Supply Air

A long time ago, when buildings still had leaky windows and the wind infiltrated the house through the shingles, the building was provided with a sufficient amount of fresh air.
Today however, most newly built houses are constructed in a well-insulated, almost completely airtight manner. That means cold air cannot infiltrate anymore, but stagnant air also cannot escape to the outside. Windows, doors, insulation material all help to save energy by conserving heat. Humidity, carbon monoxide and exhalation from the building material are transported away by extraction systems in regular intervals. The constant extraction results in a negative pressure though. In order to even out this underpressure every type of ventilation system requires some kind of air supply in the same quantity as the air that has been taken away. There are different ways to achieve this that vary in terms of feasibility and energy efficiency. In this guide we want to outline different costs, advantages and structural measures of the equipment.
If open fireplaces and furnaces are used at the same time as the electric ventilation appliances, keep in mind there will be a mutual reaction which we are going to cover in the last section of our guide.


 Possible ways to supply fresh air

The fresh breeze pouring into the house from the outside contains a a good amount of oxygen, stimulates the metabolism, balances out the CO2 content indoors and prevents any negative pressure caused by extractor fans and fireplaces. You have the following options to ensure air supply:

  • Opening windows
  • Ventilation shafts (without fan)
  • Controlled by thermostat
  • Vents inside the window
  • Centralised ventilation system with fans
  • Reversible ventilators (decentralised ventilation)

It depends on the construction of your house which type of ventilation is best for you and the easiest to implement.

In old buildings there are rarely any devices installed for controlling ventilation at all. Opening the windows is usually the easiest way to provide natural ventilation and air change. However, due to the extreme loss of heat which is literally thrown out the window, this method causes increased heating costs. In situations where the humidity content is higher than normally, for example when cooking, taking a shower or drying laundry, or if the CO2 content exceeds a healthy limit, it is best to open two windows on opposite sides of the room to allow change of air.      

Natural window ventilation:

To intensify natural ventilation in rooms without windows it is possible to implement shafts without fans. They connect the room in question directly or indirectly to a fresh air supply. One option incorporates a duct from the roof that goes directly into the room, another option indirectly ventilates by shortening doors so air can enter through the gap on the ground.
Another “method“ in the olden days was the construction of the building itself. Back then, there was no heat insulation, the building envelope had tiny cracks, meaning there was a constant airflow on doors and windows. Nowadays, it is only found in old buildings, new constructions are airtight.   

Ventilation shafts (without fan):

Supply valves controlled manually or by thermostats (without power supply) provide regular air supply through existing pipes and shafts. Due the small size of these inlets it is easy to change their position without too many structural alterations.
Supply valves of the ZLEP 100 range use a thermostat to control operation. The fully automated operation depends on the temperature and even works in winter. The basic model of the series already features an integrated coarse particle filter to ensure the quality of the incoming air is healthy. Other models come with a sound-absorbing pipe as well as heat insulation for cold incoming air.  

Ventilation openings compared to ventilation valves

Most newly-built houses and restored buildings have a way to ensure ventilation even if the residents are not at home, meaning independent of the users. Automatic ventilation appliances guarantee air circulation. The appliances are part of a thought-out concept to prevent damage due to moisture and mould. We assembled and outline of modern ventilation systems in domestic environments.

In order to ensure a sufficient vertical transverse flow system of ventilation, it is important to install several ventilation openings in the entire home. This results in structural alterations and costs for installation, however these expenses are relatively harmless compared to complex ventilation plants. The purchase price for these fans is low as well. The openings are made in the frame or sealing, many models come in a subtle, unobtrusive design and are almost not visible anymore once the window is closed. They can also be added to the window later on and can be applied to wood and PVC frames.

Ventilation openings in the window:

Central ventilation plants (with fans) are installed in the attic or cellar of a house and regulate ventilation in the entire house through various ducts. These appliances achieve outstanding results for heat recovery. They collect the energy of the warm indoor air that is now being extracted in a heat reservoir and pass it on to the incoming cold air. In models without a heat reservoir, the warmth is transferred through cross flow. Up to 95 percent of the heat in the extracted air can be transported back inside again.

The effort and costs for the installation of one of these units are quite high. In many cases it is also impossible to retrofit a system because of the various ducts that must be laid in the walls, ceiling, etc. However, in buildings where it can be done, the effort will pay off soon! Energy-efficient devices with low operational costs provide continuous, fully automated ventilation and high indoor air quality. There are even special controllers that can be added, such as a timer or humidity sensors. 


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Decentralised reversible supply and extract devices are applied to support air change in individual rooms with a lot of moisture content, for example in the bathroom or the kitchen. These devices are generally attached to ducts or shafts that are already present in the building, therefore costs and effort for installation are rather low. There is also no need for extensive maintenance work, the filters are replaced about every six months depending on the brand or can be washed. They filter incoming air and provide heat recovery. The supply and extract cycle is automatic or controlled manually on the same device. High-quality branded goods are already available at an affordable purchase price. Another benefit of the reversible devices is that there is no need for a separate supply duct.

Reversible devices in our range:

  • Controlled by step-transformer or continuously variable controller (sold separately)
  • Prevents cold air from infiltrating thanks to automatic shutter
  • High airflow rate of up to 1650m³/h and 68 Pascal total pressure  

Find out more about centralised and decentralised ventilation technology and heat recovery here.

Exception: Supply air for fireplaces and fume hoods

A warm fire in the fireplace add a cosy atmosphere in the living room. However, some safety precautions need to be made so there will be no problems.
Fireplaces absorb the oxygen they need for the combustion process from the ambient air. That means air is taken away, in a closed building this leads to negative pressure. If there is no sufficient supply of air to balance out the loss, the pressure will suck flue gas like carbon monoxide from the fireplace back into the room, where the smoke will blend with the breathing air – a toxic cocktail of gas.

In households with controlled ventilation and the use of fume extraction hoods on top of a fireplace, the loss in pressure can lead to complications with the electric ventilation system. The mutual reaction of a fireplace and a ventilation unit should be prevented by...

1. Separating the operation of both machines and ensuring an alternating operation. That means the ventilation system depending on surrounding air should no operate if the fireplace e.g. the chimney or wood burner is in use. Though a sensible guideline, this is not easily implemented in buildings with a central ventilation system.     

2 Other possibilities:

  • Window contact switches will only allow operation of a fireplace / fume hood if the window has been opened (cannot be applied to all buildings)
  • Provide a special supply air inlet just for the fireplace / fume hood
  • Fume hoods with recirculating air operation do not create any negative pressure 

Always consult your chimney sweep and landlord before installing a system! 

Fume hoods that are used as an extractor remove (moist) air from the room. This has the same effect as a fireplace, a negative pressure is generated because ambient air is extracted. The pressure then draws toxic gas from the cooking stove and brings it back into the room. Supply air from other sources such as large adjacent rooms should be provided to balance pressure.

Fume hoods during recirculated air operation pull the extracted air through grease and activated carbon filters and return it to the room after it has been cleaned, so there will be no low pressure. The filters must be replaced frequently though, usually at least twice a year, meaning there will be regular costs that you should consider.

Before the installation of a fume exhaust hood, get in touch with your landlord, architect or a licensed planner who will draw up a plan of all required wall breakthroughs for exhaust and supply air ducts, all costs incurred and who will explain everything in detail. You can also consult a chimney sweep for safety precautions. There must always be an appliance that ensures sufficient supply air no matter the type of ventilation system. 

Image sources:
Image fireplace © maho -
Image house © sunnychicka -
Image technical drawing © ViennaFrame -
Image fume hood © Juergen Faelchle -