DOAS, Radiant Cooling

How is a DOAS + Radiant System More Comfortable than a Conventional Forced-Air System?

Comfort is key. Being comfortable majorly improves performance and mental acuity.That is why HVAC systems are necessary – they should keep you comfortable, and thus, performing well.

However, not all HVAC systems are equal in their performance and functioning; some systems will be better than others, particularly in some climates. As I have mentioned previously, research is increasingly accumulating to show that the best HVAC system for efficiency, health, safety, and comfort, is the “DOAS + Radiant” system. Here, I will elaborate on why that is the case for comfort, i.e. how is a “DOAS + Radiant” system more comfortable than a conventional forced-air system?

What are “DOAS + Radiant” and forced-air systems?


Before I can say why one is better than the other, I’ll just give a quick overview of what each system is.

Radiant Cooling

Essentially, the “DOAS + Radiant” is an HVAC system that decouples the handling of sensible and latent energy loads, with the unit that handles the sensible energy comprising a radiant heating/cooling system (e.g. chilled ceiling, underfloor heating). The DOAS component tightly regulates ventilation to occupied spaces, providing 100 % fresh, outdoor air.

Conventional forced-air systems usually handle sensible and latent energy simultaneously at a single air-handling unit (AHU). This air is then delivered via ductwork, plenums, and vents to occupied space. Variable air volume units can be installed to regulate the ventilation rate to individual occupied areas. Forced-air systems will recirculate a variable percentage of air (rather than use 100 % fresh, outdoor air), and temperature control is achieved through reheating air traveling to occupants.


Why “DOAS + Radiant” improves comfort compared to forced-air


  1. Decoupling the handling of the sensible and latent energy loads allows for more accurate, precise and reliable control of temperature and humidity. Forced-air systems typically use a single AHU to control temperature and humidity, which compromises the efficiency and effectiveness of both processes. Maintaining the temperature within an accurately controlled range is the most obvious way to improve comfort. There are four thermal space comfort parameters – air temperature, humidity, air velocity, and mean radiant temperature (MRT) – and two human thermal comfort parameters – clothing and metabolism. The more parameters of space thermal comfort that can be controlled apart from each other, the better the comfort. “DOAS + Radiant” systems provide this, because MRT is controlled by the radiant systems, and an air stream with higher air temperature and lower velocity and humidity can be controlled as well. Therefore, “DOAS + Radiant” systems have far less trouble maintaining a comfortable thermal environment for each occupant than conventional systems, especially under hot, humid conditions. Accurately controlling humidity is a less obvious means to improve comfort, as people are not as sensitive to changes in relative humidity as they are in temperature. However, controlling relative humidity better will reduce discomfort from allergens such as dust mites, or spores released from fungi, growing under high humidity conditions. Controlling relative humidity within the desired range (30 – 65 % according to ASHRAE Standard 55) will also stop occupants from getting sore throats, dry skin, or feeling irritable, clammy, or too hot (high relative humidity reduces evaporative cooling capacity). It is worth adding here that the precise humidity control offered by “DOAS + Radiant” HVAC systems means that condensation is eliminated. It is a myth that buildings designed to meet building codes have problems with condensation when fitted with these systems.
  2. Radiant heating/cooling allows targeting of specific parts of the occupied space, and even of the body. Forced-air systems fail to do this because they can only control the air temperature of the ventilated air. This improves comfort because it makes it possible to evenly distribute heating/cooling to the occupied space, such that each occupant is thermally targeted. Moreover, it allows a person’s head to be kept cool, and feet to be kept warm. This will maximize comfort but maintain mental acuity. Forced-air systems struggle to evenly distribute heating or cooling, which can cause discomfort to some occupants and not others. Importantly, what occupants feel (operative temperature) is a mixture of energy radiating (radiative heat transfer) from surrounding surfaces (floors, walls, ceiling) and the air temperature (convective heat transfer). Forced air systems (and their rooms’ thermostats), however, almost exclusively (95 %) address the temperature of the air (convection) to regulate to overall room temperature, whereas radiation is almost completely ignored. Since people perceive thermal comfort as radiation to at least 50%, the MRT is an extremely important factor for thermal comfort, and therefore for HVAC systems. This includes the type of thermostat used; thermostats should measure the operative temperature rather than only the air temperature.
  3. Radiant heating/cooling is silent and can be controlled independently of ventilation rate. Therefore, desired ventilation rates can be set, without heavily compromising temperature and humidity regulation. Forced-air systems struggle to meet temperature, humidity, and ventilation rate requirements simultaneously because they are all intrinsically linked in the AHU. This component of DOAS is key to comfort because comfortable, healthy ventilation rates can be maintained at all times, almost independent of the temperature requirements. On the other hand when lowering the space temperature with a forced-air system, the temperature of the air flow can be lowered to a limit of minimum 15 °C. When this does not support achieving the desired room temperature the air velocity has to be increased, resulting in more dehumidification of the air stream. This can cause significant discomfort in certain areas of the space, due to high velocities. This is a particular problem in the context of contemporary building codes, which stipulate specific ventilation rates to maintain adequate indoor air quality (IAQ) standards. When forced-air systems have to maintain a minimum ventilation rate to occupied spaces, they can struggle to simultaneously meet temperature and humidity requirements. This, in turn, creates discomfort, as discussed in point 1. The silence of radiant heating/cooling systems compared to forced-air systems can also significantly contribute to comfort. Ambient noise can be a source of irritation or distraction, detracting from comfort. Radiant systems provide thermal comfort without being seen or heard, which is optimal for comfortable working conditions.
  4. 100 % outdoor air means no recirculation, and therefore no redistribution or transfer of contaminants which is typical of forced-air systems, even with filtration.
    Outdoor Air
    The primary feature of DOAS systems is the fact that they deliver tightly controlled ventilation of 100 % fresh, outdoor air to occupied spaces. This means that any contaminants produced within the building will constantly be removed. A lack of contaminants is healthy, and vital for occupant comfort.




Being comfortable indoors requires a combination of various factors to be satisfactory. Lighting needs to be good; the chair needs to provide good support, even the color scheme of the workspace needs to be taken into account. However, when it comes to the part of comfort governed by good indoor air quality (IAQ) and satisfactory thermal environment, “DOAS + Radiant” systems will prove to greatly enhance comfort compared to conventional forced-air systems.



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