In HVAC, two things which are required the most are energy efficiency and optimum heating or cooling with proper ventilation. These are the core working areas of HVAC (heating, ventilation, and air conditioning). HVAC functions on the property of heat exchange, where hot media transfers heat to the cold media, causing hot media to become cold and cold media to become hot.
Refrigerant gas is used in the system to achieve this task. Apart from this, there is one more system called heat recovery where instead of just extracting air and replacing it with other air, the heat from the extracted air is passed to the other air. In this post, we will see the concept of heat recovery in HVAC.
How important is heat recovery in an HVAC system?
Consider a room which is air tight and without any ventilation. In winter conditions, you will just receive cold air from outside, and in summer conditions, you will just receive hot air from outside. Due to this, there is no proper control for the temperature and moisture of the air flowing in your room. For this reason, instead of just getting air from outside, a system can be employed where the heat from both the systems, either hot or cold, will be exchanged and result in heat transfer theory.
Now, in an air conditioning system, you anyways have refrigerant gas which is the main driving component for compressors, condensing fans, and expansion valves. They flow through heat exchangers to make hot air warmer or colder inside a room. But, you can also use just the outside air quality to reduce the energy usage of such air conditioning units and make your room warmer or colder, depending upon the environment. This is heat recovery and due to this, an optimum use of outside air is done and also results in a lot of energy savings.
This means, that in air conditioning systems, heat exchange is done between refrigerant gas and air for cooling; but in heat recovery, heat exchange is done between outside air and inside air. So, heat recovery plays a very important role in HVAC systems.
How does heat recovery work?
Refer to the below image. The heat recovery works through a unit called a heat recovery ventilator or HRV. In the HRV unit, there are two blowers – one for extracting indoor air and one for extracting outdoor air. Before throwing the air to the other layer, a filter is used to remove the dust, dirt, and other impurities. Now, there will be two cases where HRV will be used.
In the first case, if the climate is cold, then the hot stale air from the indoor room will be extracted by the blower and thrown in the outside air. In return, the fresh cold air from the outside air will be extracted by the other blower and thrown into the inside room. In between, the hot air from indoors will exchange heat with the cold air from the outdoors and the cold air from outdoors will exchange heat with the hot air from the indoors.
Due to this, when the air reaches the indoor room, the temperature will be a little warmer (as the cold air temperature will increase slightly, but will be less than the present hot air inside) and when the air reaches the outdoors, the temperature will be a little colder (as the hot air temperature will decrease slightly, but will be more than the present cold air outside).
The airflow happens between these two lines without any intermixing, as the pipelines or ducts are constructed accordingly. Due to this technology, the air quality of the room has improved drastically. It will not cause a major change or dip in temperature, but continuous air quality improvement is ensured.
The second case will work in summer conditions. It will work opposite to the winter operation. Here, the fresh warm air from outside will be exchanged with the cold stale air from inside.
One thing to note is that many people ask as to why we use air conditioning or heat pump systems. Here, I will clear two doubts about it. First, these two systems are the ones which can provide you with a high increment or decrement in temperature, as per your desired setpoint. So, in heat recovery systems, you will not get a drastic change in temperature, but the air quality will be improved drastically here.
Secondly, you can also install both HRV and air conditioning/heat pump systems simultaneously. It entirely depends on the design and requirements of the temperature and humidity. Overall, due to HRV, your ventilation system improves, bacteria and foul growth in the air are eliminated, and harmful pollutants are removed.
In this way, we saw the concept of heat recovery systems in HVAC.
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