How to Choose a Heat Pump

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How to choose a heat pump - a complete guide to choosing the right type, output, and parameters of a heating source for a family house.

Why so many people choose a heat pump today

Over the past few years, the heat pump has gone from a niche technology to one of the most frequently considered heating sources for both new builds and renovations in Slovakia. The reason is simple - it takes most of the energy needed for heating directly from the surrounding environment (air, ground, or water), and needs only a fraction of this energy in the form of electricity to "pump" it up to a higher temperature. While a classic electric boiler converts 1 kWh of electricity into 1 kWh of heat, a typical heat pump produces 3 to 4.5 kWh of heat from that same 1 kWh of electricity. It's exactly this ratio that makes a heat pump worthwhile in the long run despite the higher upfront investment compared to a gas boiler.

At the same time, choosing a heat pump isn't a simple matter of "I'll take the strongest one, just to be safe". A poorly chosen type, an undersized or unnecessarily oversized output, can significantly reduce operating efficiency, shorten the compressor's lifespan, and ultimately extend the investment's payback period by years. This article will guide you through all the steps you should go through before buying - from choosing the pump type, through correctly sizing the output, to the parameters worth looking at in the technical data sheet.

An important note up front: this article is purely informational and advisory. It doesn't specify particular models or prices for specific products - its goal is that after reading it you'll know what to ask, how to calculate, and what to avoid, wherever you end up buying your pump.

How a heat pump works - in short

The principle is the same as a fridge, just reversed. The refrigerant in the pump's circuit evaporates at low temperature and pressure in the so-called evaporator, drawing heat from the surroundings (air, ground, or water) as it does. The compressor then compresses the refrigerant vapour, which significantly raises its temperature. The hot refrigerant then releases heat into the heating water (into radiators, underfloor heating, or the hot water tank) in the condenser, cooling and liquefying as it does so. Through the expansion valve, the pressure drops again and the cycle repeats. The only energy you actually supply to the whole process is the electricity needed to drive the compressor and circulation pumps - the rest of the heat is "taken" by the pump from the environment for free.

It's exactly the ratio between the electricity supplied and the heat obtained that is expressed as COP (Coefficient of Performance) for instantaneous output, and SCOP (Seasonal COP) for average seasonal output over the whole heating period. From a buyer's perspective, SCOP is a much more important number, because it also accounts for how the pump behaves in frost, when the heat source (for example outdoor air) is colder and the pump has to work harder.

Three basic types of heat pumps

Air-water (the most widespread type)

The pump draws heat from outdoor air using an outdoor unit (similar to air conditioning, just bigger) and delivers it into the heating water in an indoor unit, or directly in a combined unit. This is the most common choice for family houses in Slovakia too, since installation requires no earthworks - it's enough to place the outdoor unit on a paved area or bracket by the house. The typical seasonal coefficient of performance, SCOP, for this type runs between 3.2 and 4.5, depending on the specific model, climate zone, and the heating water temperature the system works with.

The downside is that efficiency drops as frost gets harder - on the coldest days of the year (below -10 to -15 °C), the COP is noticeably lower than at spring or autumn temperatures around +7 °C. That's why most designs account for the so-called bivalent point (more below).

Ground-water (ground collector or borehole)

Heat is drawn from the ground - either via a horizontal ground collector laid at a depth of roughly 1.2 to 1.5 metres over an area corresponding to 1.5 to 2 times the house's heated area, or via a vertical borehole 80 to 150 metres deep (depending on geological conditions and the required output). Ground temperature is far more stable throughout the year than air temperature (typically 8 to 12 °C in our conditions), so ground pumps achieve a higher and more stable SCOP, commonly in the range of 4.0 to 5.0, even in a harsh winter.

The trade-off for the higher efficiency is a significantly higher upfront investment (earthworks or a borehole) and the need for a sufficiently large plot for a horizontal collector, or a permit for a deep borehole. The payback period for the extra investment compared to the air-water variant depends on the specific location and energy prices, and generally runs over a horizon of 8 to 15 years.

