A heat pump is a device that collects heat from the outdoor air, the ground or a body of water through an electrically driven refrigerant cycle and moves it into a building. It does not make heat by burning fuel; it takes heat that is already there from the cold side and pumps it to the warm side. That is why it delivers several kilowatts of heat to the building for every kilowatt of electricity it uses. In summer the same unit reverses the cycle to provide cooling, and it prepares domestic hot water all year round.
Moving heat instead of making it
Heat flows by itself from warm to cold: in winter, the warmth of a house escapes through its walls, roof and windows. A heat pump turns that flow around; it takes heat from the cold outdoor air and releases it into the warmer interior. Like a water pump lifting water uphill, this takes energy from outside. In a heat pump, that energy is the electricity that drives the compressor.
Air at −10 °C feels cold, but it still holds heat; it is far above absolute zero, −273 °C. As long as the refrigerant is kept colder than the outdoor air, it can draw that heat in. A refrigerator works on the same principle: it takes heat from inside the cabinet and rejects it into the kitchen through the coil at the back. A heat pump scales this up to a building; the outdoor air becomes the inside of the fridge, and your building becomes the kitchen. A machine that uses the same cycle for cooling only is called a chiller; we compare the two jobs in our article on the difference between a heat pump and a chiller.
The cycle in four steps
Inside the unit, a refrigerant circulates in a closed circuit. At low pressure it evaporates at a low temperature; at high pressure it condenses at a high temperature. The heat pump uses this property and repeats the same four steps on every pass:
- Evaporator: The low-pressure liquid refrigerant passes through the finned coil of the outdoor unit, takes heat from the air blown across it by the fan, and evaporates. At this pressure the refrigerant is colder than the outdoor air, so heat flows into it on its own.
- Compressor: The vapour is compressed, and its pressure and temperature rise. Most of the electricity the unit draws is used in this step.
- Condenser: The hot, high-pressure vapour gives up its heat to the heating water in a heat exchanger (usually a plate type in residential units) and condenses back into a liquid. This is where the water for underfloor heating, radiators or the hot water cylinder is heated.
- Expansion valve: The pressure of the liquid is reduced as it passes through a valve. The refrigerant cools, returns to the evaporator, and the cycle starts again.
Which refrigerant circulates in the circuit matters too. It determines how hot the water can be at a given outdoor temperature, and what effect a leak would have on the climate. New units are moving to refrigerants with a low global warming potential; propane, R290, is the best known of them.
A control board runs the compressor speed, the fan and the expansion valve. In units with an inverter compressor, capacity is adjusted steplessly to the heat the building needs at that moment. Instead of switching on and off, the unit keeps running at a lower speed, which benefits both comfort and efficiency.
Heating, cooling and hot water from one unit
A four-way valve in the circuit changes the direction in which the refrigerant flows. In winter the outdoor coil acts as the evaporator and the indoor heat exchanger as the condenser; in summer the roles swap. The indoor heat exchanger now chills the system water, and the heat taken from the building is rejected outdoors. The chilled water goes to fan coils or to an underfloor cooling circuit.
One more difference shows in heating: a heat pump produces water at a lower temperature than a boiler, and for longer periods. Underfloor heating runs on water at 30–40 °C, so it is the system that suits this best. In buildings with radiators, the water temperature they need should be known in advance and the unit chosen for it.
For domestic hot water, the unit diverts its output from the heating circuit to the coil of a cylinder or tank. In AD systems this is handled by the indoor unit: the HİDRA indoor unit with tank carries an 80–140 litre tank in its cabinet, while the wall-hung HİD / HİT feeds an external cylinder. In homes without fan coil pipework, the HİDREM indoor unit cools through air conditioner indoor units, and in summer it transfers the heat taken from the rooms into the water in its tank.
There is one more detail in winter. When the air is around 0 °C and humid, ice builds up on the surface of the outdoor coil. The unit detects this and briefly runs the cycle in reverse to melt the ice. These defrost periods are normal, and a well-designed control keeps them as infrequent as possible.
Why does 1 kW of electricity give several kW of heat?
An electric heater turns each kilowatt it draws into one kilowatt of heat and can give no more. In a heat pump, the electricity is not the heat itself but the energy of the compressor that carries the heat. Part of the heat delivered to the building comes from that electricity, the rest from the outdoor air. The ratio is expressed as the COP (coefficient of performance): a unit with a COP of 4 delivers 4 kW of heat from 1 kW of electricity, and 3 kW of that was collected outdoors.
The COP is not fixed; it falls as the temperature difference between the outdoor air and the system water grows. The same unit is more efficient making 35 °C water at +7 °C than making 55 °C water at −7 °C. We explain the difference between COP and SCOP, and how much electricity a house really uses, with a worked example in our article on heat pump electricity consumption.
Types by heat source
Heat pumps fall into three groups according to where they take their heat from. Air source units use the outdoor air; since they need no drilling or wells, they are the most common type. Ground source systems use the heat of the soil, which changes little over the year, through pipes laid underground, and water source systems use a lake, the sea or groundwater. The second distinction is how the heat is delivered to the building: through a water circuit (air-to-water) or directly to the air (air-to-air). We also compare monobloc and split designs, hybrid systems and absorption units in our article on heat pump types.
Where are heat pumps used?
The best known use is heating homes: detached houses, villas and apartments. Hotels, schools, dormitories and office buildings choose heat pumps to combine heating and cooling in one system. Greenhouses and livestock buildings need a steady indoor temperature all winter. In factories, waste heat from production can be recovered with a heat pump to heat social areas or process water. Pool heating, product drying and ships are more specialised applications; we have gathered them all, with examples, in our article on heat pump applications.
AD heat pumps: air source, with a water circuit
The units we manufacture under the AD brand are air-to-water heat pumps. There are four residential series, and what sets them apart is the outdoor temperature at which they deliver their rated capacity: Poyraz is designed for −7 °C, Boran for −25 °C and Vaha for +2 °C, while Ayaz, with an R290 circuit and an R134a circuit working in series, produces 75 °C water even at −30 °C. Poyraz and Boran use the natural refrigerant R290, and Vaha, tuned for coastal climates, uses R32; the compressors are DC inverter.
Each residential series is built in 12, 14 and 16 kW capacities. For larger buildings there are the V and W series outdoor units from 20 to 56 kW, and for greenhouses and housing estates the Mega central system with 200–1,200 kW. You will find every model and its technical table on our heat pump products page.
Starting with the right unit
The working principle is the same in every heat pump. What determines efficiency is how well the unit is matched to the climate, the building's heat loss and the water temperature of the heating system. We explain step by step which information to start with in our heat pump selection guide. If your project is ready, send the heat loss calculation or the consumption of your existing boiler through our quote request form, and we will choose the series and capacity together.
