Delivering Good IAQ Doesn't Have to Cost the Earth
This article was first published on Modern Building Services.
It is possible to deliver good IAQ without creating significant CO2 emissions, but it requires a data- led approach that has not been commercially viable until now. In the height of the pandemic, the World Health Organization (WHO) released updated guidance on the acceptable levels of air pollution required to protect people’s health.
Approved document F – Ventilation (Part F), now requires building ventilation systems to limit the ingress of pollutants listed in the WHO guidance into buildings.
A typical ventilation system will be operated via a timeclock, with 100% of ventilation capacity being supplied during all core occupied hours. During the pandemic, building operators were advised to adjust their ventilation systems to run for two hours before occupation, and for two hours after in order to flush out potentially contaminated air. This is an inefficient way to run a ventilation system.
Demand Controlled Ventilation (DCV) is an effective way to reduce the costs associated with running an HVAC system in buildings with varying levels of occupancy. The extent to which the HVAC system runs is typically related to the Carbon Dioxide (CO2) concentration monitored within the building.
The CO2 concentration is closely correlated with the number of people in the space. The drawback of these types of systems is that people using the building outside of core hours, for example cleaners and security staff, can be put at risk from pollutants building up inside an unventilated building. This issue can be partially addressed by ensuring that the building has a minimum ventilation rate outside of core hours.
Decades old developments
Air Quality Controlled Ventilation (AQCV) is a development conceived decades ago that allows a buildings ventilation rate to vary according to multiple indoor air quality parameters. If IAQ levels are poor, the ventilation system will deliver a higher volume of outdoor air through the Air Handling Unit (AHU) to dilute the pollutants, this way of operating a ventilation has only recently become feasible due to the rise in availability of low cost IAQ sensors. The success of these systems is dependent on the filtered supply air containing lower levels of pollutants than the air already inside the building.
Air Quality Determined Ventilation (AQDV) is a recent development that allows a buildings ventilation system to modulate according to both the indoor and the outdoor air quality (OAQ). This innovation is only now possible due to the availability of both low cost IAQ sensors, accurate granular OAQ data and concessions made in Part F. OAQ varies day by day, and hour by hour, it cannot be assumed to be “fresh air” all the time. Section 2.6 of ADF states “where sources of pollution vary with the time of day, such as urban road traffic, it may be acceptable, for time- limited periods, to take one of the following actions.
A. Reduce the flow of external air into ventilation intakes.
B. Close ventilation intakes when the concentrations of external pollutants are highest.
If levels of OAQ are poor, and the IAQ is good, the ventilation system can switch to re-cycle a larger percentage of the air through the Air Handling Unit (AHU). Re-cycling the air for long periods of time with little dilution with outside air would typically result in unsatisfactory levels of odours and gaseous compounds. To address this, air cleaning technologies need to be deployed within the AHU to remove them including combined ePM10 and molecular filters. This “clean first” principle has been demonstrated in buildings built to ASHRAE 62.1 standards for decades.
IAQ sensors form an integral part of this solution. For large open plan spaces, sensors in the supply and return ducts are sufficient to measure the air quality. In buildings with varying loads throughout the space, a higher density of sensors needs to be used in conjunction with volume control dampers.
These will need to be retro-fitted in buildings that were previously operated on a timeclock. The maintenance costs of running a ventilation system effectively can be significant. On top of the energy costs of heating, cooling and dehumidifying air, regular routine on-site inspections and replacement consumables also add to the operational expenditure (OpEx).
Filters used in AHUs are typically supplied by the manufacturer of the AHU in the first instance, these are likely to have been sourced from their preferred supplier, with EN779 classifications of G4 pre-filters and F7 final filters being the typical setup. On an ongoing basis, filters are likely to be sourced from the maintenance contractors preferred supplier, with the decision typically being made on Capital Expenditure (CapEx) cost only. Little thought is usually given to the filters particle removal efficiency (to ISO 16890), the lifespan of the filter or its Eurovent energy rating.
When a contract is in place for routine replacement of filters, building operators don’t give this aspect of HVAC OpEx much thought, but there are significant savings to be made in both CO2 emissions and material waste. As an example, two “F7” filters (both rated at ISO 16890 ePM1 60%) from the same manufacturer can have wildly different energy ratings.
Compare an 8 pocket bag filter with an annual energy consumption of >2,050kWh vs a compact V filter at 808kWh. The more energy efficient filter will also require replacement 50% less often. The higher performance filter is more expensive from a CapEx perspective, but the reduced OpEx will still give a ROI a couple of months.
Pre-filters on an air handling unit require more regular replacement than final filters, but this disparity can be partially mitigated by fitting long life pre-filters. If due consideration is given during filter selection, the timing of pre-filter replacement can often be aligned with the need to replace final filters. If AQDV is utilized, then intake filters will have a longer service life, as they will not become loaded with dust during periods with high pollution levels. This saving on consumables will only be realised if the condition of filters is monitored, and their change frequencies being determined by their pressure drop rather than routinely being changed irrespective of their dust loading. Filter monitoring is particularly relevant when DCV, AQCV or AQDV systems are deployed in buildings where a hybrid-working culture means that buildings are sparsely occupied much of the time.
Newly available cloud-connected sensors can be incorporated into an HVAC system to allow performance and failures to be monitored from a remote location, even if there is no Building Management System (BMS). If only some parameters are remotely monitored, then the full reduction in OpEx cost cannot be realised, as routine inspections will still be required for some components, incurring travel and labour costs and the associated CO2 emissions.
In addition to filter load status, filter pressure drops, AHU runtime and airflow supplied, UVGI run- hours, fan energy consumption, fan status, outdoor air quality and indoor air quality can all be sent to the cloud to give a full picture of the system status.
Fitting a UVGI system to irradiate the cooling coil and drain pan removes the need for routine mould and biofilm inspections, deep cleaning of the cooling coil and improves the efficiency of the cooling coil. In accordance with Part F of the building regulations says that, in certain buildings and HVAC configurations, an Ultraviolet Germicidal Irradiation (UVGI) system should be installed within the AHU to protect occupants from the risk posed by re-cycled airborne pathogens.
Older style fans with belt drives need periodic inspections, lubrication and belt replacements. These should ideally be upgraded to energy efficient direct-drive EC fans.
Some manufacturers offer cloud- connected fans that incorporate a number of sensors to report any issues ahead of failure, they also come with sealed-for-life bearings, and don’t utilise drive belts.
All the components described can be incorporated into new air handling units, or as part of a comprehensive AHU refurbishment, which is around 25% cheaper than the cost of a brand new unit. The optimal time to upgrade to this modern way of operating is when an AHU reaches the end of its serviceable life, as the cost of replacing fans does not need to be included in the ROI calculation.
Many of these upgrades can be fitted individually to certain AHUs and many have a short ROI, but when deployed together, combined with a different approach to maintenance, the biggest reduction in OpEx can be realised. Efficient operation of AHUs is seldom high up on the list when an organisation comes up with ideas to reduce its CO2 emissions, but it should be.
There are significant savings to be made, particularly if you consider whole life carbon emissions.