For both space heating and air cooling, heat must travel. Reaching a comfortable indoor temperature is the objective in both cases of space heating and air conditioning systems. Despite the fact that heat can be transported through empty space by radiation, convection and the fluid’s bulk movement are significantly more efficient. The most often used fluids to transport heat in HVAC applications include air, water, refrigerants, and steam.
The heat distribution fluid used by an HVAC system determines numerous performance parameters because each substance has unique qualities. Also keep in mind that several fluids and intermediary heat exchange processes may be utilized in the same system.
Air Is Used To Distribute Heat
Air is already present in the atmosphere and indoor spaces, so there is no need to add more fluids to the HVAC system, which is the fundamental benefit of using air to deliver heat. Fans and ductwork are used to distribute the air once it has been directly adjusted in temperature by HVAC compressors, furnaces, or heat pumps.
Air ducts, on the other hand, take up more room than the pipes used by other heat-carrying fluids and are impractical when air must flow across a great deal of vertical space. When moving air against its natural tendency, warm air rises while cool air tends to fall below, hence the power of the fan increases significantly. This is neither useful nor energy-saving.
Air dampers are often used to regulate air distribution when various zones need the use of separate air ducts. These are automatically controlled by the thermostats in each zone and can be moved between fully open and fully closed as needed to manage airflow.
The addition of variable frequency drives (VFD) to the fans is one of the most promising system improvements for air distribution systems. When you don’t need the whole rated airflow, lowering the fan speed is far more effective than intermittent operation. Since brushless DC motors have built-in speed control, they are suggested for fractional horsepower fans instead of VFDs.
Packaged rooftop units are one type of HVAC system that primarily uses air for both heat transfer and heat removal.
Heat Transfer Using Water
Some HVAC systems use fan coils to circulate heated or cooled water, which subsequently interacts with indoor air. The installation is referred to as a hydronic system when this layout is used. Water’s higher specific heat and density allow it to hold a lot more heat per unit of volume when compared to air. In big commercial and industrial systems, hydronic piping is the preferred heat-carrying medium since it takes up significantly less room than air ducts for a given heating or cooling load.
Similar to how dampers and fans with VFDs may regulate airflow, valves and pumps with VFDs can control water flow in a hydronic system. Finding an operating point where each zone is maintained at the required temperature and humidity while using the least amount of energy is the fundamental idea.
Examples of HVAC systems that use water to transmit heat include chillers and boilers. Fan coils can then be used to heat or cool indoor air. Using larger air-handling units (AHU) connected to an air duct system, where heat exchange takes place between the hydronic piping and the air being circulated by the AHU, is another conceivable arrangement.
Refrigerant is used internally by all air conditioning compressors and heat pumps, however some HVAC systems have longer refrigerant lines connecting various pieces of equipment. In addition to air ducts, refrigerant lines are even more compact than hydronic plumbing. Similar to when water is used to conduct heat, valves and a compressor with variable speed control can be utilized to regulate the flow of refrigerant.
The condenser and evaporator units of ductless air conditioners and heat pumps are connected by refrigerant lines, and they often have very high efficiencies. The idea can also be used with a variable refrigerant flow system (VRF) to serve various zones from a single outside unit. While combining the heating and cooling systems into a single installation, VRF systems are also incredibly efficient.
Since steam is used as a utility service to a sizable area of the city, steam is used as a heat-carrying fluid in many structures as well. However, a hot water system is suggested over a steam system if you intend to install your own boiler.
The fundamental disadvantage of steam is that, in most situations, it can only be used for heating. The only way to achieve cooling using steam is through an absorption chiller, but in multifamily and commercial settings, a regular electric chiller is far more cost-effective. In contrast to when you are paying for steam as a utility service, absorption chillers are better suitable for applications where heat is available at extremely low cost or as a waste product of industrial activity.
Buildings with steam radiators frequently have window-style or through-the-wall air conditioners because steam cannot be used directly for cooling. You can think about switching to ductless units while the heating system is converted to use hot water because these typically have low efficiency.
The variety of HVAC systems is what makes them unique, and each design offers a unique set of performance features. To choose the system configuration that best meets the needs of your building, it is advised to speak with expert HVAC consultants.