Recovering heat with a central supply and extract air system

Insulated new buildings and houses with low ceilings require measures for controlled indoor ventilation that are perfectly planned right down to the last detail. There are two reasons for that: on one hand the building needs to be supplied with fresh air, on the other hand humidity, mould and odours that build up on the inside need to be evened out. So that the energy of the breathing that was heated up before and has now been used up does not get lost, it is being recycled. The most efficient way to recover heat is by implementing central ventilation systems. The supply and extract air system is operated by fans and takes over the exchange of air of the entire house and is even able to re-use up to 90% of heat in the extract air and add it to the supply air.


  • Mould and humidity in new buildings
  • Brief excursion to decentralised ventilation systems
  • Central ventilation systems with and without heat recovery
  • Winter & summer mode of central systems
  • Cleaning, maintenance and care

Mould and humidity in new buildings

Contrary to popular belief, the growth of mould is not just a phenomenon exclusively present in old buildings, instead it can also suddenly happen in newly erected buildings (low-energy, passive, zero energy and PlusEnergy buildings). In the year 2012 it was estimated that roughly 50% of all new buildings are affected! The figures are so high because during the contruction phase there is already a great risk for mould growth and other damages through humidity. If not covered sufficiently, water e.g. in the form of rain and snow in winter penetrates the structural work and accumulates in the materials. If the construction is then finished with a roof and windows without respecting an appropriate drying time, or if there has not been sufficient heating for financial reasons or nescience, mould spores that are always present in natural environments quickly reproduce unnoticed in warm and humid climate.

If your assigned building company is working according to the philosophy “Time is money“ and urges you to move in as fast as possible, be on the safe side and have a specialist examine the building. Specialised organisations actually recommend getting regular checks from an expert third-party. If constructional defects are detected in a building where residents have already settled in, you as the buyer are safe in regards to legal aspects. In that case the building has not been handed over in proper form.

But for now we shall assume that the building has been erected and handed over in accordance with the regulations. However there are still a couple of risk factors in recently finished buildings that hold a high potential for mould growth.

In new buildings (active and passive house), house walls are sealed air-tight due to their excellent heat insulation, there is no air permeability caused by joints. Still, fresh air needs to get into the house and used, humid air needs to be transported to the outside. Otherwise the lack of alternatives causes humidity in new buildings to settle on colder walls and condensate, turning into water.

A family with two children and a couple of houseplants create up to 12 litres of water vapour each day. However, vapour containing water does not only develop by showering and cooking where steam can actually be seen rising up, but also during sleep and household chores, for example when floor and shelves are cleaned with a wet cloth. Intense workouts on exercise machines and hanging up clothes to dry inside the house also lead to increased humidity levels. Of course you should not pass on these activities in your own home and live life the way you want! There are just a few precautionary measures you should take in order to keep your living space permanently as dry as possible.

Window open, fresh air in? Former habits and customs of ventilation in old buildings can, aside from legal regulations, not simply be resumed after reconstruction for more energy efficiency or moving into a new house. In old buildings, outdoor air flowed through leaky walls, the roof, joints on doors and windows completely uncontrollable. That is why there was a certain minimum air exchange going on in the house, even when manual ventilation had been neglected, and mould only occured in particularly humid rooms.
Forgetting to air out the space in new contructions would quickly take revenge on you, as it inevitably leads to mould growth because the water vapour (without ventilation system) cannot escape. Just image how it would be to open the windows several times a day in all rooms of a complete building! Save your time and your heating costs by employing heat recovering technology of automated, fan-operated systems for controlled indoor ventilation. 

Brief excursion to decentralised ventilation systems

Decentralised supply and extact ventilation systems are predominantly used to support the exchange of air in single rooms with a particularly high amount of water vapour e.g. in the bathroom, guest toilet, in kitchens and rooms that are used for drying laundry. Depending on the floor plan and structural conditions, single rooms in cellars can also be kept dry with such systems. Installing decentralised systems is costs less and is often easier to arrange than central systems. That is why they are a popular choice for old buildings that are in need of reconstruction for more energy efficiency.

Decentralised installations already create a remarkable heat recovery rate with more than 70% heating energy recovered, however the efficieny of central systems stays the front-runner amongst these equipments, they recovery 90-95%! Another disadvantage when compared directly to the central solution is that their sound level is much higher, and people with sensitive hearing might feel disturbed by it.

Central ventilation systems with and without heat recovery

Central ventilation systems can be used in three different ways. No matter which purpose the system should serve, it is always crucial to ensure sufficient air exchange in all rooms.
A central exhaust system (without heat recovery) is characterised by a fan on the inside that is connected with the rooms through vents with exhaust air valves. Decentralised air supply valves in the house wall let in fresh air. 

  • relatively little work is needed for installation
  • where required, installation in old buildings is also possible
  • central alternative with lowest costs
  • heat recovery possible with heat pump

When working with the solution of a combined supply and exhaust system (without heat recovery) supply air is centrally sucked in and distributed by the fan in addition to the central exhaust air removal.

  • feasible only in very air-tight buildings
  • requires a lot of construction work with air ducts for incoming and outgoing air
  • cannot be added to old buildings retroactively

Not every country legally requires you to use a ventilation system with heat recovery, however experience has revealed that primary energy consumption for heating and a reduced loss of heat are most likely to be reached by implementing exactly this measurement.

