CPD PROGRAMME | VENTILATION employing filters that may not potentially be needed in areas with good air quality. The nominal flowrate qnom is the declared design flowrate so the manufacturer has the freedom to determine such conditions, depending on specific design choices (for example including or excluding a pressure reserve to overcome any clogging as the unit becomes dirty). The nominal flowrate is delivered at the maximum-rated fan speed. These ecodesign requirements do not apply to VUs that operate below 30W power consumption per air stream or under special conditions, such as corrosive environments or high temperatures. A greater level of heat recovery performance typically provides an increased pressure drop, owing to the narrower passages in the heat exchanger unit. This leads to a higher electrical energy use, for both the intake and extract fans. However, greater heat recovery performance will recover additional heat energy that can reduce space heating loads. The energy balance between both is aside from system characteristics strongly affected by climate. In an effort to provide increased energy efficiency, hybrid ventilation systems have been developed that integrate both natural and mechanical ventilation modes. These employ high-efficiency fans and optimised, segregated, automatically switchable internal air paths designed to benefit from maximising the application of natural ventilation, hybridmixing ventilation and low-energy mechanical ventilation with heat recovery. These include low pressure-loss heat exchangers to utilise space heat gains to temper incoming outdoor air. The heat exchanger trades thermal effectiveness for a low pressure drop, as illustrated in Figure 2. During the summer months, hybrid ventilation such as the example of Figure 3 employs the fan to boost the natural ventilation and optimise indoor comfort. During winter, the hybrid system mixes warm internal air with outdoor air to deliver Variable speed drive fan Required Required Heat recovery system (HRS) with thermal bypass facility Required Minimum required thermal efficiency of heat recovery, t Plate heat exchanger or thermal wheel 73% Run-around coil system 68% Efficiency bonus (E) Wm-3s-1 Plate heat exchanger or thermal wheel (t-73) x 30 Run-around coil system (t-68) x 30 Both medium and fine filters fitted 0 Missing medium filter (EN 779 M5)* 150 Missing fine filter (EN 779 F7)* 190 Both filters are missing Maximum PSFP (Wm-3s-1) (using reference configuration) 340 Plate heat exch., thermal wheel qnom < 2m3/s 1,100 + E-300 x qnom /2 - F qnom 2m3/s 800 + E - F Run-around coil system qnom < 2m3/s 1,600 + E-300 x qnom /2 - F qnom 2m /s 1,300 + E - F 3 *Note that the filter correction references the obsolete EN 779. Filters would now be specified to BS EN ISO 16890.2 Examples3 may be ePM2.5 65% for a fine filter and ePM10 55% for the medium filter. Table 1: ErP requirements and maximum SFP value for VY with supply and extract air (bidirectional airflow) mixed tempered air, and can also utilise the heat recovery function to save energy. The mechanical ventilation combines with openable windows to expel stale air and ventilates the space with outdoor air. Units can also include low air pressure drop cooling and/or heating coils (potentially linked to a heat pump) to control the supply air temperature during periods of higher cooling and heating loads. As with traditional recirculation fan-coil units, these can be used for pre-occupation preheat and night cooling. However, as might be suspected, such systems do not conform with the standard operation that would be expected of a BVU by EU regulation 1253/2014. To determine the optimum trade-off between higher electrical energy use and higher heat recovery, dynamic simulation models may be used. A study4 undertaken on behalf of a manufacturer compared the performance of hybrid and traditional MVHR units, by modelling systems for a newly constructed school in London. The study included models both with and without heat recovery. The calculation was completed using both the simplified building energy model (SBEM) and a dynamic simulation model (DSM) to evaluate the sensitivity of the applied modelling methods on the CO2 emissions and to produce an energy performance certificate (EPC) rating. The characteristics of a selection of Lower thermal efficiency Lower pressure drop Higher thermal efficiency Higher pressure drop Return from space Supply fan Medium filter Intake air Heat recovery system ErP 2018 Filter pressure switch Filter correction (F) Wm-3s-1 Extract fan Fine filter ErP stage 10.3 watts fan power @ 0.130m3.s-1 Figure 1: Sketch of generic bidirectional ventilation unit (BVU) 70.2 watts fan power @ 0.130m3.s-1 Figure 2: Average fan power consumed by plate heat exchanger as used in a hybrid unit, compared with that used in a traditional MVHR unit, at stated airflow rate and thermal efficiency, modelled for Birmingham, UK, simulated year-round operation (Image source: Monodraught) 18 November 2022 www.cibsejournal.com CIBSE Nov 22 pp17-20 CPD 206 Monodraught Supp.indd 18 21/10/2022 17:04