✦ Convective Fluid Dynamics

Air Changes per Hour (ACH) & Infiltration Calculator

Calculate the thermal heating load caused by air leakage and ventilation drafts (Q = 0.33 × n × V × ΔT). Convert 50 Pa Blower Door test results (n50) into natural infiltration (ACHnat) and simulate MVHR energy savings.

Ventilation Strategy:
Units:

📐 1. Building / Room Air Volume

💨 2. Airtightness & Air Changes per Hour (ACH)

🌡️ 3. Design Temperatures

Ventilation Thermal Load
1,650 W
1.65 kW · 5,630 BTU/hr
Air Flow Rate: 200 m³/h (118 CFM)
Effective Air Changes: 0.80 ACH
Recovered by MVHR: 0 W (0%)
Formula: Q = 0.33 × n × V × ΔT = 0.33 × 0.8 × 250 × 25 = 1,650 W
💨 Blower Door Infiltration Modeling

Interactive Blower Door (n50) to Natural Infiltration (ACHnat) Converter

Apply the Sherman-Grimsrud Lawrence Berkeley Laboratory (LBL) 'Rule of 20' model with terrain and height adjustments.

Natural Background Infiltration
0.25 ACH_nat
Applied LBL Divisor: N = 20.0
Thermal Impact: For a 300 m³ house at ΔT 25K, natural air infiltration represents 618 Watts of continuous heat loss.
♻️ Heat Recovery Technology

MVHR Heat Recovery Energy & Cost Savings Simulator

Calculate how much thermal energy and fuel cost a heat recovery ventilation unit salvages from exhaust air.

Raw Ventilation Heat Loss 1,238 W
Recovered Heat Energy 1,089 W
Net Residual Heat Loss 149 W
Annual Gas / Elec Saved £320 / yr
📊 Compliance Benchmarks

Airtightness Standards & Ventilation Requirements

National building regulations and international performance benchmarks for air permeability.

Standard / Era Blower Door Metric (n50 / q50) Estimated Natural ACH Required Ventilation Strategy
Pre-1980s Traditional Home n50 = 10.0 – 20.0 ACH 0.8 – 1.5 ACH Natural uncontrolled drafts + chimney flues.
Part L 2021 Limiting Threshold q50 ≤ 8.0 m³/(h·m²) 0.4 – 0.6 ACH Intermittent extract fans + window trickle vents.
Future Homes Standard (2025) q50 ≤ 4.0 m³/(h·m²) 0.2 – 0.3 ACH Continuous Mechanical Extract (dMEV) or MVHR.
Passivhaus Classic n50 ≤ 0.60 ACH ≤ 0.04 ACH_nat 100% Mandatory Heat Recovery Ventilation (MVHR ≥ 80%).
⚠️ Infiltration Traps

5 Fatal Infiltration Mistakes in Heating Sizing

Avoid these common ventilation modeling errors that cause inaccurate heating calculations.

❌

1. Using Blower Door n50 Directly in Heat Loss Formulas

The Blower Door test measures air changes under an artificial 50 Pascal hurricane-level pressure. Plugging n50 directly into $0.33 \cdot n \cdot V \cdot \Delta T$ overestimates ventilation heat loss by 2,000%. You must divide by the LBL factor (~20).

❌

2. Sealing an Old House Without Mechanical Ventilation

Fitting airtight foam and silicone without providing continuous background ventilation traps moisture, raising indoor humidity and causing black mold growth.

❌

3. Ignoring Open Fireplace Chimney Flues

An unblocked chimney hearth draws warm air out of the room at over 40 m³/hr, acting as a permanent heat siphon.

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4. Assuming Zero Air Leakage in New Builds

Even new builds experience background infiltration through loft hatches, downlight penetrations, and service pipe entries.

Ventilation Suite

Related Ventilation & Heat Loss Tools

Connect infiltration modeling with room fabric heat loss, total building energy demand, and winter outdoor design temperatures.

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Whole Building

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Aggregate whole-building heat demand to size central heating boilers and heat pumps.

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80+ Cities Database

Design Temperature Lookup

Search regional 99.6% winter design outdoor temperatures across the globe.

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Infiltration, Blower Door & MVHR FAQs

Building science answers on n50 conversions, natural ventilation rates, stack effect, and heat recovery savings.

Convective heat loss from air infiltration is calculated as: Q_vent = 0.33 × n × V × ΔT. Where 0.33 is the volumetric heat capacity of air (ρ × cp / 3600 in Wh/m³K), n is the air change rate per hour (ACH), V is the heated internal air volume in cubic metres (m³), and ΔT is the temperature difference between inside and outside.
Under the widely accepted Sherman-Grimsrud / Kronvall 'Rule of 20' (LBL model), natural air changes per hour under normal ambient conditions is estimated as: ACH_nat ≈ n50 / N, where the divisor N typically equals 20 (adjusted to 15–25 depending on building height, wind shielding, and leak distribution).
A certified MVHR system extracts stale, humid air from kitchens and bathrooms while supplying fresh outdoor air to living rooms and bedrooms. A high-efficiency counter-flow heat exchanger transfers 85% to 92% of the heat from the exhaust air to the incoming fresh air, reducing ventilation thermal heat demand by ~85%.
Stack effect (or chimney effect) is the buoyancy-driven movement of air through a building. Warm indoor air is less dense than cold outdoor air, creating positive air pressure near the top of the house (pushing warm air into the attic) and negative suction pressure at the ground floor (drawing cold air in through floorboards and doors).
UK Building Regulations Part L (2021) sets a maximum limiting air permeability of q50 ≤ 8.0 m³/(h·m²) at 50 Pa (with typical new builds achieving 3.0 to 5.0). The international Passivhaus standard requires ultra-airtight performance of n50 ≤ 0.6 ACH at 50 Pa.
Buildings need continuous fresh air to evacuate moisture (from breathing, showering, cooking), carbon dioxide (CO₂), and volatile organic compounds (VOCs). If an older home is sealed without controlled mechanical ventilation (such as continuous MEV or MVHR), relative humidity climbs above 65%, causing toxic black mold (Stachybotrys) and condensation.