Controlling humidity - and costs

“Humidity control is expensive to install and operate and I think I can get away with cutting it out of the spec without anyone noticing, so everyone wins.”

Well not quite.

The Oxford English dictionary definition of air conditioning is: "A system for controlling the humidity, ventilation, and temperature in a building or vehicle, typically to maintain a cool atmosphere in warm conditions."

This definition has humidity in it for a reason, because to maintain a comfortable environment for humans, humidity control is an essential part of indoor air quality. 

To maintain good health in an office, school or care home, for instance, it is important to ensure that a stable humidity of around 50% relative humidity (RH) is created. Lower than this level (<40%) enables germs and pathogens to remain suspended in the air for long periods, so they can travel further and spread infection across a wider area. Viruses such as influenza and norovirus (the ‘vomiting bug’) survive longer at an RH of 20-30%, while a mid-range RH between 40% and 70% will minimise their survival rate. Also, tests indicate the infectivity of the influenza virus is increased by both low and very high RH, with minimum infectivity at 50%.

RH has been shown to have a similar effect on airborne bacteria, with intermediate levels increasing the mortality rate of airborne pneumococci, streptococci & staphylococci. Furthermore, a stable and appropriate RH eliminates static in the air, prevents sore eyes, reduces eczema, improves concentration and limits the loss of hydration among the workforce.

High humidity, above 60%, will cause people to feel uncomfortable out of all proportion to the actual indoor temperature, with reduced capacity to focus on their work. If comfort cooling is installed, occupants will typically reduce the set point to alleviate that discomfort, resulting in increased energy costs.

High humidity also encourages mould growth and consequent damage to the building’s fabric and fittings. It may also result in condensation on cold surfaces, potentially causing damage or creating a slip hazard.

Problems relating to RH in the UK tend to be because of low humidity. During the winter months buildings are usually heated to around 20°C and as the temperature rises, the RH falls dramatically. For example, outdoor air at -5°C & 100% RH has a moisture content of 0.0025kg moisture per kg of dry air. When raised to 21°C dry bulb, with no humidification control, the resultant RH is a mere 18%.

This brings the focus back to what we do with outside air as we pull it into buildings. Firstly, we need to ensure that pollutants are removed by applying effective filtration that is dictated by appropriate IAQ regulations.

Second, in addition to tempering incoming air to either raise or lower the temperature, we need to control the RH to ensure that it contributes positively to both thermal comfort and IAQ. 

What about the cost?

When talking about humidity control, ‘too expensive’ often refers to energy efficiency, and it is true that if you look at a humidifier in isolation it does consume energy - and in the case of a steam humidifier, a lot of energy. However, if you look at the air conditioning system as a whole, then the humidifier can not only be energy efficient, but it can actually save energy as well.

This is mainly thanks to the nature of the human body, which regulates its temperature by sweating, allowing moisture to sit on the surface of our skin before evaporating. The process of evaporation creates adiabatic cooling. This is where heat energy is required to allow this liquid to change state. This heat energy is taken from the body so cooling it down. The greater the differential between the humidity of the air and the body the greater the cooling effect. So, in a dry atmosphere the evaporation process becomes highly efficient and makes us feel cooler. In the winter this means that when the humidity is low, we tend to turn the heating up to help us feel warm - using more heat energy than you would need to humidify the air. 

A building’s energy consumption can actually be reduced with effective humidification. If a site has a high cooling load (like a datacentre, a busy office or a print room), then adding an adiabatic solution means you are humidifying and cooling the air at the same time. An adiabatic humidifier uses a tiny amount of energy (approximately 2kw to humidify 100 litres of moisture) compared to steam that may require 1kw / litre of water boiled. In addition, the process achieves cooling of approximately 20 watts to provide 2kw of cooling compared to a significant amount of additional energy if the cooling is provided using standard air conditioning equipment. All this means that you can control your environment more efficiently without adding to your energy bill.

Installing a humidifier into a building that’s still being designed is a simple process – just add the humidity system via the AHU, or through a standalone system like the new HomEvap humidifier (which can be used with fan coils or as a self-contained unit). With this approach the standard power, water and drainage services can all be integrated into the building management system along with the humidifier. Retrofitting is equally simple, and can often be done with minimal disruption, so the capital cost of the humidifier is often lower than expected.

So why would a building not have humidity control? it helps to reduce illness, eliminates static, increases human wellbeing and can reduce heating energy bills. This sounds like a no brainer to me.