When it comes to humidity, energy efficiency is key

When an existing building lacks effective humidity control it may be necessary to retrofit humidification. This needs to be done in the most energy-efficient fashion, says John Barker, a director of Humidity Solutions Ltd.

In recent years there has been growing emphasis on the importance of maintaining indoor air quality (IAQ) in the workplace. However, the focus is very often on avoiding a build-up of pollutants in the air, with less attention to relative humidity (RH).

As a result, many older buildings (and even some relatively new ones) were not fitted with specific humidity control systems, which are often ‘value-engineered’ out of original specifications. These buildings are therefore vulnerable to fluctuations in RH caused by factors such as natural ventilation/ leaky buildings, air conditioning, operation of the heating system etc.

For example, when the building is constructed to modern standards of airtightness there may be problems because of reliance on mechanical ventilation systems. The effectiveness of these depends on the outdoor air having a suitable moisture content. In fact, outdoor air in cold weather is often very low in moisture, and heating it when it enters the building will create very low RH levels.

Consequently, in the UK it is far more common to experience problems with low RH than with high RH, especially in the winter when RH levels may well fall below 20 per cent in a heated building during the winter.

Increasing the amount of outdoor air being introduced the building will not help as the moisture content is low - plus the need to temper this air and increase the fan power required to move it around would greatly increase energy consumption.

Humidity control via the addition of a humidifier is therefore the only way to achieve comfortable conditions.

The acceptable RH range for commercial workplaces is 40-60 per cent and this suits both human and machine occupants. When the RH is either side of this range, problems can occur. Above 60 per cent RH people feel hot and sweaty out of all proportion to the actual temperature. This means that where comfort cooling is installed people are likely to lower the room set point temperature so that energy consumption increases. If there is no comfort cooling, people’s capacity to focus on their work is impaired under high RH conditions.

Below 40 per cent RH people will feel colder than is justified by the actual temperature and will often turn the heating up to compensate – once again wasting energy. Ironically, raising the temperature also lowers the RH even further, thus exacerbating the problem.

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 will inject water vapour directly into the space.

In the majority of workplaces, it also needs to be a system that can be used in conjunction with relatively low ceilings.

There are several ways of achieving direct injection of moisture into a room and it is important to be aware of the different characteristics of these to ensure the best solution is specified.

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

An alternative, that is discreet, does not take up floor space and is easy to retrofit, is a pressurised water system using compact multi-directional, fan-assisted nozzles, so that the water is atomised and absorbed into the air (adiabatic humidification).

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.

Nozzles can be controlled in small groups in a zone, with multiple zones possible from one central controller that maintains the water treatment plant and pumping station. The fan in each nozzle ensures the water is atomised and absorbed within 1.5m 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 required for the humidifier and free cooling is provided, which can generate additional energy savings by reducing the load on comfort cooling systems.

The design of pressurised water systems facilitates centralised water treatment at the pumping station which can be housed in an appropriate equipment plant room or store area so that access for maintenance is straightforward and does not disrupt the workplace.

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 environmental and economic sense.

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