What are HVAC parameters and why are they calculated?

Before we start, we need to understand what HVAC is. HVAC (Heating, Ventilation, & Air Conditioning) is a technology system for maintaining the necessary requirements for temperature, humidity and air purity in an indoor space. This definition is applicable for any closed premises in which it is meant the presence of a person, animals, plants. Also, various equipment and art objects cannot exist or work propery without the operation of HVAC systems.

HVAC systems are divided according to the principle of location into central and local. Central heating and air conditioning units are usually located outdoors, and the delivery of cooled / heated air or water is carried out using duct systems and pipelines. Local systems can be located in close proximity, on a wall right behind the window, or in each separate room for example. The list of heating and cooling systems also include various fan coil units, ventilation systems, humidifiers and dehumidifiers, ionizers and even underfloor heating (both electric and water). Please note, that the organization of the HVAC system does not have to include all of the above. The main task for these devices in various combinations and layouts is to maintain air parameters within the required limits.

The list of estimated parameters in the design of such systems includes a set of few fundamental factors for maintaining a comfortable state in any enclosed space.

• air temperature
• radiant temperature
• air velocity
• humidity

We know for sure that any heated body is capable of radiating heat into the environment. The temperature of this radiation significantly affects the air temperature in the room, in its individual zones, and also directly affects how a person receives or loses heat while in this room.

So how does the air speed in a room affects the overall feeling of comfort? – imagine a small office room, about 35-55 square meters, in which 4-6 people work at computers. With minimal air movement into the room, there is a feeling of "stagnation" and "stuffiness", which naturally negatively affects the performance and general condition of a people that live or work in the room. Several open windows in the “ventilation” mode can increase the speed of air movement inside the room, which will significantly affect the forced convection of heat from a person into the environment and we will feel a draft. This effect is especially noticeable in the cold season.

On the one hand, air humidity is not the most difficult parameter in our comfort formula, however, it takes a lot of effort to maintain it within comfortable or specified limits. The optimal limit of the level of humidity in the room for a comfortable stay of a person is defined as 40 - 70%. High relative humidity values ​​prevent sweat from evaporating from the human skin, while too low values can lead to a feeling of dryness, which mainly affects the mucous membranes. I think that many people are familiar with the feeling of dryness in the nose, unpleasant itching, and so on.

Despite the fact that most of the examples are given in accordance with the feeling of comfort specifically for a person, these parameters, within completely different limits and assumptions, are also valid for animals, plants, various equipment and art objects in the room.

The thermal resistance of clothing directly affects the maintenance of a comfortable state of a person in environmental conditions in a room or transport. Too much clothing can be a major cause of heat stress, even if the environment is not considered warm or hot. Conversely, if the clothing does not provide sufficient thermal insulation, hypothermia can occur, and sometimes more serious injuries associated with frostbite of the limbs.

Our metabolism is another important factor. The more we move, the more physical activity is present in our activities, the more heat we produce. The body must dissipate this heat into the environment. Thus, even sitting at the table, with minimal physical activity, we heat the air around us with body heat. The metabolic rate actually describes the rate of conversion of chemical energy into heat and into mechanical work due to metabolic activity within the body. For example, a person walking at a speed of 5 km/h releases 110 W/m2 of heat from the body surface. Sedentary work (office, remote work from home, school, university) causes our body to release approximately 70 W/m2 of heat.

Now try to combine all these parameters into a single mathematical model. How to account for everything at once? Even with the use of averaging and simplifications, the task does not look trivial and requires serious calculations. When designing living and workspaces heating and cooling specialist should use proper skills and tools for modeling and calculating HVAC systems. Combinations of the factors described earlier, as well as taking into account various medical requirements, allow specialists to predict and model various situations using engineering analysis tools to achieve the best possible results.

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