🌡 Heating Infiltration
❄ Cooling Infiltration
| Component | Value | Heating BTU/hr | Cooling BTU/hr | % of Envelope Load* |
|---|
*Estimated % of total envelope heating load. Run the Heat Load Calculator for the full picture.
How to Calculate HVAC Infiltration Loads
Use the ACH Method if you know the construction type or have a natural ACH estimate. Use the Blower Door Test tab if you have an ACH50 result from an airtightness test. Use the Crack Method for a more detailed analysis of specific leakage paths using standard crack-length infiltration data.
Enter the total conditioned floor area and average ceiling height. Volume = Area × Height, calculated automatically. For multi-storey buildings, enter the total floor area of all conditioned floors combined. This volume drives the infiltration flow rate calculation: Q = ACH × Volume ÷ 3600.
Enter indoor setpoints and outdoor design temperatures. Use our design temperature lookup for ASHRAE values for your city. The infiltration sensible load is directly proportional to the temperature difference: Q_sensible = ṁ × Cp × ΔT.
The calculator outputs both sensible (temperature) and latent (moisture) infiltration loads. Latent load is often overlooked but significant in summer — hot humid outdoor air infiltrating must be dehumidified. The tightness rating compares your building to NBC Canada benchmarks. Feed results into the heat load calculator for the complete picture.
Understanding HVAC Infiltration Loads
Air infiltration — the uncontrolled movement of outdoor air into a building through cracks, gaps, and openings — is one of the most significant and often underestimated components of HVAC load. In older Canadian homes, infiltration can represent 25–40% of total heating load. In modern tight construction, it drops to 5–15%. The difference directly impacts energy bills, comfort, and equipment sizing.
The Three Calculation Methods
ACH Method: The simplest approach — express infiltration as air changes per hour (ACH) and multiply by building volume. Natural ACH values range from 0.03 for net-zero homes to 0.85+ for pre-1960 leaky construction. This is the method used in the heat load calculator and load estimator.
Blower Door Test Method: A blower door test pressurizes the building to 50 Pascals and measures air flow. The result (ACH50) is divided by a factor (typically 20 for single-storey, 17 for two-storey) to estimate natural ACH. This is the most accurate method when test data is available. NBC Canada requires new homes to pass a blower door test at ≤2.5 ACH50.
Crack Method: Using standard residential infiltration methodology, infiltration is calculated from the sum of individual crack lengths (windows, doors, wall penetrations) multiplied by crack coefficients that depend on wind exposure and building height. More detailed but requires more input data.
Sensible vs. Latent Infiltration
Infiltration adds both sensible (temperature-driven) and latent (moisture-driven) loads. The sensible heating load is calculated as: Q_s = ρ × Cp × Q_flow × ΔT, where ρ is air density (1.2 kg/m³), Cp is specific heat (1,005 J/kg·K), Q_flow is flow rate in m³/s, and ΔT is the temperature difference. The latent load uses: Q_l = ρ × h_fg × Q_flow × Δω, where h_fg is the latent heat of vaporization (2,430,000 J/kg) and Δω is the humidity ratio difference. Use the psychrometric calculator and humidity ratio calculator for detailed moisture analysis.
How to Reduce Infiltration
Improving air tightness is one of the most cost-effective energy upgrades available. Common improvements include: sealing electrical boxes and pipe penetrations, weatherstripping doors and windows, caulking wall-to-floor and wall-to-ceiling junctions, and adding an air/vapour barrier during renovation. Each of these reduces ACH and the infiltration load. Use the payback period calculator and load comparison tool to quantify the energy savings from air sealing.
Frequently Asked Questions
NBC Canada 2015 requires new homes to achieve 2.5 ACH50 or less (tested at 50 Pa). This converts to approximately 0.12 ACH natural. Energy Star Certified homes target ≤1.5 ACH50 (≈0.075 ACH natural). Net Zero Ready homes must achieve ≤0.6 ACH50 (≈0.03 ACH natural). For reference, older pre-1980 homes often test at 10–20 ACH50 (0.5–1.0 ACH natural) — 10–30 times leakier than modern code. Each province may have additional requirements beyond NBC minimums.
Divide ACH50 by 20 for single-storey homes (LBL method — Lawrence Berkeley National Laboratory). Use 17 for two-storey homes and 15 for three-storey or taller. The divisor accounts for the fact that natural wind and stack pressures are much weaker than the 50 Pa used in the blower door test. For example: ACH50 of 5.0 ÷ 20 = 0.25 ACH natural for a single-storey home. This is the industry standard conversion used in residential load calculations.
In modern tight homes (ACH50 ≤2.5), infiltration typically represents 10–20% of total heating load — less significant than ceiling and wall losses. In older leaky homes (ACH50 of 10+), infiltration can represent 30–50% of total heating load, making it the single largest heat loss component. This is why air sealing is often the most cost-effective energy retrofit in older homes — it can reduce total heating loads by 20–30% at a fraction of the cost of adding insulation. Use the load comparison tool to model before/after scenarios.