Humidity control without high energy bills

Humidity control is vital in maintaining workplace comfort but often overlooked. John Barker of Humidity Solutions discusses energy-efficient retrofit options.

There can be no question that indoor air quality (IAQ) is critical to maintaining the health and productivity of the workforce. However, when addressing IAQ there is a tendency to focus on indoor pollutants and overlook the relative humidity (RH). However, failing to control RH can cause a wide range of health problems for occupants, interfere with the proper operation of office machines and damage the building fabric.

In addition, if the RH isn’t controlled properly (40-60% RH is the acceptable range) it can increase energy consumption. If it is too high people feel hot and sweaty out of all proportion to the actual temperature and are likely to use comfort cooling unnecessarily. If it is too low, people feel colder than is justified by the room temperature and turn the heating up. Ironically, raising the temperature also lowers the RH even further, thus exacerbating the problem.

These phenomena are well known and understood but humidity control is often removed from a design during a ‘value engineering’ exercise, leaving building operators with the prospect of retrofitting humidity control after handover.

Low RH is far more common than high RH in the UK and often results from the ventilation system being separate from the heating and cooling systems, such as when VRF or split system heat pumps are in use. This arrangement relies on the outdoor air used for ventilation having suitable moisture content. In the winter, though, cold outdoor air may have an RH as low as 25%.

One option would be to greatly increase the amount of outdoor air being introduced but the need to temper this air, and increase the fan power required to move it around, would greatly increase energy consumption.

In our experience the most effective and energy-efficient way to deal with low humidity, particularly when retrofitting to an existing building, is to introduce a separate humidification system that injects water vapour directly into the space.

The options available include atmospheric steam generators with fan boxes on top, or ultrasonic humidifiers spread around the perimeters of the building. However, both take up valuable floor space and do little to complement the aesthetics. A less intrusive option is to use wetted media placed above the ceiling, but this will require extra ductwork and diffusers.

An alternative that avoids these issues is a pressurised water system using compact multi-directional fan assisted nozzles mounted strategically in the space. The nozzles are about the same size as a CCTV camera and are served by a high pressure water ring main that follows cable routes and uses mechanical joints so no fire certification is required for the site.

Each nozzle can be controlled individually or collectively in a zone, via a central controller that maintains the water treatment plant and pumping station. The fan in each nozzle ensures the water is atomised and absorbed within 1.5 metres of the nozzle, enabling this system to be used in spaces with ceiling heights as low as 2.4m. The fact that cold water is used means there is no additional heating energy.

Clearly, such systems require appropriate water treatment and their design facilitates centralised water treatment at the pumping station so that access for maintenance is straightforward and does not disrupt the workplace. This is an important consideration, as all humidification systems, as with most building services systems, require regular maintenance if they are to continue to deliver the required performance.

In taking these steps to gain effective control over RH in winter, building occupiers don’t just get a healthy and comfortable workplace and reduce absenteeism, they are also able to turn the temperature set point down to reduce energy usage. So, when the full picture is taken into account, humidity control makes perfect sense.