Conceptual planning

The conceptual planning of a ventilation system for high-performance housing includes the following.

Definition of ventilation/air quality requirements. This joint action by the planner and client establishes the purposes and the expected properties of the ventilation system and sets the boundary conditions. Standards and applicable codes have to be clarified. The conclusions of this process should remain constant for the rest of the planning process.

Specifying zones by air quality and temperature requirements. Fresh air should always be supplied to the living and sleeping spaces (for example, living rooms, children's rooms, study room, bedrooms). From there it flows through circulation spaces (corridors, entry hall or stair) to wet rooms (kitchen, bathrooms, toilets, utility rooms) where it is extracted. Placement of the supply and exhaust ducts to serve these zones has to be considered with the goal of keeping duct runs as short as possible.

Determining the nominal air flow rates and distribution. The total air change rate comes from the addition of all supply rates (or exhaust rates) for the interconnecting rooms between the point of supply and extraction. Measures have to be provided that allow for such air flows. Supply rate and extraction rates should be balanced.

Deciding on the level of air filtration (normal or for allergies). Different classes of air filters concerning efficiency, pressure drop and dust holding capacity are defined in ventilation standards (for example, the German DIN EN 779). The lowest acceptable filter class is G3 for coarse dust. To prevent pollution by very fine dust (down to about 1 jum), filter classes F6, F7 or F8 should be applied (see Table 10.3.1). The cleaning of air by filters is necessary when high concentrations of particles are present or when the outside air is contaminated. As the ambient air is usually rather clean, filtration of outdoor air serves mainly to keep the supply branch of the duct free from dust. If the indoor air contains pollutant aerosols (fat particles, tobacco smoke, etc), filters are also mounted in front of exhaust ducts, fans and heat exchangers, as the exchange of filters is cheaper than cleaning the ducts. On the other hand, filters cause pressure drop (typically 50 Pa to 300 Pa) and thus increase the electrical power demand of fans. Careful dimensioning is required to maximize the useful operating time of the filters while keeping the pressure drop to acceptable values. Because filters have to be inspected, cleaned or exchanged about every three to six months, they have to be easily accessible.

Table 10.3.1 Air filter classes (degree of retention versus particle size)

Particle size Percentage of retention

Table 10.3.1 Air filter classes (degree of retention versus particle size)

Particle size Percentage of retention

G 1

G 2

G 3

G 4

F 5

F 6

F 7

F 8

0.1

-

-

-

-

0-10

5-15

25-35

35-45

0.3

-

-

-

0-5

5-15

10-25

45-60

65-75

0.5

-

-

0-5

5-15

15-30

20-40

60-75

80-90

1

-

0-5

5-15

15-35

30-50

50-65

85-95

95-98

3

0-5

5-15

15-35

30-55

70-90

85-95

> 98

> 99

5

5-15

15-35

35-70

60-90

90-99

95-99

> 99

> 99

10

40-50

50-70

70-85

85-98

> 98

> 99

> 99

> 99

Average values of dust concentrations in urban outdoor environments vary between 0.1 mg/m3 and 0.5 mg/m3, and particle sizes with maximum concentration range between 7 ¿u,m and 20 ¿u,m. For rural outdoor environments, dust concentrations are much less, ranging from 0.05 mg/m3 to 0.10 mg/m3, with most existing particles being smaller than 1 ¿u,m to 2 ¿u,m.

Specifying the duct geometry and construction. The ductwork geometry strongly influences the required fan power, as well as the sound level of the ventilation system. Duct cross-sections should be chosen to allow as slow an air velocity as possible so that it never exceeds 3 m/s. This is in order to limit pressure drops, electrical consumption and noise production. Space limitations or costs often lead to small diameter ducts. This results in turbulent air flow with all of the above disadvantages. Duct lengths should be kept short and with minimal bends. The duct interior should be smooth. Folded spiral seam tubes of tin-plated sheet steel are acceptable. Flexible spiral tubes made of aluminium are less favourable. They require special attention during installation to avoid pinch blockages. Spiral tubes of synthetics should be avoided. For straight ducts, a pressure loss of less than 0.5 Pa per metre length is desirable. The air ducts should be air tight in order to prevent penetration of external dust and to avoid heat losses in between the heated zone and the heat exchanger. Particularly during the building construction, ducts must be kept sealed.

