Mechanical ventilation

Mechanical ventilation by electricity-powered fans can provide fresh air for a whole building or for special rooms only. In the latter case, such 'decentralized ventilation units' are not treated here. This section describes 'central ventilation systems' for a house, apartment or whole apartment building.

In the case of the simplest mechanical ventilation, air is extracted only. This creates an internal under-pressure that draws supply air through planned openings in the building envelope. At the next level, fresh air is mechanically supplied and 'used' air is extracted.

Mechanical extract ventilation permanently ensures the required air change rate independent of occupants opening windows and independent of the weather. As a result, building condensation of humid air inside of exterior walls can be avoided, contaminants are removed and outside noise can be kept outside. During summer nights, the housing can be flushed with cool, fresh outdoor air. The fresh air is supplied to the living and sleeping zones, flows across the entrance hall and is exhausted via the kitchen, the bathroom and the toilets. For multi-family dwellings, every apartment should have its individual extract ventilation system to avoid transmission of noise and/or odour among neighbours.

The under-pressure caused by the fans should be adequate to dominate over wind and stack effects. However, the negative pressure difference between the building and its environment should not be excessive - otherwise, high velocity draughts in the indoor air and/or back-draughts from the outside air may occur. Additionally, buildings with a mechanical extract system have to be as air tight as possible in order to avoid air flows through other openings than those which are provided for that purpose.

In summary, the advantages of mechanical extract ventilation are:

• controlled ventilation rates are possible;

• when carefully designed, the air flows along the planned paths;

• pollutants can be extracted at their source and entry into occupied spaces can be avoided;

• condensation of water vapour inside exterior walls is likely eliminated; and

• heat recovery from the exhaust air stream is possible by using a heat pump.

Disadvantages include that:

• investment costs are higher compared to natural ventilation;

• electrical energy is needed for system operation;

• system noise may be a problem;

• regular cleaning and maintenance are necessary;

• extreme wind may affect the adequacy of the ventilation because intakes and exhaust vents are at fixed locations on the building;

• there is a risk of back-draughts from flues or the intake of radon (or any other soil gas) being drawn from the below-grade spaces; and

• occupants adjusting individual air inlets can affect the air flow through other branches of the system.

Mechanical supply ventilation drives the outdoor air mechanically into the building where it mixes with the existing air. This process creates a positive pressure difference against the atmospheric pressure, which inhibits the entry of infiltration air from outdoors. In this way, all incoming air can be precleaned and thermally conditioned. Such systems are mainly used in areas where the outdoor air is polluted (for example, in city centres), for industrial clean rooms or for occupants with allergies. This system type is usually not recommended for dwellings since it increases the risk of indoor-generated water vapour penetrating and condensing in the exterior walls and roof insulation.

Mechanical balanced ventilation combines extract and supply systems that are implemented as two separately ducted networks. Typically, the air is supplied and mixed into the 'dry' or 'occupied' zones (living rooms, bedrooms and study rooms) and is extracted from the 'wet' or 'polluted' zones (kitchen, bathrooms and toilets). An air flow pattern is established between the supply to the extract areas, which should be supported by appropriately sized air slots between these rooms. This may occur under doors, or, for acoustical reasons, special sound-deadening slots can be built in.

Systems with an accurate balance of mass flows are usually 'pressure neutral' and, therefore, not resistant to infiltration/exfiltration driven by wind velocity and temperature differences. Consequently, the building must be air tight to ensure that the air flow volume and direction occur as planned. There are systems available on the market that are able to maintain an almost perfect mass balance either by velocity sensors or by using the characteristics of built-in fans (power versus flow rate). Sometimes an intentional imbalance of air flows is introduced to put the building in a slight negative pressure, with the extraction rate up to 10 per cent higher than the supply rate (for dwellings). This allows for some limited infiltration and prevents moisture migration into the exterior walls and interstitial condensation there.

Mechanical balanced ventilation systems offer the same advantages as mechanical extract ventilation regarding a guaranteed supply of fresh air and avoidance of excessive ventilation heat losses (as can occur with uncontrolled natural ventilation). Moreover, direct heat recovery from the exhaust air to the supply air by heat exchangers can drastically reduce the amount of purchased heat, which must be provided to temper incoming air. Furthermore, filtering and thermal conditioning of the supply air improve indoor air quality and comfort. These arguments can help to justify the additional capital investment and operating costs of such a ventilation system.

The disadvantages of mechanical balanced systems are similar to those of a mechanical extract system. The problem of noise (here with two fans) can be minimized by installing the ventilation units away from the living and sleeping zones near functional rooms such as the kitchen or bathroom. The air handling unit can be sound insulated. Impact sound has to be reduced by mounting the units with sound-absorbing materials. In any case, the sound level in living or sleeping rooms should not exceed 30 dB(A) according to German codes (DIN 4109, Part 5 and VDI Richtlinie 2081). In highperformance houses, an even lower sound level of 25 dB(A) is recommended. For single family houses, suitable space in the attic or the basement can often be found for the central ventilation unit.

In these cases, however, the duct runs for supply air as well as for exhaust air may be longer and, in any case, the duct runs in unheated spaces have to be kept short and well insulated.

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