from outdoor conditions to indoor setpoint, and this load is often underestimated relative to its actual energy and capacity impact, especially in tightly sealed modern Canadian buildings where envelope loads have shrunk but ASHRAE 62.1/62.2 ventilation requirements haven't.
Sensible ventilation load follows: Q_sensible = density factor × airflow × (T_indoor − T_outdoor). Latent ventilation load (relevant primarily for summer cooling) follows: Q_latent = density factor × airflow × (humidity ratio difference) × latent heat of vaporization. In winter, latent load is typically negligible to even reversed (since cold outdoor air carries almost no moisture), so heating calculations focus on sensible load alone. Use the humidity ratio calculator if you need to verify the moisture content driving the latent term.
Winter: the sensible-only Canadian challenge
In Canadian winters, the sensible heating load from ventilation air can be substantial. At a Winnipeg design temperature of -29°C, every 100 CFM of outdoor air needing to reach 21°C indoor requires roughly 5,400 BTU/hr of heating capacity, purely for the ventilation air, before accounting for any envelope load. For buildings with significant outdoor air requirements (schools, healthcare, assembly spaces), this ventilation load can rival or exceed the envelope heating load. Cross-check your total outdoor air requirement with the outdoor air calculator before sizing heating equipment.
Summer: sensible plus latent cooling load
In humid Canadian summer climates (southern Ontario, Quebec, the Maritimes), outdoor air carries substantial moisture that must be removed by the cooling coil. This latent load often exceeds the sensible cooling load from the same airstream, particularly when outdoor humidity ratio is much higher than the target supply air humidity ratio. This is precisely the scenario where an ERV with latent recovery provides its biggest advantage over a sensible-only HRV.
The heat recovery impact
This calculator lets you directly compare the ventilation load with no heat recovery, with HRV (sensible-only) recovery, or with ERV (sensible + latent) recovery, at your specific climate design conditions. The percentage reduction shown represents the direct capacity and energy benefit of installing heat or energy recovery ventilation, which is why this technology has become standard or mandatory in most current Canadian building codes for both residential and commercial construction. For the dollar value of that reduction over a full heating season, run the same airflow through the HRV calculator's annual savings estimate.
Frequently Asked Questions
Sensible heating load = 1.08 × CFM × (T_indoor − T_outdoor) in BTU/hr, or 1.23 × L/s × ΔT in watts metric. For 100 CFM heated from -22°F (Calgary winter design, approx -30°C) to 70°F indoor: 1.08×100×92 = 9,936 BTU/hr — substantial for one airstream, on top of the building envelope load. HRVs reduce this proportionally to their rated sensible recovery efficiency. Use this calculator with your specific airflow and design temperatures for an exact value.
A 70% efficient HRV reduces ventilation heating load by 70% at rated conditions. For 5,000 CFM at -25°C Winnipeg design heating to 21°C, unheated load might exceed 250,000 BTU/hr; with 70% recovery, this drops to roughly 75,000 BTU/hr — a 175,000 BTU/hr reduction at design conditions. Over a full heating season this translates into substantial fuel savings, which is why heat recovery ventilation is now standard in most new Canadian construction. Use the HRV calculator for annual dollar savings.