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How does the heat pump work in heating, cooling and defrost mode?
Heat pumps work similar to an air conditioner (AC) or refrigerator but in reverse (for heating cycle). It is actually smarter alternative to an AC, because it is designed for heating, cooling and dehumidifying. As the heat pump delivers more heat output than how much of electricity it uses, they are considered very efficient – up to four times more efficient than electric heaters.
Before explaining how does the heat pump work, let’s see what the main components of the air-to-air HP are. A heat pump uses several components to extract the heat and pass it further: evaporator, condenser, compressor, expansion valve, fan, refrigerant and few more. Some of the parts are described in this article.
First, let’s see how refrigerator works. At the back of this home appliance is the condenser which is always warm to the touch. This is because the unit’s refrigerant, in liquid state, while flowing through the evaporator, evaporates into gas, as it was absorbing the heat from inside the fridge, and cooling it down. Now the compressor pushes the gas to the condenser located outside where the heat is released into the room converting gas into liquid (condensation) again.
This type of heat pumps has three cycles: heating, cooling and defrost cycle.
In simple words: The outside component extracts the heat from the outside air and transfers it inside the house through the ductwork (ducted models) or directly into the room, such as ductless models.
Heating cycle simply explained is below, while thermodynamics and more here:
The refrigerant that is in liquid state with the low pressure and temperature goes through the evaporator (outdoor coil) where it absorbs the heat from surrounding, starts boiling and changing its state to low-pressure vapor (gas).
The heated vapor now goes through the reversing valve and the accumulator that collects any excess liquid refrigerant to prevent entering the compressor, where it is compressed (reducing its volume) into the high-temperature and high-pressure gas. Note that not all the heat pumps have the accumulator.
From the compressor the vapor is moved into the condenser lowering the temperature and changing its state from vapor back to liquid, resulting in the heat release, absorbed by indoor air, passing over the condenser coil and further used to heat the room.
The high-pressure liquid goes through the expansion valve where it gets expanded while decreasing the pressure to low - and from there the low-pressure and low-temperature refrigerant moves back into the evaporator, and from there the cycle is repeated.
The cooling cycle is similar to the heating cycle, but it is in reverse. The liquid refrigerant passes through an indoor coil-evaporator, absorbs the heat from the indoor air, boils and becomes the vapor. Now it goes through the reversing valve and accumulator and then to the compressor where it gets compressed causing it to heat up. From there, the hot vapor goes through the reversing valve and the outdoor coil-condenser causing the vapor to condense back to liquid state. Now the liquid refrigerant passes through the expansion device and the cycle is repeated.
As both the condenser and evaporators are heat exchangers, they change roles; the evaporator becomes condenser and condenser becomes evaporator. The evaporator coils extract the heat from the indoor air while passing over the coils, while the condenser rejects it outside. The refrigeration cycle in heat pumps is based on the second law of thermodynamics.
Another benefit in the cooling mode for the homeowners is when the moisture found in the air passes over the indoor coil, it gets removed, dehumidifying the air, while the condensate is collected at the bottom of the coil in a pan.
Air-to-air heat pumps, when operating in the heating mode can be affected by the freezing weather and therefore it is important to explain the defrost cycle too. When the outdoor temperature is below 32 F (0 C), the moisture in the air passing over the outdoor heat exchanger coil (evaporator) will condense and freeze up. The defrost cycle is then activated, melting the formed ice on the outdoor unit and allowing an unobstructed heat transfer between the air and refrigerant inside the coil.
When the heat pumps switches into the defrost mode, the reversing valve switches the heat pump to cooling mode so hot vapor goes into the outdoor coil to melt the ice buildup. During that time some cooling air can be noticed coming from the ductwork which is negligible because the outdoor fan is turned off.
Defrosting in heat pumps is very important because if the outside unit freezes, the efficiency of the unit significantly drops and it loses its ability to extract the heat. Defrosting can occur as needed (demand mode) or in set time-intervals regardless of whether freezing happened or not.
Today’s heat pumps are very efficient that can work not only in regions with the moderate temperatures such as older types, but in colder areas too, supply enough heat for home or water heating. If the heat pump cannot meet the required temperature levels, back-up heating is used, either in a form of electric or gas heating - found in more advanced models.
The heat can be extracted not only from the air, but from the ground and water too. Since it does not generate heat, but moves the heat from one place to another, heat pumps are very efficient in the heating mode.