Simulation results

During heating period, the whole building background ventilation is provided by the infiltration rate and an additional ventilation rate which value is assigned to satisfy the required background ventilation rate. In practice, this additional ventilation rate could be supplied by trickle ventilators and partially opening windows or the use of mechanical ventilation. Space heat loads for 4 different orientations with the improved building fabric factors in comparison with those consumptions of the initial construction are shown in Figure 6-11.

Figure 6-11: Heating loads and overheating degree hours for improved fabric criteria via four main orientations

0 2000 4000 6000 8000 10000

0 10 20 30

South North East West

Overheating degree hours

Energy consumption (kWh/m² pa)

Heating load

Overheating - Windows kept shut Overheating - Windows are opened

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The difference in total space load (in kWh/m² per annum) between 4 main orientations is not significant with the lowest consumption for east case of 39.1and the highest consumption is for the west case with 41.1 thus around 5% consuming more. Total space load consumption for the base case (initial construction state) ranges best for south case and worst for north case amongst 4 main orientations with 3.5%

consumption difference. It can be concluded that orientation contributes a very little in the thermal performance of highly insulated building envelope. In comparison with the initial construction, the modular dwelling which envelope meets Passivhaus standard criteria is around 36.5 % heating savings (36% for west case to 37% for east case).

However, increased thermal insulation slows down the release procedure of the heat built up in the space when windows are kept shut. The overheating degree hours in this case is four times higher than the base case, due to the length of time that the room operative temperature stayed higher than the 28°C benchmark. Figure 6-12 describes the temperature profile in different thermal zones via four main orientations. The simulation outputs included infiltration rate and background ventilation for indoor air quality. However, the peak temperature could be up to 46°C if ventilation is not sought through. It could results in energy savings from heating improvement are insignificant to cooling energy by electricity consumption (mechanical fan or air conditioning system).

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Figure 6-12: Temperature profile in different zones via four orientations 6.3.2 Mechanical Ventilation Heat Recovery system

The transition towards airtight buildings where uncontrollable ventilation is minimised means that there is a need for providing controllable ventilation to ensure healthy and comfortable indoor environment. Thus alternatives means to supply adequate air exchange rates through the building envelope are sought. It could be passive ventilation in winter through trickle vents on windows or by passive stack ventilation which mainly rely on stack and wind effects to push air through the dwelling in order to maintain

0%

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> 46

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whole building ventilation background. However, simulation results on ventilation performance in Chapter 5 Section 5.3.1 show that trickle vents in the building unit do not meet the criteria thus lower ventilation rate is supplied. Also with respect to the current design of the building unit, there is no room to develop passive stack ventilation so as to fulfil the whole building ventilation rate demand regarding polluted and stale air replaced by outdoor fresh air. By using natural ventilation through opening windows partially in winter (e.g. opening angle of window is 15°), the heat carried with warm air leaving the building escapes that could defeat the purpose of tightening the building envelope. Also, opening windows in winter is undesirable as it involves cold drafts directly affects thermal comfort and increases condensation risk. Moreover, there are some periods of high moisture production (e.g. bath, shower and cooking) that passive ventilation cannot respond thus extract fans are used in wet rooms like bathroom and kitchen.

For these reasons, mechanical ventilation options with the ability to recover heat from the extracted warm air in wet rooms have an obvious attraction. It is recommended to be used in the building unit as the whole building ventilation rate is not met by the use of trickle ventilators during heating season according to results in Chapter 5 Section 5.3.1.

A whole-house mechanical ventilation is then recommended for use as it could remove polluted air and adequately ventilate every room rather than individual rooms like bathroom and kitchen where extract fans are installed. This system normally combines supply and extract ventilation in one. Fresh air is supplied to living areas and bedrooms by a supply fan and duct system while stale air and/or with high moisture content is removed from kitchen and bathroom by an extract fan and duct system (Riffat and Gillott, 2002). A heat exchanger can be incorporated into the whole house mechanical ventilation to preheat the incoming air, namely Whole house mechanical ventilation heat recovery (denoted herein as MVHR system). The extracted air which is warm indoor air from wet rooms like kitchen, bathroom via a duct system passes through a heat exchanger before being exhausted. The supplied air which is fresh outdoor air is preheated whilst passing across the heat exchanger and ducted to living areas and bedrooms (CIBSE, 2005a).