Basics of ventilation for highperformance housing

The conventional ventilation systems described in the previous sections potentially cause high heat losses corresponding to the air change rate n. The only difference between natural and mechanical ventilation is that the mechanical ventilation rate n can be known (n = nmech + nf whereas for the natural ventilation it is largely unknown. Keeping in mind that the air change rate required for hygiene and resulting from leakage totals at least 0.5 h-1, this results in ventilation heat losses of about 36 kWh/m2a. This is a high value, given that the target heating demand for high-performance housing is 25 kWh/m2a or less. Accordingly, a ventilation system with heat recovery is necessary to meet the target. Heat recovery can be accomplished by:

• mechanical extract ventilation with a heat pump;

• mechanical balanced ventilation with a heat exchanger;

• mechanical balanced ventilation with heat exchanger and ground heat exchanger; and

• mechanical balanced ventilation with all of the above.

For any of these systems to operate effectively, two prerequisites must be fulfilled:

• an air tight building envelope; and

• an 'all-inclusive' planning concept for the ventilation system.

Building air tightness is measured by the so-called n50 value. This is the average air change rate of a building that has a positive and negative pressure difference of 50 Pa against atmospheric pressure. The common measuring technique is by fan pressurization (blower door). The infiltration rate n^, which is achieved for natural pressure differences (typically between -10 Pa and +10 Pa) is estimated from the n50 value by:

where the factor e describes how exposed the building is to its surrounding wind field. The following values for e can be assumed (German DIN 4108, Part 6):

• 0.07 for moderate conditions.

A measured n50 value of 3.0 h-1 for a moderate site can accordingly be expected to have an air exchange rate due to infiltration of about 0.2 h-1. The European standard EN 832 also offers values for the wind factor.

Air tightness ensures that:

• draughts due to external wind and temperature differences are prevented;

• noise and odour transmissions through the exterior walls are minimized;

• indoor-generated water vapour does not migrate into the exterior walls; and

• the air change rate and indoor air flow directions are under mechanical control and minimally affected by weather.

Moreover, some countries (for example, Germany, Switzerland, The Netherlands, Belgium, Sweden, Norway and Canada) have introduced air-tightness requirements as best practice standards.

Tight envelope construction is essential for the proper functioning of heat recovery in a mechanical ventilation system. If eHR is the effectiveness of heat recovery and only some fraction of the total exchanged air flow is driven through the corresponding devices, the effectiveness of this real system

This means that if 25 per cent of the total air exchange rate n occurs via the infiltration path (ninf = 0.25 • n), it will reduce the nominal heat exchanger effectiveness eHR to a real value of £HRREAL =

The decision to make the building tight and to have mechanical ventilation has several planning consequences. Due to the volume taken up by air ducts, their placement must be considered early in the design phase. The space and volume needed where all the ducts come together - namely, the equipment room - must also be planned for.

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