How To SAVE MONEY Understanding Ventilation Heat Loss?

What is Ventilation Heat Loss? 

In every home, heat is lost in two main ways. Fabric heat loss occurs through walls, windows, floors, and other building elements. Ventilation heat loss occurs when warm indoor air is replaced by colder outdoor air through chimneys, doors, windows, and ventilation ducts.

In well-insulated homes, ventilation heat loss becomes a larger share of the total heat loss.

This effect becomes clear when looking at real-world heat loss figures.

For example:

• A home with 4 kW total heat loss may lose around 1 kW (25%) through ventilation

• A home with 10 kW total heat loss may lose only 1–1.5 kW (10–15%) through ventilation
  

This means that ventilation heat loss plays a more important role in energy-efficient buildings and must be considered carefully during system design.

If you want to explore this topic further, we recommend the video Do Not Overestimate Heat Loss.

How to Calculate Ventilation Heat Loss? 

The standard formula for ventilation heat loss is: 

Q_vent = 0.33×Volume×Air Changes per Hour × ΔT 

Where:

• 0.33 is a constant based on air density and specific heat capacity

• Volume is the room volume in cubic meters

• Air Changes per Hour (ACH) depends on regulations

  – bathrooms: 1.5 ACH

  – other rooms: 0.5 ACH

• ΔT (Delta T) is the temperature difference between indoor and outdoor air

While this formula is widely used, it relies on assumptions that may not always reflect real-life conditions.

Are We Overestimating Ventilation Heat Loss? 

In most calculations, we assume constant air exchange rates, even at extreme outdoor temperatures (e.g. –10°C).

However, in reality:

• People do not keep windows open for long in freezing weather

• Air change rates vary throughout the day, especially in occupied homes
  

The most accurate way to determine actual ventilation heat loss is through a blower door test, which measures air leakage under controlled conditions.

Blower door test used to measure air leakage and ventilation heat loss.

How MVHR Reduces Ventilation Heat Loss  

One of the most effective ways to reduce ventilation heat loss in practice is the use of MVHR systems. Many new homes do not install MVHR, even though it can significantly improve energy efficiency.

MVHR systems work by:

• Supplying fresh outdoor air while removing stale indoor air

• Recovering heat from the outgoing air using a heat exchanger

Mechanical Ventilation with Heat Recovery (MVHR) system recovering heat from exhaust air.

For example, in a home with 1000 W of ventilation heat loss:

• Without MVHR: 1000 W lost

• With 80% efficient MVHR:

• 800 W recovered

• Only 200 W lost

This means MVHR cuts ventilation heat loss by 80%, improving both energy efficiency and indoor air quality. 

Conclusion

• Ventilation heat loss is often overestimated in design calculations

• MVHR systems significantly reduce heat loss and improve air quality

• Regulations vary by country, but real-world testing leads to better system design

For a real-world perspective on ventilation heat loss and air change assumptions, watch the video below.

Watch the video