Modern Building Services

18 MODERN BUILDING SERVICES MARCH 2023 FEATURE WORKING BUILDINGS F irstly, when AHUs are procured, particularly via the Design and Build route, minimising the ongoing maintenance and running costs for the AHU is normally less of a priority than lowest initial CapEx costs at tender stage. Even with the traditional design route, consultants with the best intentions are often asked to compromise on measures that will perform better in the long term, for those that are initially cheaper. Secondly, once a building is in the operational phase of its life, FM contractors who maintain the HVAC systems may not have the same priorities as the end client. Billing regular site visits and consumables is better business for them than investigating how much can be saved by looking at a holistic change to the way the existing HVAC is operated and maintained. Their limits of expertise may also mean that they won’t directly be able to offer clients the fan upgrades/optimised filtration and Indoor air quality sensors required to achieve the maximum energy savings possible. Guidance for specifiers and building owners Here at Arm Environments, we aim to guide specifiers and building owners through available options that specifically suit the way their building is or is due to be operated. There are battling priorities for building operators at present. A rapid increase in awareness around the importance of air quality is driving building operators to provide higher ventilation rates and better quality air. This, when delivered with a standard ventilation system will increase energy use and maintenance requirements. It is possible to optimise HVAC systems to provide good indoor air quality without using toomuch energy, but it does involve taking a deep look at the HVAC design. There are two main aspects of an air handling unit that will affect how much energy it uses; the first is the grade of filtration, and the second is the efficiency of the fan and motor driving the air flow. While these items can be looked at in isolation, it is best to take a combined approach when looking to upgrade an air handling unit. Standard filters in air handling unit systems are primarily designed to protect and keep the ductwork clean, and typically selected on a rule of thumb basis (G4 prefilter/ F7 Final filter in old money EN779 terminology) rather than selected to provide a particular quality of air (see Eurovent REC4-23 for guidance1.). The grade of filters required depends on the outdoor air quality (see Eurovent REC4-23 for guidance 1) . It is possible to gather local air quality data to feed into selection software when modelling a filters’ longevity or selecting the appropriate grade. Unfortunately, the type of filter used in an air handling unit is normally chosen based on initial CapEx only, with no regard to the OpEx costs of that filter both in terms of replacement intervals, the labour to change, nor the energy used by that filter as it starts to clog. There is an industry-wide assumption that filter grades cannot be upgraded in air handling units because of an inevitable increase in pressure drop. In the example shown in table 1, we demonstrate that both the filtration efficiency can be increased at the same time as reducing the pressure drop by using a different specification of filter. Based on the selection shown in table 1, based on current energy costs, the annual energy cost for the filters in scenario 1 is £8,280, for scenario 2, the energy cost is £5,447. It is important to highlight that despite the higher energy efficiency, the filters selected in scenario 2 are a higher grade and offer superior filtration performance. Standard bag filters with low numbers of pockets are cheaper (~£25) than long life mini pleated ECO filters (~£70), but the latter has a higher dust holding capacity. For PM1 it’s 41g vs 68g, and for PM2.5 it’s 57g vs 89g, so the ECO filter will last longer, resulting in reduced maintenance visits. The additional saving in the compact filter comparison shown in table 2, comes from the lower initial pressure drop of the compact filter (145Pa vs 65Pa). This results in an energy saving of £1,404 (17%) between the two selections. It will be difficult to realise the full potential cost savings if the maintenance regime is not altered to match the new filters. The unknowns surrounding when to change a filter means that many filters will either be changed too often or too infrequently. Few AHU’s have working pressure drop sensors fitted to them. They may have magnehelic gauges in the plant room or on the roof but these are not regularly checked, as this requires an expensive specialist visit. Pressure differential sensors can be connected to the building management system to alert AHU design and operation, a modern approach AdamTaylor , CEO at ARM Environments, explores the two main reasons why HVAC systems cost more than necessary when it comes to energy use and planned maintenance. Table 1 Compact filter comparison – 2” panel and E rated 535mm ePM1 60% 8 pocket bag vs. 2” panel and A+ rated ePM1 60% compact filter. Product Basic Design Nominal Airflow Rate [m£/h] Energy Class Annual Energy Consumption [kWh] Total Energy Cost [GBP] PM1, particle removal efficiency [%] Stage 1 ePM10 80% Panel 0.944 E >1400 3360 N/A Stage 2 ePM1 60% 8P Bag 0.944 E >2050 4920 64 Stage 1 ePM10 80% Panel 0.944 E >1400 3360 N/A Stage 2 ePM1 85% 10P Bag 0.944 A+ 882 2117 85