Ventilation heat losses occur when warm indoor air leaves the building and is replaced by cold outdoor air. They typically account for 20–40% of the heating load – in very well-insulated buildings even more.

The Principle

Air is a heat carrier. When 20°C warm indoor air is replaced by -10°C cold outdoor air, the heating system must compensate for this temperature difference:

• Warm air out = Heat energy is lost
• Cold air in = Must be heated up
• Result: Additional heating demand

The Calculation Formula

Ventilation heat losses are calculated according to DIN EN 12831-1:

Air Change Rate: The Decisive Factor

The air change rate n indicates how often the room volume is exchanged per hour:

n	Meaning	Example
0.5 1/h	Half the volume per hour	Well-sealed building
1.0 1/h	Entire volume per hour	Average
2.0 1/h	Double volume per hour	Draughty old building

Minimum Air Change According to DIN EN 12831-1

The standard defines a hygienically necessary minimum air change:

Room type	Minimum n	Reason
Living rooms	0.5 1/h	CO2 removal, moisture
Bedrooms	0.5 1/h	Moisture from occupants
Kitchen	1.0–1.5 1/h	Cooking fumes, humidity
Bathroom	1.5–2.0 1/h	High humidity
WC	1.5 1/h	Odours

The Two Types of Air Exchange

1. Infiltration (Uncontrolled)

Air exchange through leakage in the building envelope:

• Joints around windows and doors
• Penetrations (pipes, cables)
• Leaks at connections
• Porous building materials

Infiltration depends heavily on building airtightness, measured as the n50 value (air change at 50 Pa pressure difference):

Building type	n50 value	Infiltration in windy conditions
Unrefurbished old building	6–10 1/h	0.5–1.0 1/h
Refurbished with new windows	3–5 1/h	0.2–0.4 1/h
New build to Building Regs	1.5–3.0 1/h	0.1–0.2 1/h
Passivhaus	< 0.6 1/h	< 0.05 1/h

2. Mechanical Ventilation (Controlled)

Targeted air exchange through ventilation systems:

System	Description	Heat recovery
Extract-only system	Only extract air is mechanical	None
Balanced system (MVHR)	Both supply and extract mechanical	Possible (HRV)
Decentralised units	Single-room solutions	Often integrated

Heat Recovery (HRV)

Modern ventilation systems can recover 60–95% of the heat from extract air:

HRV efficiency	Effective temperature difference	Heating load reduction
0% (without HRV)	32 K (at -12°C outside)	Reference
75%	8 K	-75%
85%	4.8 K	-85%
95%	1.6 K	-95%

Worked Example

A living room with:

• Volume: V = 60 m³
• Air change: n = 0.5 1/h
• Indoor temperature: θi = 20°C
• Outdoor temperature: θe = -12°C

Without Heat Recovery

With 85% Heat Recovery

Ventilation Concepts Compared

Concept	Advantages	Disadvantages	Effect on heating load
Window ventilation	No investment, flexible	Heat loss, user-dependent	High (full ventilation losses)
Extract-only system	Inexpensive, moisture protection	No HRV	Medium
MVHR with HRV	Energy-efficient, comfortable	High investment	Low (up to -90%)
Decentralised units	Retrofittable, HRV possible	Noise, appearance	Low to medium

Effect on Building Heating Load

For the building heating load, ventilation heat losses are treated differently from transmission losses:

Standard Method (Conservative)

Sum of all room ventilation losses = Building ventilation loss

Extended Method (More Realistic)

Takes into account:

• Simultaneity: Not all rooms are ventilated at the same time
• Heat transfer: Warm extract air from one room preheats others
• Infiltration balance: Pressure effects

Common Mistakes to Avoid

1. Underestimating Ventilation Heat Losses

In well-insulated new buildings, ventilation losses can account for over 50% of the heating load!

2. Not Retrofitting After Window Replacement

New, airtight windows drastically reduce infiltration. Without controlled ventilation, problems arise:

• Moisture damage
• Mould growth
• Poor air quality

3. Overestimating HRV Efficiency

The manufacturer's efficiency rating applies only under laboratory conditions. Realistic deductions:

• Frost protection: -5 to -10%
• Leakage: -2 to -5%
• Lack of maintenance: -5 to -15%

Measures to Reduce Losses

1. Improve Building Airtightness

Measure	Typical cost	n50 improvement
Seal windows	£40–80/window	-0.5 to -1.0 1/h
Insulate roller shutter boxes	£25–50/unit	-0.2 to -0.5 1/h
Seal penetrations	Variable	-0.5 to -1.0 1/h

2. Retrofit Ventilation System with HRV

System	Typical cost	HRV efficiency
Decentralised units (pair)	£650–1,200	70–85%
Central MVHR	£6,500–12,000	80–95%

3. Optimise Ventilation Behaviour

• Purge ventilation rather than constant trickle
• Cross-ventilation for rapid air exchange
• Demand-controlled ventilation (CO2 sensor)

The Heating Load Calculator

Our heating load calculator considers all ventilation aspects:

• Room-by-room air change rates according to use
• Various ventilation concepts (natural, mechanical)
• Heat recovery with adjustable efficiency
• Infiltration calculation based on building airtightness

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Further Reading

• What is Heating Load? – The fundamentals of heating load calculation
• Transmission Heat Losses – Heat losses through the building envelope
• The U-Value Explained – The key building component metric
• Renovation Recommendations – Measures to reduce heating load

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Sources

• DIN EN 12831-1:2017-09 – Method for calculation of the design heat load
• DIN 1946-6 – Ventilation of dwellings
• DIN EN 13829 – Blower door test