A supply and exhaust air system with heat recovery has a centrally installed fan that controls both air currents. Used up and humid air is extracted by suction and transferred to the outside via an aperture in the roof, for example. Fresh air from the outside is then sucked into the house at the central place and lead through air ducts into all rooms that need to be supplied with air. The amount of incoming air is equal to the amount of outgoing air in order to prevent a vacuum. Rooms that need to be supplied with air are living rooms or bedrooms for instance, typical humid rooms with air that gets extracted are bathrooms, kitchen and WC. The directions of both streams cross each other in a cross flow heat exchanger, sometimes called heat transmitter or recuperator, so that the temperature of the incoming cool air is adjusted to the warmth of the outgoing air. It may sound simple, but this method is incredibly effective. At least 75% of heat from the exhaust air can be recovered, depending on the manufacturer and model this figure rises up to 95%.


  • outstanding amount of recovered heat
  • heating energy is conserved, heating costs decrease
  • CO2 reduction for environmental protection
  • humidity reduced considerably
  • filtered supply air without allergy-triggering particles and pollutants
  • manual ventilation of the entire house is not necessary
  • windows stay closed preventing thieves and noise from getting inside
  • long service life of the system if maintained correctly
  • where applicable: comfortable, individual control via remote control
  • bothersome smells in the living area are extracted by suction
  • correct planning prevents draught
  • low energy consumption of the system, particularly when operated by direct current motors
  • multi-level fan performance can be adjusted as needed
  • noise insulated parts for very quiet operation
  • ventilation of the rooms is independently adjustable
  • improved quality of living conditions by fully automated controllers


  • higher purchase price and installation costs
  • requires more effort for regular cleaning
  • annual costs for cleaning and replacing filters (about 100 Euro)
  • requires space and elaborate construction work
  • can rarely be added to old buildings

You can find a comparison of benefits and drawbacks of different ventilation systems here.

What you should pay attention to when making a purchase:
Before the system is built, a specialist needs to plan and calculate the sufficient air exchange in all rooms of the house. Usually, the system is installed in places that are easy to access and not furnished, e.g. cellar, attic, storage room and can also be set up in boiler rooms of restaurants, offices and other commercially used premises. Take details about the defined space in cubic metres into consideration that are stated in each item description. The air tightness of the house should also be confirmed by a specialist in advance to ensure that the ventilation system can work without any draught effects and reach the best possible yield of heat recovery. With the pressure differential, also called blower door test, the air exchange rate can be determined.

To ensure hygienic operation over the course of several decades, it is beneficial to install the ventilation ducts in a position where they can be maintained and cleaned easily. Please also pay attention to specific instructions given by the manufacturer in the manual. Depending on the model they can be installed in various positions, e.g. lying, standing, horizontally or vertically. Details about filters and required ventilation pipes as well as additional controls and sensors also vary between brands. If not already included in the delivery, add sound insulating parts to your system if required.

Winter & summer mode of central systems 

The question whether a supply air system that takes fresh air from the environment and transfers it into the house will subject you to strong climatic fluctuations, is only justified. Furthermore, heat recovery technology during hot summer months would be downright fatal! You should neither suffer from sub-zero temperatures in winter, nor from summer heat. For instance, during summer you can set your system to suck in more air from the outside during night when it is more cool and refreshing outside than during daytime. A “summer bypass“, an additional technical component, prevents a direct cross flow, avoiding heat recovery in the months with increased temperatures.

During winter however, heat recovery fully comes into effect. The integrated humidity and temperature sensors (not all manufacturers) automatically react to rain, strong wind or very cold outdoor air. A frost protection valve makes sure that sensitive inner parts like the heat exchanger are not affected by ice formation. In areas that are known to suffer from extreme weather conditions it is also possible to add an optional electrical air heater inside the pipes.

Cleaning, maintenance and care

A central ventilation system requires a lot of care and maintenance from specialists. That includes replacing and cleaning the filters respectively twice a year, and if required cleaning the ventilation ducts. The actual wear and tear of the filters depends on the pollutant contents of the outdoor air (environement with sandy ground, fine dust from busy main roads, pollen near forests and meadows, cobwebs on the outside ducts) and indoor air (mites, pet hair, grease and other kitchen vapours, house dust). Fine types of filters for indoors keep away dust and small particles of dirt, even bacteria and germs, ensuring healthy breathing air. Smells from kitchens, smoker’s rooms and WCs are bound by active carbon for instance, and cannot spread out to the other rooms.
Supply air pipes are usually protected from dirt and intruding insects by pre-installed filter flaps and coarse filters or pre-filters that are mounted behind it. In reality, however, gravel, twigs and leaves manage to get in from time to time when positioned at the right angle. That is why you should not rely only on the filter replacement, but check the pipes and ducts regularly as well.

Exhaust air ducts also suffer from the pollution of everyday life, e.g. dust and grease from the air in the kitchen. In order to prevent impurities inside the pipe clogging the air way, the pipes need to be kept clean. Replacing the filters is a good opportunity to discover impurities before the air flow is reduced. If filters are clogged or very contaminated, unhygienic dusts might be transported through the entire house, and in the worst case the whole ventilation system stops working properly. A study of the University of Lucerne (Switzerland, 2012) criticises the hygienic conditions of ventilation systems. Half of the systems that were examined revealed filters that were attached the wrong way round or clearly visible water damage like rust. Yet, in more than 90% of the cases the indoor air was still in good condition, the increased quantity of germs did not lead negative effects on the residents‘ health. It is still recommended to have the installation of the required filters and checking the system done by a certified specialist.

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