If you've ever wondered why some hot tub owners seem to spend a fraction of what you do on heating costs, the answer is almost certainly a heat pump. Air source heat pumps have quietly transformed the way people heat their hot tubs and swim spas — delivering warm, consistent water temperatures for significantly less energy, less money, and far less noise than traditional electric heating.
But how do they actually work? And are they right for your setup? This guide walks you through everything — from the basic science to what to look for when choosing a model.
"For every 1kW of electricity a heat pump uses, it can deliver up to 16kW of heat in return."
The Hortus CollectionThe Basic Principle
A heat pump doesn't generate heat the way an electric element does. Instead, it moves heat — extracting warmth that already exists in the surrounding air and transferring it into your water. Even on a cool UK day, the air outside contains usable thermal energy. The heat pump captures it, concentrates it, and delivers it to your spa.
This is the same fundamental technology used in your refrigerator — but in reverse. Rather than removing heat from inside a box and expelling it into the room, a heat pump takes heat from outside and brings it in.
The key measure of efficiency is called the COP — Coefficient of Performance. A COP of 16 means you receive 16kW of heat for every 1kW of electricity you consume. By comparison, a standard electric immersion heater has a COP of 1. The economics are dramatic.
The Four Stages of the Heating Cycle
Inside every heat pump, a refrigerant fluid circulates continuously through four key components. Understanding these stages helps explain why heat pumps are so remarkably efficient.
Evaporation — Absorbing Heat from the Air
A fan draws outside air across a large coiled surface called the evaporator. The liquid refrigerant flowing through these coils has an extremely low boiling point, so even cool air is warm enough to cause it to evaporate into a gas. As it evaporates, it absorbs the thermal energy from the surrounding air — capturing free heat from the environment.
Compression — Amplifying the Heat
The refrigerant gas is then passed through a compressor — the heart of the system. Compressing the gas dramatically raises its temperature, concentrating the heat energy it's carrying. In advanced inverter heat pumps like the Mr. Silence, the compressor runs at variable speeds, adjusting its output intelligently to match demand rather than simply switching on and off at full power.
Condensation — Transferring Heat to Your Water
The now hot, pressurised gas passes through the heat exchanger — a component submerged in your water circuit. Here it releases its heat into the water flowing from your hot tub or swim spa, warming it to your set temperature. As it gives up its heat, the refrigerant condenses back into a liquid.
Expansion — Resetting the Cycle
The liquid refrigerant passes through an expansion valve, which rapidly drops its pressure and temperature back down — ready to absorb warmth from the air all over again. The cycle repeats continuously and silently, maintaining your water at precisely the temperature you've set.
Inverter Technology — Why It Matters
Older heat pumps operated like a light switch — fully on or fully off. This meant they would blast at maximum power until the water reached temperature, then cut out entirely, only to restart again when the water cooled. This constant cycling wastes energy, creates noise spikes, and wears components faster.
Modern full inverter heat pumps — like the Aquark Mr. Silence — work more like a dimmer switch. The compressor continuously adjusts its speed based on how much heating is needed at any given moment. When your spa is cold at the start of the season, it runs at higher speed to heat quickly. Once it reaches temperature, it slows right down to a quiet, efficient hum to simply maintain it.
What is InverPad® Technology?
InverPad® is Aquark's proprietary inverter system, engineered specifically for pool and spa heat pumps. It combines stepless DC inverter control, a unique low-noise airflow design, and active noise-cancelling engineering to deliver a unit that runs at just 36.5dB — quieter than a normal conversation — while achieving a COP of up to 16. It's the technology behind the Mr. Silence range.
Heat Pumps vs Other Heating Methods
To understand the value of a heat pump, it helps to see how it compares directly to the alternatives most hot tub owners use.
| Heating Method | COP | Running Cost | Noise | Best For |
|---|---|---|---|---|
| Air Source Heat Pump Recommended | Up to 16 | Very low | Near silent (36dB) | All hot tubs & swim spas |
| Standard Electric Heater | 1.0 | Very high | Silent | Backup / top-up only |
| Gas Heater | 0.9 | Moderate | Moderate | Pools (requires gas supply) |
| Solar Heating | Varies | Very low | Silent | Summer supplementary only |
Does a Heat Pump Work in the UK Climate?
This is the question we hear most often, and the answer is a confident yes. The UK's mild, damp climate is actually well-suited to air source heat pumps. Modern units like the Mr. Silence range are designed to operate effectively in air temperatures as low as −10°C, which comfortably covers even the coldest British winters.
Efficiency does reduce slightly in very cold weather — a COP of 16 at 27°C ambient drops to around 6–7 at 15°C — but even at its least efficient, a heat pump is still six to seven times cheaper to run than a standard electric heater. Year-round hot tub ownership becomes genuinely affordable.
How Does It Connect to My Hot Tub?
A heat pump is installed externally — typically on a flat surface near your hot tub — and connected into the water circulation system. The water from your spa flows out through the filtration system, passes through the heat pump's heat exchanger where it's warmed, then returns to the tub.
In most setups, the heat pump replaces the hot tub's built-in electric heater entirely, though it can also work alongside it. Professional installation ensures the flow rates, pipe connections, and electrical supply are all correctly configured — protecting both performance and your warranty.
Will it work with my hot tub?
The vast majority of hot tubs and swim spas are compatible with an external heat pump. At Hortus Collection, we'll confirm compatibility with your specific model before recommending a unit — and handle all installation ourselves, so you don't need to worry about the technical details.
Sizing — Choosing the Right Heat Pump
Getting the right size heat pump for your hot tub or swim spa makes a significant difference to both performance and running costs. Too small and the unit struggles to maintain temperature in cooler weather. Too large and you're paying for capacity you don't need.
As a general guide:
Standard Hot Tubs
Suitable for most 4–6 person hot tubs up to approximately 2,000 litres. The Mr. Silence MS70 (6.8kW) and MS90 (9kW) cover this range perfectly.
Swim Spas & Large Spas
Swim spas typically hold 10,000–20,000 litres and require significantly more heating capacity. The Mr. Silence 30 (30kW, COP up to 22) is purpose-built for this application.
What to Look for When Buying
Not all heat pumps are equal. When comparing models, these are the specifications that matter most:
COP rating — the higher the better. Look for units rated at COP 10 or above. Be aware that manufacturers often quote COP at ideal conditions (27°C air temperature); check the COP at 15°C for a more realistic UK figure.
Full inverter vs on/off — always choose a full inverter model. They're quieter, more efficient, and last longer than on/off units.
Sound level — measured in dB(A). Below 40dB is considered very quiet. The Mr. Silence runs at 36.5dB — roughly equivalent to a quiet library.
Operating temperature range — confirm the unit works effectively down to at least −5°C for UK year-round use.
Warranty — look for at least 5 years on the full unit. The Mr. Silence carries a 7-year full warranty and 15 years on core components.
Smart controls — Wi-Fi app control lets you pre-heat your spa before you arrive, monitor energy usage, and adjust settings remotely. A genuinely useful feature, not just a gimmick.