Air-air (air conditioning units in heating mode)

These are essentially air conditioning split units that can also work in reverse, heating mode. Heat isn't delivered into water, but directly into the room air. The investment is the lowest of all three types and installation the simplest, but the system can't heat domestic hot water or supply classic radiators or underfloor heating - it's more suitable as a supplementary or local heat source (for example for a single room, a cottage, or as support for existing heating), not as the sole central source for an entire family house. Seasonal COP here commonly runs between 3.0 and 4.0.

Visual comparison of the three types

COP and SCOP - the most important figures in the data sheet

When comparing specific models, you'll come across several COP values, always given for a specific combination of outdoor temperature and heating water temperature (for example "COP at A7/W35" means outdoor air of +7 °C, water outlet temperature of 35 °C). The lower the required heating water temperature, the higher the COP the pump achieves - which is why heat pumps work significantly better with underfloor heating (typically 30-35 °C) than with high-temperature radiators (45-55 °C).

A key characteristic of pumps using air as the heat source is that their efficiency changes throughout the year based on the outdoor temperature. At around +7 °C (a typical autumn/spring temperature), a good-quality pump achieves a COP of around 4.0. As it drops to 0 °C, COP falls to approximately 3.2. At -7 °C (a typical frosty day in Slovakia), it's already only around 2.6, and in extreme frost of -15 °C, COP can approach 2.0. That's exactly why seasonal SCOP is more important than a one-off COP value at a single temperature - it realistically captures how the pump behaves throughout the whole heating period, not just in ideal weather.

How COP changes with outdoor temperature

The graph shows why it's important to know the climate zone your house is in, and why suppliers use the so-called bivalent point - the temperature below which the heat pump alone can no longer cover the house's entire heat loss and a supplementary (bivalent) source kicks in, most often an electric heating rod built directly into the pump. For most locations in Slovakia, the bivalent point is set somewhere between -5 °C and -10 °C - in practice this means the pump alone covers about 95-98% of annual heat demand, and a supplementary source helps out only on the few coldest days of the year.

How to correctly size a heat pump's output

The most common mistake when choosing a heat pump isn't the wrong type, but the wrong output - either undersized (the pump can't keep up in frost and has to rely extremely often on the electric heating rod, which significantly raises consumption), or oversized (the pump starts and stops too often - so-called cycling - which shortens the compressor's lifespan and reduces real efficiency compared to catalogue values).

The correct approach to choosing the output is always based on calculating the house's heat loss, not on estimating it "by eye" from its floor area. Heat loss is calculated according to standard STN EN 12831 and accounts for the area and orientation of the envelope structures, the thickness and type of insulation, the size and type of windows, ceiling height, and the climate zone. As a rough guide (and truly only a rough guide - nothing replaces an exact calculation), for a typical family house in Slovakia's central climate zone, heat loss runs:

A new build insulated to current standards (low-energy or passive standard): approximately 30-50 W per m² of floor area
An older insulated building (subsequently insulated, quality windows): approximately 50-70 W per m²
An older uninsulated building: 80-120 W per m² or even more

For a sample house with a floor area of 120 m², this means a heat loss of roughly 5-6 kW for a new build, 7-8 kW for an insulated older house, and 10-12 kW or even more for an uninsulated older house. These figures serve only for an initial orientation before deciding roughly what output you'll need - the binding calculation should always come from a designer or installation company based on a specific energy rating and the house's actual condition.

Why you shouldn't design for the coldest day of the year

The common layperson's approach of "let me have a pump for the worst possible frost, just to be safe" leads directly to oversizing. The design outdoor temperature used for the heat loss calculation (for example -12 °C or -15 °C depending on location) occurs only a few days a year during the heating season - the vast majority of the season runs at temperatures between 0 °C and +10 °C, where the pump has its highest efficiency. That's exactly why pump output is usually designed to cover 90-100% of the heat loss at the design temperature, with a bivalent source taking over the rest of the extreme peaks - it's a more economical solution than buying a bigger, more expensive pump just for a few days a year.

