Installers and spec sheets throw around terms like «condenser,» «buffer tank,» and «expansion valve» as if everyone already knows what they mean. If you’re planning an aerothermal installation and want to actually understand what you’re paying for, here’s every major component explained in plain language — no engineering background required.
The Outdoor Unit: Where Heat Is Collected
1. Fan
Pulls outside air across the outdoor coil so the system has a constant supply of air to extract heat from. Think of it as the «breathing» part of the system — the more air that moves across the coil, the more heat the unit can capture.
2. Outdoor Coil (Evaporator in Heating Mode)
A network of thin metal tubes and fins that the refrigerant flows through. As outdoor air passes over it, the cold refrigerant inside absorbs heat from the air and turns from liquid into gas — this is the very first step of the whole process.
3. Compressor
Often called the «engine» of the system. It squeezes the refrigerant gas, which raises its pressure and temperature significantly. This is the component that does the actual work of concentrating low-grade heat from the air into heat that’s hot enough to warm your home — and it’s also the main thing that uses electricity.
4. Reversing Valve (if the system has cooling)
A valve that can flip the direction refrigerant flows through the system, switching the unit between heating and cooling mode. Only present in reversible systems — heating-only units don’t need it.
The Refrigerant Circuit: The System’s «Bloodstream»
5. Refrigerant
A special fluid that boils and evaporates at very low temperatures — often below -25°C. It’s what actually carries heat from outside to inside, changing between liquid and gas as it moves through the cycle. It’s a closed loop: the same refrigerant is reused continuously and never runs out under normal operation.
6. Expansion Valve
A small but critical component that drops the refrigerant’s pressure and temperature right before it re-enters the outdoor coil, «resetting» it so it can absorb heat again. Think of it as the opposite of the compressor.
The Indoor Side: Where Heat Reaches Your Home
7. Condenser (or Plate Heat Exchanger)
Where the hot refrigerant gives up its heat — either to water (in air-to-water systems) or directly to indoor air (in air-to-air systems). This is the handoff point where «outside heat» officially becomes «your home’s heat.»
8. Circulation Pump
Moves heated water from the heat pump through your home’s pipework to radiators, underfloor heating, or a hot water tank. Without it, the heat generated would have no way to travel through the building.
9. Buffer Tank (sometimes optional)
A water tank that stores a reserve of heated water, helping the system avoid switching on and off too frequently (called «short cycling»). Not every system needs one, but it can improve efficiency and equipment lifespan, especially in smaller homes with low heat demand.
10. Hot Water Cylinder (DHW Tank)
Stores heated water for showers, taps, and household hot water use, separate from the heating circuit. Most air-to-water heat pump systems include one, since the heat pump can heat both your rooms and your water.
The Heat Emitters: Where Heat Enters the Room
11. Radiators
The most common way to release heat into a room. Heat pumps work best with radiators sized larger than what a gas boiler would need, since heat pumps typically run at lower water temperatures.
12. Underfloor Heating
Pipes embedded in the floor that release heat gently across a large surface area. This is considered the ideal emitter for heat pumps because it works efficiently at low water temperatures (30–35°C).
13. Fan Coil Units
Compact units (often wall- or ceiling-mounted) that use a small fan to blow air over a heat exchanger, delivering heat or cold air into a room quickly. Common in systems that also provide cooling.
The Brain of the System
14. Thermostat / Controller
The interface you actually interact with — lets you set target temperatures, schedules, and modes. Modern systems often include smart, app-connected controllers that can optimize running times based on electricity tariffs or weather forecasts.
15. Outdoor Temperature Sensor
Feeds real-time outdoor temperature data to the controller, allowing the system to automatically adjust water flow temperature — a feature called weather compensation — so it runs no hotter than necessary at any given moment.
Quick Reference: What Each Part Does
| Component | Simple role |
|---|---|
| Fan | Pulls in outside air |
| Outdoor coil | Absorbs heat from that air |
| Compressor | Concentrates the heat (the «engine») |
| Reversing valve | Switches heating/cooling direction |
| Refrigerant | Carries the heat through the system |
| Expansion valve | Resets refrigerant pressure |
| Condenser | Hands heat off to water or air |
| Circulation pump | Moves hot water through the house |
| Buffer tank | Smooths out system cycling |
| Hot water cylinder | Stores hot water for daily use |
| Radiators/underfloor/fan coils | Release heat into rooms |
| Thermostat | Lets you control the system |
| Outdoor sensor | Enables automatic efficiency adjustments |
Why Understanding These Parts Matters
Knowing these components helps you ask sharper questions during quotes and installation: Is a buffer tank included, or will the system short-cycle in my small flat? Are the radiators being resized, or reused as-is? Does the quote include weather compensation controls? Installers who explain these details clearly are usually a good sign of quality — and now you’re equipped to follow along.
Frequently Asked Questions
Do I need a buffer tank for my heat pump?
Not always — it depends on your home’s heat demand and zoning setup. Smaller, open-plan homes are more likely to need one to prevent frequent on/off cycling.
Can I keep my existing radiators with a new heat pump?
Sometimes — it depends on their size and your home’s heat loss. A proper heat loss calculation will tell your installer whether they’re adequate or need upsizing.
What’s the difference between the compressor and the condenser?
The compressor pressurizes and heats the refrigerant; the condenser is where that heat is released into your home. They’re two separate steps in the same cycle.
Is a hot water cylinder always included with a heat pump system?
Most air-to-water systems include one, since they typically handle both space heating and household hot water, but it depends on the exact package installed.