Trickle ventilator performance

The adopted ventilation strategy for the dwelling unit is the combination of naturally ventilation (e.g. whole building ventilation or background ventilation is provided by trickle ventilators and purge ventilation for reduce overheating risk through opening windows) and mechanical ventilation system (e.g. intermittent extracting fan). This is known as “mixed-mode ventilation” or hybrid ventilation (DCLG, 2010b). It is suggested in Approved Document part F - Ventilation that for the air supply to the habitable spaces in a dwelling that the whole dwelling ventilation rate is more than 13 l/s as for a dwelling with one bedroom (Table 5.1 in (DCLG, 2010b)). Additionally, it requires that the minimum ventilation rate per each room is not less than 0.3 l/s per m² internal floor (DCLG, 2010b). Applying these requirements, the ventilation rates in different zones in the building unit are calculated as shown in Table 5-6.

Table 5-6 : Required ventilation rate per floor area

Living space Bedroom Office Bathroom Hall Whole building Floor area (m²) 20.43 10.98 5 4.86 4.6 45.87 Ventilation (l/s) 6.13 3.29 1.5 1.46` 1.38 13.76 An approximate annual air infiltration rate was determined through the known value from the airtightness test given as one twentieth of the measured number of air changes per hour at the pressure difference of 50Pa using empirical values in CIBSE technical memoranda about testing buildings for air leakage rate (CIBSE, 2000). The airtightness results were presented in the previous chapter (See Chapter 4, Section 4.4.1), with the number of air change per hour at 50Pa noted as n50 = 2.1 ac/h (Air leakage test result shows Q50 = 283 m³/h and building volume V = 135 m³) thus air leakage rate is 0.1 ach, equating to 1.67 l/s with the volume of the living space. By comparing the required ventilation rate indicated in Table 6-6 whilst accounting of the air leakage rate of 1.67 l/s (or 0.1ac/h), the required ventilation rate for the living space is then equal to 4.46 l/s.

In addition, the term “equivalent area” was introduced to replace the “free area” when sizing the background of ventilator (i.e. trickle ventilators). “Free area” is defined as the physical size of the aperture of the ventilator while “equivalent area” reflects the air flow performance that the trickle ventilator will achieve (DCLG, 2010b). Trickle ventilators installed on large and small windows of the building unit were supplied by

the VELFAC Ltd manufacture. The manufacture statement indicated that the product in use namely click vent of 40 cm² free area possessed the equivalent area of 27.6 cm² (VELFAC, 2009). Seven windows with click vents could make up to 7 x 27.6 = 193.2 cm² or 19320 mm². This is below the guidance for background ventilators as indicated in Table 1.2a in Approved Document part F - Ventilation of the Building Regulations 2000. For a one bedroom single storey dwelling above the ground level with more than one exposed facade, the total "equivalent area” for the total floor area of less than 50 m² should not be less than 25,000 mm² (DCLG, 2010b). Regarding building background ventilation performance as a whole, the current numbers of trickle ventilators in the building unit were below the guidance. Additionally, it is worth notify that in order to provide minimum ventilation, trickle ventilator maybe oversized for the more common external conditions and this could lead to draughts and wasted energy in heating the coming air ((White and Perera, 1998). Therefore, the simulation results regarding ventilation performance by trickle ventilators in providing air exchange rate would be looked at closely in both heating load and the required air flow rate.

Assumption that the required background ventilation was maintained during occupancy period, 2 simulation scenarios were developed. The first case employed air flow model reflecting click vents on windows that are active while in the second case, a fixed rate equivalent to 13 l/s was assigned to the model. In fact, a value of 0.27 ac/h was used which added to the infiltration rate of 0.1 ac/h to make up 0.37 ac/h equating to 13 l/s.

In the two simulation scenarios, the ventilation profile was available during occupancy period. In the second case, the air exchange is at the fixed value on continuous basis that could be supplied from the mechanical ventilation system or mechanical ventilation with heat recovery which will be discussed in Chapter 6-Section 6.3.1. Figure 5-3 shows the predicted energy consumption for heating in these two cases.

Figure 5-3: Heating energy consumption for 2 cases, one with fixed air exchange rate and another with operated trickle ventilators.

It is observed from Figure 5-3 that the heating requirements of the second case that assigning a fixed air exchange rate is significantly higher than that of the first case which modelled trickle ventilators in use. Taking the living space as an example, Figure 6-4 shows the air flow rate supplied by operating trickle ventilators built within the simulation modelling. It was assumed that the trickle ventilators were used since the start of the occupancy period and closed when the building was unoccupied by occupant’s manual control. This flow rate profile driven by wind and stack effect, already included the air infiltration rate. The recommended background ventilation was not met by the provision of air exchange rate through the three trickle ventilators on 3 external facades of the living space as shown in Figure 5-4.

Figure 5-4: Air exchange rate in litre per second supplied from three trickle ventilators on three exposed facades of the living space

0 10 20 30 40 50 60

South North East West

Energy consumption (kWh/m² pa)

Whole building heating load

Heat load with the fixed air change rate as required Heat load with ventilation supplied from trickle ventilators

By resizing the trickle ventilators in order to provide adequate ventilation (e.g. the total equivalent area of 25,000 mm²), the equivalent area of each trickle ventilator is 25000/7

= 3571 mm² or 35.71 cm². As the ventilation performance of trickle ventilators in dwelling was not the main focus in the study, therefore it was impossible to address the use of trickle ventilators in supplying adequate background ventilation or improving indoor air quality. For this reason, the fixed air exchange rate to maintain background ventilation during occupancy period was assumed for simulations. The difference in heating energy demands in these two cases as shown in Figure 5-3 could be illustrative for the effect of heat loss by ventilation. A much higher heating consumption would be expected if relying on opening windows to provide fresh air into room during winter months.