Air conditioners have long been seen as devices for cooling the air on hot days. Some remember the period when the air conditioner was adapted only for cooling. For many years, air conditioners can also be used for heating rooms. Virtually every split device is equipped with a reversible heat pump, which means that its operation can be reversed. It is this function that opens the door to new applications of air conditioners, especially during periods of colder temperatures.
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The construction and operating principle of an air conditioner
The most important elements that allow the devices to perform their basic cooling or heating function are in the external unit:
-
four-way valve, - compressor,
- expansion element,
- exchanger (evaporator/condenser)
- fan.
In the indoor unit:
- exchanger (condenser/evaporator)
- fan.
The exchangers of the internal and external units of a room air conditioner , depending on the operating mode implemented, are alternately called evaporator or condenser.
- Evaporator – in it the agent evaporates into a gaseous state, releasing cold into the air
- Condenser – in it the agent condenses into a liquid state, releasing heat to the air
In practice, for the end user, the selected operating mode (cooling/heating) is assigned to the unit using the appropriate button on the controller.
Depending on the selected scheme, the four-way valve directs the refrigerant to the condenser. In the case of cooling, the external unit is the condenser. In the case of heating, the internal unit becomes the condenser.
Principle of operation of air conditioning in the cooling process
The refrigerant is compressed in the compressor, where both the pressure and temperature of the refrigerant increase. Then, the warm refrigerant is redirected to the 4-way valve, which determines the direction of the refrigerant flow, and thus the cooling/heating mode. In the case of the cooling mode, the refrigerant is pumped to the exchanger (condenser) of the external unit. The air circulation on the exchanger (condenser) is forced by the fan, the heat from the refrigerant is released to the outside air, and the gas condenses and turns into a liquid state.
The liquid enters the expansion element (capillary or electronic expansion valve EEV), through which the agent is throttled - its pressure and temperature are significantly reduced. The cooled agent in the form of a liquid is fed to the internal unit exchanger (in this case the evaporator). The internal unit is equipped with a fan, which forces air circulation. The agent evaporates on the exchanger (evaporator) and releases the cold into the air inside the room, thus performing the cooling function. At the same time, the agent is heated by the warm air drawn in by the fan from the room, therefore it initially changes its state to gas. Then it returns to the compressor and the process is repeated.
Principle of operation of air conditioning in the heating process
In heating mode, the home air conditioner works in the opposite direction. The refrigerant is compressed in the compressor of the outdoor unit, where its temperature increases. Then, the warm agent is redirected to the 4-way valve, which determines the direction of the flow of the agent, and thus the cooling/heating mode.
In the case of heating mode, the medium is pumped directly to the internal unit exchanger (condenser). The fan built into the internal unit forces air circulation on the condenser, and the heat from the medium is released into the room, while the gas condenses and turns into a liquid state (still under high pressure). The condensed gas in the form of a liquid enters the expansion element of the external unit, which may be a capillary or a thermostatic expansion valve EEV. The liquid is throttled - to reduce its pressure and temperature accordingly. The medium is fed to the evaporator of the external unit. The fan forces air circulation on the evaporator. The medium is heated by the external air and turns into a gaseous state. The heated medium in gaseous form returns to the compressor of the unit, where it is compressed again, increasing its temperature. The process repeats.
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Air conditioning heating in the transition period
Air conditioners with a heating function are an excellent solution for transitional periods (spring/autumn), when the outside temperature is not extremely low. Using such a solution, we can significantly delay the start of the main heat source in the fall, or turn it off earlier in the spring.
Air conditioning as a supplement to an alternative heat source
This solution will also work great as a supplement to an existing heat source. For example, after a long absence in winter, before the main heat source heats the rooms (especially with underfloor heating, which is characterized by high thermal inertia), air conditioning will quickly raise the temperature to a comfortable one. In the event of a gas boiler, solid fuel boiler or heat pump failure, an air conditioner with a heating function will save the situation for the time of repair.
Proper installation of the outdoor unit
It is worth mentioning that the outdoor unit should not be placed too low. While this is of little importance in the summer for the cooling process, in the winter there is a risk of being covered with snow. In addition, water draining from the drip tray equipped with a heater may freeze under the device. As a result of long-term frosts, ice accumulates, which threatens the device located above. It is worth using an additional heating cable in the pipe draining condensate from the drip tray.
Selecting air conditioner parameters for heating
First of all, you need to answer the question of how you intend to use the device. We will choose an air conditioner for cooling only differently than for heating as well. Here, we will focus on the aspects that we should consider when choosing an air conditioner that will heat rooms even at temperatures below zero.
The all-season heating kit is a heater kit that includes a compressor crankcase heater and a drip tray heater. The main purpose of the crankcase heater is to ensure that the oil or grease in the compressor is at the right operating temperature, especially in low ambient temperature conditions. Heating the oil helps to ensure fluidity and flexibility for the mechanisms inside the compressor, which can be especially important when starting the compressor in cold conditions. It works when the device is not working and the outside temperature is below 1°C. This heater helps to prevent moisture and condensation from accumulating inside the compressor, which could lead to corrosion and problems with the efficiency or durability of the device. Regardless of the selected air conditioner operating mode, condensate is formed in the system (evaporator), which must be drained from the system.
In the heating process, the evaporator is the external unit. And here, in winter, a drip tray heater is needed, which prevents condensation from freezing. Condensation is also, or perhaps primarily, created in the process of defrosting the exchanger. The air conditioner switches to the cooling function and heats the exchanger fins with warm air. Without a heater, the risk of freezing and ice accumulation is high and can damage the device. When choosing an air conditioner, it is worth checking whether such a set of year-round operation is standard with the device or whether it should be purchased separately.
Energy class of the device and efficiency
When examining these parameters, it is best to analyze the catalog card and energy label of the device. The Energy Efficiency Class for air conditioning ranges from D to A+++. Class A means the most energy-efficient device. It is worth mentioning here that the same device may have a different energy class in the cooling function and a different one in the heating function. The second important parameter to consider when choosing an air conditioner is the SCOP value.
It can be found in the device's catalog card and defines the ratio between heating efficiency and electricity consumption. To calculate the SCOP indicator, the air conditioner's operation at 4 different outside temperatures -7°C / 2°C / 7°C / 12°C is taken into account and by taking a weighted average from the appropriate formula, we obtain the seasonal energy efficiency for heating. So if SCOP is 3.6, it means that on average from 1 kW of electricity, the air conditioner will produce as much as 3.6 kW in heating energy. In short, the higher the SCOP, the more efficient and energy-saving the air conditioner is.
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