Heat Pump

What is a heat pump and how does it work?

A heat pump is a machine that moves thermal energy from one place to another using electricity as a driver, rather than burning fuel to generate heat. It captures heat that already exists in outdoor air, groundwater, or soil and transfers it indoors for space heating or domestic hot water. This transport mechanism means a heat pump delivers more thermal energy to a building than the electrical energy it consumes, typically between two and six units of heat for every unit of electricity input. This ratio is called the coefficient of performance or COP.

How does the vapour compression cycle move heat?

A heat pump uses a sealed refrigerant circuit to capture and release thermal energy. Low-pressure refrigerant at the outdoor unit absorbs heat from air, ground, or water in an evaporator coil. A compressor pressurizes this gas and raises its temperature. The hot high-pressure gas flows to an indoor condenser coil where it cools and releases heat to the building's water circuit. The cycle repeats continuously. By switching valve positions, the same machine can extract heat indoors and reject it outside for summer cooling.

Why is the temperature difference between source and sink the single fact that governs everything downstream?

A heat pump's efficiency depends entirely on how far apart the two temperatures are. When the outdoor air is cold and the heating water must be hot, the pump must work harder and consumes more electricity per unit of heat delivered. This is why the same heat pump achieves different COP values at different times of year. Winter design conditions in Slovakia at minus 15 degrees outdoor temperature create large temperature lifts, especially if the heating system is sized for 55-60 degree water flow. Conversely, a system designed for 35 degree water flow to underfloor heating achieves much higher COP because the gap is smaller. Low-temperature heating systems were invented specifically to maximize heat pump efficiency.

The table below illustrates how the same air-to-water heat pump model behaves at different outdoor and flow temperatures, showing the dramatic efficiency penalty from large temperature lifts:

At minus 15 degrees (common in January in Slovakia), a system feeding 60-degree radiators barely outperforms electric resistance heating; one feeding 35-degree underfloor heating still achieves useful efficiency. This table is why retrofitting a heat pump onto old radiators without first reducing building heating demand is nearly always disappointing.

This relationship is so dominant that it overrides nearly every other design choice. A poorly insulated building needs high water temperatures to reach comfort, which makes any heat pump inefficient. A building with minimal heating demand (from passive house design or deep renovation) can use small water temperature lifts and pairs beautifully with any heat pump. The single path to successful heat pump retrofits in Slovakia is to reduce heating demand first through insulation, then add the heat pump to work against a much smaller temperature difference.

What are the main heat source types and how do they compare?

Heat pumps in residential settings extract heat from three environments: air, ground, and groundwater. Each offers different trade-offs in efficiency, space requirement, permitting complexity, and capital cost.

In Slovak practice, air-to-water heat pumps dominate new buildings because installation is fast and permitting is trivial. Ground-source heat pumps deliver superior efficiency but require drilling budget and face legal barriers on small plots (many Slovak parcels are under 500 m², making horizontal collectors impossible). Water-source systems are rare except in regions with clean groundwater, as extraction rights add cost and uncertainty.

What is the difference between instantaneous COP and seasonal SCOP?

The COP printed on a heat pump nameplate (often A7/W35) represents performance at a single test condition: outdoor air at seven degrees Celsius, heating water at thirty-five degrees. This is a mild day in October or April, not a January morning at minus 15 degrees. Real buildings experience the full spectrum of outdoor temperatures during the heating season, and the heat pump works less efficiently on every degree colder than the test point.

SCOP (Seasonal COP) averages the pump's performance across the entire heating season (October to April under European standards), accounting for all weather transitions, defrost cycles, and electric backup heating that activates on very cold days. SCOP is always significantly lower than the test-point COP and far more accurate for predicting annual energy costs. A system with a nameplate COP of 4.5 at A7/W35 might achieve a real annual SCOP of 3.2 to 3.8 in Slovak climate conditions. This is why comparing products requires checking SCOP, not marketing COP figures.

How does Slovakia's continental winter affect heat pump design and operation?

Slovakia experiences extended cold spells with outdoor temperatures dropping below minus ten degrees, sometimes reaching minus twenty degrees. At these extremes, an air-source heat pump's capacity to extract heat from outdoor air drops sharply. Frost accumulates on the outdoor coil, and refrigerant temperature differentials become unfavourable. To maintain heating output, modern systems activate a defrost cycle that reverses the refrigerant flow and briefly uses heating energy to melt ice on the coil. In January and February, defrost penalties represent a real loss of heating output and efficiency. Most air-to-water systems also include an electric immersion heater that engages when outdoor temperature falls below the bivalent point (typically minus 5 to minus 10 degrees), and this backup heating increases winter electricity consumption significantly on the coldest design days. Ground-source systems avoid these penalties almost entirely because soil and groundwater temperatures remain stable well above freezing.

Zelená Domácnostiam (Green Households) and Obnov Dom (Renovate House) are the main subsidy schemes for heat pump retrofit in Slovakia, co-financed by the Environmental Fund. Eligibility and funding change annually; check your local authority for current 2025-2026 conditions before purchase.

Why do heat pump noise and outdoor placement matter in residential settings?

Outdoor units produce 40-50 decibels during operation, similar to continuous refrigerator noise. Most Slovak municipalities require 3-5 meter distance from neighbours' windows or acoustic barriers. Neighbour disputes over noise are the single most common source of complaint. Before purchasing, check your local town hall (stavebný úrad, mestský úrad) for noise regulations for your exact address and confirm distance to neighbouring windows. Vibration mounts and acoustic cabinets reduce noise by 3-10 decibels but add cost. Poor planning has led to forced removals in high-density areas; this conversation must happen before installation, not after.