📐 Q (Watts)
The energy lost - what the boiler must replenish.
Calculating Ventilation Losses in kWh: How Much Do Your Gaps Cost?
To heat the air in a room, we must give it energy. In physics, air has a specific heat capacity. It takes approximately 0.34 Watts of energy to heat 1 cubic metre of air by 1 degree Celsius. This number comes from air density (~1.2 kg/m³) multiplied by its specific heat (~1,005 J/kg·K) divided by 3,600 (Joules → Wh).
If we know how much air enters our home every hour through gaps (the famous ACH we discussed in the first article) and the indoor-outdoor temperature difference, we can calculate the lost energy with absolute accuracy!
1. The Loss Equation
The formula engineers use to find the power of ventilation losses (in Watts) is:
Q = V × ACH × 0.34 × ΔT
📏 V (m³)
The net volume of the home. E.g. 100 m² × 3 m = 300 m³.
🌪️ ACH
Air changes per hour - how many times the entire air volume is replaced.
🔢 0.34
The specific heat capacity of air - a physics constant.
🌡️ ΔT (°C)
The temperature difference between indoors and outdoors.
2. The 10x10 Model Experiment (January, 5°C)
Home 100 m² × 3 m = 300 m³. Outside 5°C, thermostat 21°C. ΔT = 16°C. Let's see three scenarios over 24 hours:
❌ Scenario A (The Old "Sieve" - ACH 3)
The home has old timber windows and gaps everywhere. The air changes
3 times per hour.
Q = 300 × 3 × 0.34 × 16 = 4,896 W
(≈ 4.9 kW).
In 24 hours: 117.6 kWh lost just through air! At average
electricity/gas cost, we’re talking about
€15 to €20 thrown away every single day!
✅ Scenario B (Sealed - ACH 0.5)
We installed new windows and silicone. The air changes half a time
per hour (just enough for breathing).
Q = 300 × 0.5 × 0.34 × 16 = 816 W
(≈ 0.8 kW).
In 24 hours: 19.2 kWh. An enormous saving (we saved
almost 100 kWh per day), simply by sealing the
gaps!
🏆 Scenario C (Passive + MVHR 85%)
The home is airtight (ACH 0.5), but we have installed Mechanical
Ventilation with Heat Recovery that retains 85% of the heat. So we
lose only 15% of the already reduced losses.
19.2 × 0.15 = 2.88 kWh/day!
Negligible cost.
The Final Conclusion: The maths never lies. An uninsulated home with poor windows can lose over 100 kWh per day just through the air that penetrates it. Investing in proper seals, correct silicone filling and (if feasible) an MVHR system is not a luxury. It is the most mathematically certain payback of money you can make on your home!
Related Articles
Air Changes per Hour (ACH): What are they and why do they matter? Case Study: The 10x10 Model in 3 Greek cities Mechanical Ventilation with Heat Recovery (MVHR) Heating Degree Days (HDD)
Ventilation, Airtightness & Real Consumption: Climate Control
Return to category.
Go to categoryCentral Guide
Return to the central guide.
Go to guide