Targeting ventilation to where it's needed

Maintaining acceptable indoor air quality in existing buildings with older ventilation systems can be a challenge. David Bradbury, product manager for ventilation at SAV Systems, explains how direct, zone-specific, demand-controlled ventilation can provide good solutions.

Poor indoor air quality (IAQ) in any building has a detrimental effect on the occupants, so it is essential to ensure that IAQ remains acceptable – and to do so as efficiently as possible.

In new buildings this issue is addressed by Part F of the Building Regulations. However, there are many offices, schools and other types of building that were built to less demanding standards and that now suffer from poor IAQ. This has an inevitable impact on the comfort, well being and productivity of occupants.

In existing buildings, tackling this issue can be challenging. If there already is an existing ventilation system based on centralised air handling units, there will be a limit to the additional load that can be imposed. In some cases, the re-design of windows can offer an alternative solution, but not where there are potential noise and pollution issues from nearby roads.

Yet the majority of refurbishment projects require an improvement in IAQ and it will be necessary to find an acceptable solution.

One approach that is growing in popularity in the UK is the use of direct, zone-specific mechanical ventilation with heat recovery units (MVHR). These are installed within the space to be ventilated and are ducted directly to the outside through an external wall or roof. Indoor units can be mounted at high wall or floor level. In the case of high wall-mounted units, the Coanda effect ensures that air moves across the ceiling and is evenly distributed, making ductwork unnecessary. This has positive implications for capital costs and installation time.

Each local MVHR unit uses separate fans for supply and extract, with a counter-flow air heat exchanger to recover energy from the extract air. With a thermal efficiency of 84 per cent (on dry bulb) or 92 per cent (accepting humidity), demand control air handling units have an energy requirement of approximately 8 per cent of a typical natural ventilation system.

Further energy savings are achieved by the use of electronic commutation (EC) fan drives so that specific fan power is within the range 0.7 – 1.2 W/l/s.

Modern designs operate very quietly (35dB(A) at 1m at full throughput, falling to 30dB(A) at 80 per cent) This has proved to be particularly important in applications such as drama and music rooms, as well as Special Educational Needs (SEN) facilities.

In keeping with current best practice, MVHR units lend themselves to demand-controlled ventilation. This is achieved by linking fan speed to either CO2 levels or relative humidity in the room space. CO2 sensors can be wall mounted or built in to the master air handling unit. Using demand control in this way means that CO2 can be held within a prescribed limit of, say, 1,000ppm, which is generally taken to be an indicator of good air quality. If CO2 levels fall below, say, 500ppm the unit can be set to switch off or go to maximum turndown, as this indicates low occupancy. The unit will then come back on again should CO2 concentrations rise 10 per cent above this level.

Where required, the control strategy can also be linked to movement sensors so that the units only start on signs of occupancy, with a limited runon period (e.g. 30 minutes) following the last detected movement.

Centralised control is also possible, as up to 20 decentralised MVHR units can be slaved to a single controller or be controlled by a building energy management system (BEMS). At the same time the heat recovery units retain their ability to operate independently of each other.

Additionally, decentralised MVHRs can be used to provide free cooling by automatic bypass around the heat exchanger, so that the bypass portion of inlet air remains unheated. By operating on full bypass at night, the system can take advantage of lower ambient nighttime temperatures during heat waves, to cool the room’s thermal mass (night cooling). This gives a fresh feel to a room in the morning.

Efficient filtration of both supply and extract air is also a necessity to protect occupants from outdoor pollutants and also to ensure heat exchanger surfaces are kept clean. M5 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, to remove outdoor pollutants from supply air. The filter surface area is optimised by using a pleated design, combined with a rigid frame to prevent bypass leakage. Maintenance of air filters is an important factor, helping to ensure that these units continue to operate efficiently and effectively through their projected 15-year life span.

Direct, decentralised heat recovery units have been proven in a wide variety of refurbishment projects, encompassing many different building types. They are ideal for bolstering IAQ in spaces where the existing ventilation is underperforming, as well as for areas that are not connected to an existing centralised system. Similarly, they provide an effective solution for windowless rooms within a natural ventilation cluster and for areas where windows cannot be opened because of proximity to roads or other sources of pollution.

Have your say...

Would you like to write your own Comment?

Your Comment

Your Name*
Please enter Your Name
Email Address*
Please enter an Email Address
Comment Subject*
Please enter a Comment Subject
Please enter your Comments
RefreshPlay AudioHelp
I agree to the terms of use.
Please agree to the terms

There were errors. Please see the messages above.