We have all felt that sinking feeling when opening the electricity bill or hearing the heating oil price at the start of winter. Searching for a money-saving solution, you have surely been bombarded by advertisements and discussions about Air-to-Water Heat Pumps.
Everyone says they are "the future", that they "consume almost nothing" and that they are generously subsidised. But behind the technical jargon and the marketing, what exactly is this machine? How does it manage to heat an entire house in winter using the freezing ambient air?
Let us set aside the complex mechanical engineering terms. In this guide, we will explain in plain language how the technology that changed heating in Greece forever actually works.
The simplest explanation is this: Think of a heat pump as a refrigerator working in reverse. Your kitchen fridge removes heat from the food (its interior) and rejects it to the environment (through the warm grid on its back). A heat pump does the exact opposite: it draws free heat from the outdoor air (even at -5°C) and transfers it to the water circulating in your radiators or underfloor heating.
The big secret behind its success lies in a fundamental principle: a heat pump does not burn electricity to create heat (unlike an electric heater or a water heater). It uses electricity merely to run its compressor, so it can transfer the heat that already exists outdoors, indoors.
It sounds absurd to "pump heat" when it is snowing outside. Yet, from a physics standpoint, thermal energy exists in the air all the way down to absolute zero (-273°C). To capture this heat, the machine uses a special refrigerant (such as the modern R32) that has the property of boiling at very low temperatures.
The process (refrigeration cycle) is completed in 4 steps, through which the heat pump converts the cold outdoor air into hot water for your radiators.
Outdoor air passes through the unit. The refrigerant, which is ice-cold, absorbs the heat from the air and turns into a gas (it evaporates).
This is where electricity comes in! The compressor (the heart of the system) compresses this gas. When you compress a gas, its temperature skyrockets. Suddenly, the warm gas becomes scorching hot (e.g. 80°C).
The hot gas meets the heat exchanger. There, it delivers its high temperature to the water heading to your radiators. Losing its heat, the gas turns back into a liquid.
The liquid passes through a narrow valve, its pressure drops sharply, it freezes again, and it is ready to go back to the Evaporator to start the cycle all over again.
It is no coincidence that nearly 90% of new homes in Greece - and the bulk of renovations through the "Exoikonomo" subsidy programme - choose this technology. The advantages are overwhelming.
The Coefficient of Performance (COP) of a modern heat pump is approximately 4.0. This means that for every 1 kW of electricity you pay for, the pump delivers 4 kW of heat. In practice, you get 75% free energy from the environment and pay only 25%.
Unlike northern Europe where temperatures drop to -20°C (where ground-source heat pumps are preferred), Greece's mild winters allow air-to-water pumps to operate at the absolute peak of their efficiency.
An air-to-water heat pump, when combined with fan coils or underfloor piping, can reverse its operation in summer. Instead of heating the water, it chills it, delivering excellent cooling to the home and eliminating the need for multiple individual air-conditioners.
Forget oil tanks, odours, gas leak risks, flues and smoke. It is a 100% clean and safe technology.
Reading about compressors and refrigerant, it is perfectly logical to wonder: "Isn't this just an air-conditioner?" The answer is: Yes, they are based on exactly the same technology, but they differ fundamentally in how they deliver heat inside the house.
It takes heat from outside and transfers it directly to the room air. This means instant heating but with significant drawbacks: it dries the atmosphere, creates annoying draughts (that stream hitting the back of your neck) and has no thermal inertia. The moment you turn the unit off, the room freezes within 10 minutes because air does not retain heat.
It takes heat from outside and transfers it to water. The hot water travels through piping and reaches the underfloor heating or your radiators. Heating occurs through radiation, just like a traditional oil boiler. Sweet, uniform warmth everywhere, no air dryness, and huge thermal inertia (the house stays warm for hours after the machine turns off).
There is also a huge bonus: an Air-to-Water Heat Pump can be connected to a storage tank and provide you with abundant Domestic Hot Water (DHW) for your bathroom - something a simple wall-mounted air-conditioner can never do!
The machine and installation cost significantly more than a simple gas boiler. However, the payback (ROI) is typically achieved within 3 to 5 years thanks to the difference in running costs.
If your house is completely uninsulated, with old draughty wooden windows, the heat pump will struggle (and consume electricity) to maintain the temperature. Proper thermal insulation is the first step.
Heat pumps love low water temperatures (e.g. 35°C for underfloor). If you have old, small radiators (AKAN type) that need 75°C water to heat the room, the pump must be a special High-Temperature model or you will need to upsize your radiators.
⚠️ As responsible professionals, we must point out that a heat pump is not a "magic box" you simply place in the yard. It requires proper engineering, proper installation, and a properly insulated building envelope.
Without a doubt, yes. Air-to-Water Heat Pumps are the most mature, ecological and economical heating solution available today. They are an investment that dramatically upgrades your property's energy class and shields your wallet from the wild fluctuations in fossil fuel prices.
Now that you understand the basic working principles, the next step is choosing the right type of machine for your space. In our next article, we analyse the immediately most critical question: What to choose between a Monoblock and a Split heat pump?
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