Refurbishment projects — how to achieve optimum indoor air quality

The need to provide healthy Indoor Air Quality (IAQ) in existing buildings creates unusual challenges for building services engineers. David Bradbury of SAV Systems explains how direct, zone-specific, demand-controlled ventilation can provide a valuable option.

Poor air quality in existing buildings has a significant impact on the comfort, wellbeing and productivity of occupants. With insufficient ventilation, offices or teaching areas will suffer from high CO2 levels and feel stuffy. There is mounting evidence that such environments cause drowsiness and lack of concentration. Poor temperature control is another important issue.

Remedial action to improve indoor air quality (IAQ) may be less than straightforward. If existing ventilation is based on centralised air-handling units, there are limits on additional load. Opening windows may be an alternative solution, but not where nearby roads pose noise and pollution issues. Room depth may also conspire against natural-ventilation solutions.

However, it is almost always the case that improvement in IAQ will be required in a refurbishment project.

Decentralised air-handling units such as these in a school (below right) and office (above) can be an effective approach to improving indoor air quality in refurbishment work.

Local response for maximum control

Solutions based on direct, zone-specific, heat-recovery air-handling units (AHUs) are gaining acceptance in the UK refurbishment market. ’Direct’ means that local AHUs are installed within each space to be ventilated — mounted at high wall or floor level and connected directly to the outside through an external wall or roof. Distribution makes use of the Coanda effect to move air across the ceiling, so that distribution and return ducting become unnecessary. This greatly reduces installation time.

Each unit has fans for supply and extract. There is a counterflow heat exchanger to recover up to 95% of the heat from the extract air, thereby reducing heating bills. Electronically commutated (EC) fan drive technology means that specific fan power (SPF) is kept within the range 0.7 to 1.2 W/l/s, with a favourable effect on energy ratings.

Crucially, given their location within the space, modern designs operate very quietly [35 dB(A) @ 1 m at full throughput, falling to 30 dB(A) at 80% throughput]. This has proved to be particularly important in applications such as classrooms, especially those used for special educational needs.

Demand control of supply air is achieved by linking fan speed to either CO2 level or relative humidity in the room space. CO2 sensors can be wall mounted or built in to the master air-handling unit (when multiple units are provided). Demand control means that CO2 can be held within a prescribed limit of, say, 1000 ppm, which is generally taken to be an indicator of good air quality.

Nor does this arrangement sacrifice the benefits of centralised control, as up to 20 decentralised AHUs can be slaved to a single controller or be controlled by a building-management system (BMS). At the same time they retain their ability to operate independently of each other.

To maintain good performance over the life of the equipment, efficient filtration of both supply and extract air must be maintained. Protecting supply flow is of benefit to occupants, while heat exchanger surfaces are kept clean by filtration of both supply and extract pathways. F5 class is the minimum recommended requirement to prevent deposits building up within the heat exchanger.

Ventilation standard EN 13779 recommends that filters should be specified to at least M5 (previously known as F5), to remove outdoor pollutants from supply air. A pleated design increases filter surface area and the rigid frame keeps a firm grip on bypass leakage. Reasonable care with maintenance of air filters is the best guarantor that units deliver on their projected 15-year life span.

By far the greatest contribution to energy saving comes from heat recovery. With a thermal efficiency of 84% (on dry bulb) or 95% (accepting humidity), demand-controlled air-handling units have an energy requirement of only 8% of a typical natural-ventilation system for tempering incoming cool air.

Associated control systems can ensure the AHUs are kept out of service at appropriate times. With movement-sensor (PIR) control, units have a limited run-on period (e.g. 30 minutes) following the last detected movement. With CO2 demand control, the unit is aware of low occupancy when CO2 content falls below, say, 750 ppm, at which point the unit ceases operation.

Decentralised AHUs can also be used to provide free cooling by operating an automatic bypass around the heat exchanger, so that the bypass portion of inlet air is unheated. By operating on full bypass at night, they can take advantage of lower ambient night time temperatures to cool the room’s thermal mass (night cooling).

Decentralised ventilation units have been used in a variety of refurbishment projects. They can deal with windowless ‘landlocked’ rooms within a natural-ventilation cluster, or serve rooms not connected to an existing centralised system. They can also bolster IAQ in areas where existing ventilation should be doing better, and ventilate rooms adjacent to roads, where windows must remain shut.

By helping to improve indoor air quality, direct ventilation units ensure that room occupants remain alert, more productive and can maintain good concentration right through the working day.

David Bradbury is product manager for ventilation at SAV Systems

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