Design and system selection and simulation information input

For the MVHR to work, the building needs to be well sealed. It is recommended an air leakage index (i.e. air leakage rate at reference pressure of 50Pa: Q₅₀ (m³/h) divided by the building envelope S (m²) of 8 m³/(h.m²) at 50 Pascal for good practice and 4

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m³/(h.m²) at 50 Pascal as best practice standard (Table 1 in (CIBSE, 2000)). In fact, the airtightness test result of the building unit conducted in June 2011 obtains the air leakage index of 1.82 m³/(h.m²) at the reference pressure of 50Pa which meets the best practice standard thus installation of MVHR is suitable to work in the building unit.

Although it will add into the total electricity consumption but potential savings through better control of ventilation as well as means of reducing heat loss through heat recovery from the outgoing air are to be explored. The MVHR chosen for modelling work is an air to air heat recovery system in which heat is extracted from the exhaust air and transferred to the supply air using plate heat exchanger (See Figure 6-13).

Figure 6-13: An illustration of a heat recovery ventilation unit (REUK, 2012).

The comparative analysis between passive ventilation and MVHR in Passivhaus houses conducted by AECB using Passhivhaus design and SAP packages assumed electrical consumption of MVHR rated about 0.36 W/h.m² (AECB, 2009). With the gross floor area of the building unit of 46 m² (e.g. it includes external walls but excludes roof), the power consumption of MVHR is approximately 17 W. The MVHR could operate on continuous mode like 24 hours basis or could be switched off during unoccupied period.

From energy efficiency aspect, the study considers the 2^nd option so the MVHR system only operates when the building is occupied. It means during weekend and public holidays, the MVHR operates on 24 hours mode and during weekdays, it runs on 15 hours per day mode that excludes 9 hours of work and commuting time (See Chapter 5, Section 5.2.1).

The extract ventilation rates recommended in Approved Document part F – Ventilation indicates a minimum amount of 8 l/s for bathroom and 13 l/s for kitchen on continuous basis whilst intermittent extract requires a minimum extract rate of 15l/s in bathroom and for the kitchen 30 l/s if adjacent to the hob – 60 l/s elsewhere (DCLG, 2010b).

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Regarding the established profile of MVHR’s operation time, it is reasonable to assume the extract rate on continuous basis, thus minimum extract rate for the building unit through bathroom and kitchen are 8 l/s +13 l/s = 21 l/s. The supply air rate in liveable spaces of the building unit (excludes wet rooms, here is only bathroom as kitchen area is half of the living space) with regards to Table 5-6 is of 12.3 l/s. It satisfies the requirement of the extract air rate is at least equal to the supply air rate as in Approved Document part F (DCLG, 2010b).

The MVHR system was modelled in IES<VE> using Apache HVAC tool. The energy associated with ventilation includes energy required to heat the space and the fan power required to drive the ventilation. Heating was provided to the room through radiators in the space at the room temperature of 19ºC. And the flow rates were the sum of minimum ventilation rate, plus any boost ventilation if required, that would be translated into fan power of the system to achieve this flow rate.

Regarding the power consumption of the system, the term specific power consumption is introduced which is defined as the ratio of air flow divided by fan power to measure the efficiency of a mechanical ventilation system. In accordance to Energy Saving Trust

“best practice” MVHR units have been set certain standards and must have specific power consumption of 1 W/ (l/s) or less and a heat recovery efficiency of 85% or higher (EST, 2008). From the assumptions for the power consumption and the extract rate above, the specific power consumption is then calculated as 17W / 21 (l/s) = 0.8 (W/l/s) which satisfies this requirement. A highly efficiency value of 90% is assumed for heat recovery model with respect to CIBSE guide B statement for heat recovery unit to be able to effectively transfer up to 90% of heat from warm air otherwise will be lost outside (CIBSE, 2005a).

The operation time of MVHR is when the building is occupied (Chapter 5, Section 5.2.1) over a year is calculated below:

During weekend and public holidays, the unit runs on 24 hours mode:

24x [52 weeks x 2 days (weekend) + 8 days (public holidays)] = 24 x 112 = 2688 hours.

During weekdays, it operates during 15 hours then:

15 hours x [365 days - 112 days] = 15 x 253 = 3795 hours.

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The total operation time MVHR over a year is 2688 + 3795 = 6483 hours. With the operation time of 6483 hours and building floor area of 46 m² the MVHR electrical use is calculated as: 0.36 W/h. m² x 6483 hours x 46 m² = 107358 Watts or 107 kWh.

6.3.2.2 Simulation results

It is important to clarify that the MVHR developed in this section is for exploring the improvement of heating performance in comparison with the initial design without installation of such system. Besides, its operation time is determined throughout a year to maintain whole building ventilation rate to extract polluted air and supply fresh air for healthy indoor environment during the year. Regarding electricity consumption as well as internal heat gain, the difference between the use of MVHR to provide extracted air continuously for kitchen area and bathroom and the use of mechanical fans to supply intermittent extract rate in these two spaces are not significant. Indeed, the average power consumption of MVHR per day is as:

17 Watts x [(15 hours x5 + 24hours x2)/7] = 298.71W.h = 0.3 kWh, while the daily average consumption of extract fans in use in kitchen and bathroom in Table 5-2 and Table 5-3 is 0.25 + 0.25 = 0.5 kWh. The difference between these consumption figures in proportion with overall household electricity consumption of 2744 kWh could be negligible.

Regarding the cooling performance, passive ventilation is still the main mean of alleviating overheating risk. It is possible that the continuous use of MVHR system could contribute in eliminating the overheating issue. In fact, this followed suggestion in the design guidance of Energy Efficiency Best Practice in Housing that the air flow rate of 5 air changes per hours during the night could help reducing overheating in a very important way, representing most significant effect of any measures (EEBP, 2005: p16).

However opening windows could provide an exchange rate up to 2000 (l/s) (data given from the simulation results in IES<VE> on 20^th August with wind speed data at 8.7 m/s) which offers instant effect in removing the heat built up before (e.g. when arriving home from work) thus lessening the period of overheat suffering. The predicted space heating energy requirements for 4 cases of orientations when incorporating with the use of MVHR unit are given in Figure 6-14. This demonstrates an improvement in heating energy savings of approximately 40% in reference with the initial design without installation of MVHR unit in all 4 cases.

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Figure 6-14: Space heat loads without and with installation of MVHR unit via 4 main orientations.