Chapter 5 — Understanding Modern Heat Pumps

Introduction

The thermodynamic principles of a heat pump have not changed since the 1980s. What has changed radically is the way these principles are implemented: power electronics, microcontrollers, and new refrigerants have transformed a relatively simple mechanical machine into a complex, high-performance system.

This chapter is aimed at readers who want to understand what actually happens inside a modern heat pump — not to reproduce Suominen’s work, but to be able to discuss with an installer, read technical documentation, or approach the diagnosis of a fault with confidence.

Visualise the complete architecture of a modern heat pump: diagrams — Air/Water and Air/Air split versions.

The refrigeration cycle: the foundation of everything

A heat pump is a thermodynamic machine that moves heat from a cold medium to a warm one — the opposite of the natural direction of heat transfer. To do this, it uses a refrigerant that changes state, absorbing or releasing heat in the process.

The basic cycle has four stages:

1. Evaporation — The refrigerant, under low pressure, absorbs heat from the cold source (outdoor air, ground, water) and vaporises. This is the role of the evaporator.

2. Compression — The compressor draws in the vapour and raises it to high pressure and temperature. This is the stage that consumes electrical energy.

3. Condensation — The hot, pressurised fluid transfers its heat to the building’s heating circuit and liquefies. This is the role of the condenser (or indoor heat exchanger).

4. Expansion — The liquid refrigerant passes through an expansion valve that sharply reduces its pressure, causing significant cooling. The cycle can begin again.

The key point: the heat delivered to the building is the sum of the heat extracted from outside and the electrical energy consumed by the compressor. This is why the COP¹ is greater than 1 — we are not creating energy, we are moving it.

Direct expansion (DX) vs intermediate circuit

There are two main architectures for geothermal heat pumps, which correspond exactly to the debate that Suominen had opened in the 1980s.

Intermediate circuit (glycol water)

This is the most widespread architecture today for geothermal systems:

• A circuit of buried collectors contains glycol water (antifreeze)
• This glycol water circulates between the ground and a heat exchanger in the heat pump
• The exchanger transfers heat to the refrigerant circuit

Advantage: safety (the refrigerant remains confined within the machine), ease of maintenance.
Disadvantage: double heat exchange = additional losses = slightly reduced COP.

Direct expansion (DX — Direct eXpansion)

This is the principle that Suominen recommended:

• The refrigerant circulates directly in the buried collectors
• Evaporation takes place in the ground itself
• One heat exchange instead of two

Advantage: better COP, shorter collectors (the exchange is more efficient).
Disadvantage: larger quantity of refrigerant in circulation, stricter safety constraints, more complex maintenance.

Direct expansion remains a technical niche — used by a few specialist manufacturers — but it confirms forty years later that Suominen had identified the thermodynamically optimal solution.

The Inverter compressor: the heart of the modern heat pump

How the inverter drive works

An Inverter compressor is a compressor whose rotation speed is variable, controlled by an electronic inverter drive (or variable frequency drive).

The inverter operates in three stages:

1. Rectification: the alternating current from the mains (120/240 V AC) is converted to direct current (170–400 V DC depending on supply voltage)
2. Filtering: high-capacity capacitors smooth the DC bus voltage
3. Switching: power transistors (IGBTs) reconstruct an alternating current of variable frequency to drive the compressor motor

It is this DC bus at 170–400 V that is the point of vigilance during maintenance — exactly like the high-voltage capacitors in a valve amplifier or cathode-ray TV set, they remain charged for several minutes after the mains supply is cut.

Note for North American readers: single-phase residential supplies in North America are typically 120 V or 240 V AC split-phase. The DC bus voltage on 240 V systems is similar to European 230 V systems (around 340 V DC peak). The same safety rules apply: wait, then measure before touching.

Why it matters

A fixed-speed compressor runs at full power or stops — this is the on/off approach that Suominen criticised. An Inverter compressor can run at 30%, 60%, 100% of its rated output, and any point in between.

Direct consequences:

• COP maximised at all times: the machine operates at its optimal efficiency point
• Improved comfort: indoor temperature is stable, without oscillations
• Reduced wear: fewer full-load starts, increased service life
• Operation at very low temperatures: the machine can run slowly rather than stopping

Controls: the brain of the system

General architecture

A modern heat pump has two levels of control:

Internal control (outdoor unit electronic board):

• Inverter compressor management
• Defrost management
• Component protection (pressures, temperatures, currents)
• Communication with the indoor unit

System control (thermostat or control panel):

• Weather compensation curve — flow temperature calculated as a function of outdoor temperature
• Mode management (heating, domestic hot water, cooling)
• User interface
• Home automation connectivity

The weather compensation curve

This is the most important control principle to understand. Instead of reacting to an indoor temperature (like a simple thermostat), the heat pump anticipates demand based on outdoor temperature.

A typical weather compensation curve might be:

• At +50 °F (10 °C) outdoor → circuit flow at 95 °F (35 °C)
• At +32 °F (0 °C) outdoor → circuit flow at 113 °F (45 °C)
• At +14 °F (−10 °C) outdoor → circuit flow at 131 °F (55 °C)

This curve is adjustable according to the type of emitters (underfloor heating, low-temperature radiators, standard radiators) and the building’s insulation level. Incorrect weather compensation settings are one of the most frequent causes of underperformance in an otherwise well-sized installation.

Communication protocols

The indoor and outdoor units communicate via proprietary or standardised serial protocols. A few examples:

• Daikin: S21 or D-BUS protocol
• Mitsubishi: CN105 protocol
• Atlantic / Hitachi: proprietary protocols

These protocols are increasingly documented by the open source community, opening up interesting possibilities for home automation integration — in particular via ESP32 gateways that allow reading the heat pump’s data and integrating it into Home Assistant, Node-RED, or MQTT.

Refrigerants: evolution and constraints

The trend is clear: tomorrow’s refrigerants will have low GWP, at the cost of increased flammability that imposes new safety rules for installers and maintenance technicians.

What a Papy Maker should remember

• The refrigeration cycle is unchanging — what evolves is how it is controlled
• The Inverter is the key to modern performance — understanding how it works means understanding 80% of electronic faults
• The weather compensation curve is the most impactful setting — a well-sized but poorly set heat pump will systematically underperform
• Communication protocols are increasingly accessible — an ESP32 can read and control a modern heat pump
• The inverter DC bus remains live after shutdown — same reflex as with a valve amplifier: wait, then measure before touching

The next chapter addresses diagnosis and troubleshooting — with the necessary safety precautions.