Locating the outside air intake, distribution and room air extraction. Obviously, outside air should be taken where the best air quality is available (distant from sewer vents, chimneys, exhaust air vents, waste containers, car parking, etc). The fresh air intake should be protected, ideally inaccessible to avoid manipulations and to be shielded from heavy rain. The opening geometry should minimize pressure losses. The path from the point of supply in the living spaces to the point of extraction must also be ensured by closed doors. This can be achieved by horizontal slits at the upper or lower part of the door leafs or built-in air grilles. They should be designed so that pressure drops across the doors should not exceed approximately 1 Pa and air velocities should be under 1 m/s (to avoid air draught and noise). These pressure differences should be almost the same between all rooms of the supply zone and the transition zone, as well as between this zone and all rooms that belong to the extract zone of a building. This ensures the same quality of fresh air supply for all rooms. For volume rates up to 40 m3/h, a horizontal slit with 15 mm height is sufficient. The exhaust vents should also cause minimal pressure losses (being adjustable along a correlation between volume rate and pressure drop). They should be mounted well away from heating appliances and should avoid short-circuiting with the air supply inlets. In rooms with high humidity, the exhaust vents should be located near the ceiling. In the kitchen area, a separate exhaust hood with volume rates between 150 m3/h to 200 m3/h is recommended to capture high water vapour and fat particles. During the operation of kitchen hoods, when exhausting air directly to the outside, the supply rate for the overall ventilation system has to be increased to keep the house balanced, otherwise a lower pressure within the building would increase the uncontrolled infiltration of air from outside. Prevention and suppression of noise is a common engineering task for the entrance, distribution and removal of air. For that, specific data and design advice are available from component suppliers.

Detailing thermal insulation. Cold and warm air ducts should ideally be within the tempered zone being served. Fresh cold air should not withdraw heat from the heated zone, and stale warm air should not lose heat to the colder ambient air. If this is not feasible, ducts must be insulated (thermal conductivity: 0.35-0.40 W/mK). This typically leads to an insulation thickness of between 5 cm and 10 cm. The central ventilation unit should be near where the ducts penetrate the boundary of the heated zone of the building - either just inside this zone or just outside. The central ventilation unit itself should be thermally insulated, too, to avoid the heat exchange between exhaust and supply ducts and the surroundings of the unit. Supply ducts providing heating should always be within the heated zone of the building.

Planning sound insulation, dampers and fire/smoke breaks. The sound level of the ventilation system should not exceed 25 dB(A) for the supply zone and 30 dB(A) for the rooms of the transition zone and the extract zone. These values are each better by 5 dB(A) (lower) than those required by the German standard DIN 4109. Experience has shown that designs just meeting the limits can lead to occupant complaints. To obtain such rather low sound levels, silencers have to be built into ducts between the fans and the served rooms. Depending on the ambient noise levels at the outside air intakes or air outlets, additional silencers may be needed. Rigid or flexible tubular silencers containing sound-absorbing material, which is covered on the inner surface of the tube by perforated metal sheets, are well suited for this purpose. The propagation of sound between rooms through the ducts or through slits or air grilles has also to be considered. Sound protection requires engineering! Finally, dampers, and fire and smoke protection have to be planned for local building codes.

Programming the control system. Optimization of indoor air quality with the constraint of ventilation heat losses requires the ability to control the ventilation system for the conditions that are occurring. The following aspects should be considered:

• Operation must be kept simple and easy to understand (user acceptance!).

• In multi family houses, air change rates should be adjustable individually for each apartment.

• For every apartment, the air change rate should be variable between several steps (for example, 'off', 'low', 'normal' and 'high'). This can be done manually or automated.

• Freeze protection of heat exchanger must not be achieved by switching off the supply fan, otherwise increased infiltration and ventilation heat losses, as well as cold air draughts, may result when the exhaust fan is still operating.

• Summer operation needs no heat exchanger. Therefore, either the system is switched off totally (window ventilation only), or there exists a bypass for the heat exchanger, or an interchangeable summer module for the ventilation unit is used.

Controls unfortunately require many sensors, actuators and other electronic equipment that cause additional pressure drop and electric power consumption. Moreover, additional costs arise due to investment, operation and maintenance. Therefore, the benefits of more sophisticated controls have to be traded off against these drawbacks.

Adjusting air flows and performance testing. Planning is one aspect; quality of workmanship is another! Therefore, prior to the call for tenders, a qualified procedure of acceptance tests has to be defined. This includes a complete system description, selected measurements, comparison with design targets, adjustments of devices and corresponding protocols for documentation. The main task is to establish the planned air change rates for every room and then, as part of the commissioning, test the functioning of the system and its controls and operating rules to verify that the requirements are fulfilled.

Documenting the operation and maintenance plan. Long-term performance depends on routine inspections and maintenance. A ventilation system has to be operated, inspected and maintained. Checking the operation is a short-term action and can happen in a sporadic way (for example, checking the power supply and on-off status of equipment). However, quality surveillance and maintenance must be carried out at regular intervals, at least every six months.

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