Trying to think and work when you’re uncomfortable is difficult. Indeed, studies have shown that poor comfort leads to lowered productiveness. That’s why you have a HVAC system, to keep you comfortable, right? Well, maybe not.
Your HVAC system could be the problem!
While the job description of HVAC systems is to regulate the indoor environment to keep you comfortable and healthy, they don’t always do a good job of it. And it is the perception that HVAC systems are always good that may have stopped you from asking the question, is your HVAC system making you uncomfortable? Here are the reasons why your HVAC system may be a foe rather than a friend:
HVAC systems could be the cause of discomfort through three main pathways:
- Poor regulation of temperature
- Poor humidity control
- Poor filtration / source air
There are a lot of ways that these can happen with a HVAC system, but they can each contribute to your HVAC system making you uncomfortable:
Ventilation is generally the key component of good indoor air quality (IAQ). The importance of good ventilation toward providing a comfortable indoor environment is reflected by the fact that ventilation rate is the one aspect of HVAC systems that is most often mentioned in legislation on IAQ.Poor ventilation can mean that the rate of air turnover is too slow, air flow does not reach the space actually occupied by people working, too much air is recirculated, or the source of the air is inadequate. Poor ventilation can cause discomfort through all three avenues mentioned above.
Ventilation that delivers air unevenly in the occupied space can cause thermostats and humidistats to inaccurately gauge temperature and humidity, and if measurements are inaccurate, then regulation will be as well. Source air for ventilation that is contaminated or inadequate in some way can also cause discomfort.
Bad systems settings
This may be obvious, but it can be a significant source of discomfort for occupants. The system has to be set to heat, cool, or dehumidify air to within a range that maximises the number of people comfortable. To achieve this, the ideal indoor air temperature is generally considered to be between 20 – 24 °C in the winter, and 24 – 27 °C in summer, while relative humidity should be maintained between 30 % and 65 % at all times.
With that said, you must keep in mind that HVAC systems are designed to minimise discomfort and health and safety concerns, not eliminate them. Under ASHRAE standard 62.1, occupant satisfaction should be the vast majority (> 80 %), and contaminants must be kept below levels where they will adversely affect health. That means that some people will always be uncomfortable. What is good for some will not be good for others, given how different people dress, or differences in thermal physiology. So while good system setting will have most people comfortable, there will almost never be complete satisfaction.
In many HVAC systems, air is recirculated to save energy. If the recirculated air is not filtered properly, then occupants may be exposed to high concentrations of contaminants produced inside the building. Even if air is sourced primarily, or entirely, from outside, it may need to be filtered. In many areas, outside air is not better than inside air. This is particularly the case in many Asian cities, where pollution has created a highly unhealthy outdoor air quality.
In these cases, a high quality filter (MERV rating 6 or higher) will be necessary to maintain the comfort of the occupants. In addition, even good filters have a lifespan of optimal functionality before they need to be cleaned or replaced. Be sure that the guidelines for different filter types are followed accurately to ensure that they do their job.
Design and Components: The design and components of an HVAC system are what determine its ability to adequately regulate and control the overall indoor environmental quality, and thus the comfort of occupants. This is a broad category that would need wide discussion. However, for the purposes of this article suffice it to say that not all HVAC systems are made equal.
Over the last two decades, research has steadily shown that HVAC systems equipped with radiant temperature control (as opposed to all-air systems), as well as decoupled temperature and humidity control (as opposed to HVAC systems that have an air handling unit that deals with everything simultaneously) will maximise comfort. Particularly in hot, humid climates, your HVAC system may not be able to adequately deal with the temperature and humidity load, and therefore deliver inadequately regulated air.
It is worth noting here that components also require regular maintenance and cleaning to function properly. Your HVAC system may be making you uncomfortable simply because filters are overdue for cleaning, or some components are outdated and need to be replaced.
Furthermore, there is a common myth that contemporary HVAC systems such as “DOAS + Radiant” systems experience problems with condensation when designed to meet building code standards. The precise humidity control offered by these systems eliminates any problems with condensation, and they should be considered entirely appropriate for modern buildings, even in very high humidity locations.
The underlying causes of discomfort
The underlying reasons that each of these aspects of HVAC systems make you uncomfortable is because temperatures and humidity levels outside of a particular range begin to become uncomfortable. Humidity that is too low will cause skin dryness and sore throats among other things. Humidity that is too high will reduce your capacity to dissipate heat, and begin to make you feel clammy or too warm. Temperature is the most obvious criteria for comfort. Being too hot or too cold significantly contributes to discomfort; indeed, studies have shown that temperature is the most important factor in the indoor environment for perceived comfort (Figure 1).
Figure 1. The proportion of office workers who were comfortable across a range of indoor air temperatures. Taken from Nicol J, Humphreys M (2004) Adaptive thermal comfort standards in the hot–humid tropics. Energy and Buildings 36 (7):628-637.
In terms of temperature it is critical to note that the temperature people feel, the operative temperature, is not just the air temperature (convective energy transfer). Operative temperature also includes energy transfer from contact with surfaces (conductive energy transfer) and from energy emitted from the surfaces (radiative energy transfer). The operative temperature people perceive can be made up of over 50 % the mean radiative temperature (MRT). That means that if your HVAC system does not account for radiative energy (as is the case for conventional all-air systems), the thermal comfort of occupants can be considerably reduced.
Beyond the obvious physical discomfort that comes from poor humidity or temperature control, the other major underlying cause of discomfort stems from HVAC systems that do not properly control humidity levels, filtration, and recirculation. All of these can allow the proliferation and distribution of allergens and disease causing pathogens.
High humidity levels allow the growth of dust mites and fungi, which are significant sources of allergens that cause severe discomfort. High humidity that leads to condensation can also allow mould to grow that adds to visual or odour discomfort (in addition to damaging building materials). Poor filtration and recirculation also allow contaminants to be brought in and delivered to occupants, making them sick or uncomfortable.
How to get comfortable again
The solution to a HVAC systems that is making you uncomfortable is to ensure that all these factors are accounted for and incorporated into the HVAC design and components. For example, a dedicated outdoor air system (DOAS) coupled with a radiant cooling system will deliver optimal thermal comfort to occupants (incorporating the MRT aspect of the thermal comfort environment), while ensuring that ventilation rates are adequate, and that air is delivered directly to occupied spaces as needed. Moreover, this system will decouple the handling of sensible (heat) and latent (moisture) energy, which allows more accurate and efficient control of both.