Output design process - a clear diagram

Other parameters worth looking at

Heating water temperature and the type of heating elements

As already mentioned, the lower the heating water temperature, the higher the COP. Underfloor heating working with a water temperature of 30-35 °C is therefore an ideal partner for a heat pump. If the house is heated with classic radiators designed for a higher temperature (for example 55/45 °C), you either need to allow for lower overall system efficiency, or (if the budget and the house's condition allow) consider replacing them with larger low-temperature radiators that can deliver the same heat output at a lower water temperature thanks to a larger heat-exchange surface.

Outdoor unit noise

The outdoor units of air-water and air-air pumps contain a fan and compressor, which produce noise. Typical acoustic output values run between 55 and 65 dB(A) measured directly at the unit, which at a distance of a few metres, with the natural drop in noise over distance, falls to the level of ordinary conversation or below. Nevertheless, it's important when planning the placement to account for the distance from a bedroom (your own or a neighbour's) and from the plot boundary - night-time noise limits have their own legal thresholds, and a neighbour with a bedroom under the window the unit's air discharge points toward may reasonably be unhappy. Manufacturers today also offer so-called silent operating modes, which reduce the fan's speed and noise by a few decibels in exchange for slightly lower output at that time.

Refrigerant and its environmental profile

Older models used refrigerants with a high GWP (Global Warming Potential); newer models increasingly switch to low-GWP refrigerants (for example R290 - propane), which are more environmentally friendly in the event of a leak, but require stricter safety measures during installation due to their flammability. When comparing models, it's worth checking what refrigerant is used and what its GWP number is - lower is better in the long run, also considering future European F-gas regulation.

Power input and the method of output control (on/off vs. inverter control)

Modern heat pumps almost exclusively use an inverter compressor, which smoothly varies its speed (and therefore output) according to the house's current need, instead of the older principle of simply switching on and off at full output. Inverter control means less cycling, more stable indoor temperature, and generally also higher real (not just catalogue) efficiency during the transitional periods of the year, when the house needs only part of the pump's maximum output.

Domestic hot water preparation

Most air-water and ground-water heat pumps can, alongside heating, also heat water in a domestic hot water (DHW) tank. However, it's worth knowing that DHW heating happens at a higher temperature (typically 50-55 °C to prevent legionella) than underfloor heating, which slightly lowers the average COP during water heating compared to normal heating mode. Some systems therefore combine the heat pump with supplementary electric DHW heating, or with a larger tank that's heated outside peak times (for example at night on a cheaper electricity tariff).

Indicative comparison of running costs

The calculation below is purely an illustrative model, not an offer for a specific product or current energy price list - it serves only to show, roughly, how much more advantageous a heat pump can be compared to other common heat sources. Let's consider a model family house with an annual heat demand for heating of 10,000 kWh (a typical value for a medium-sized, adequately insulated family house in Slovakia):

A gas condensing boiler with an efficiency of approximately 92% needs about 10,870 kWh of gas to deliver 10,000 kWh of heat.
Direct electric heating (an electric boiler or direct heating) with an efficiency close to 100% needs 10,000 kWh of electricity directly.
A heat pump with a seasonal SCOP of 3.8 (a typical realistic value for a quality air-water pump under our climate conditions) needs only approximately 2,630 kWh of electricity (10,000 / 3.8) to deliver the same 10,000 kWh of heat.

Even without tying ourselves to specific current energy prices (these change and vary by supplier and tariff), this calculation clearly shows the principle: a heat pump needs less than a third of the amount of electricity for the same resulting thermal comfort compared to direct electric heating. It's exactly this physical ratio (not marketing claims) that's the reason pumps pay off in the long run - we can work out the specific savings in euros for your house and current energy prices together, individually.

Illustration - energy consumption for the same amount of heat

Common mistakes when choosing a heat pump

Over years of experience with heating technology, we repeatedly come across a few typical mistakes that can be avoided in advance:

Choosing output based on house area without a heat loss calculation. As shown above, two houses of the same size can have heat losses differing by as much as 100%, depending on the quality of insulation and windows. A "by eye" estimate often leads to oversizing.

Ignoring the heating system's temperature. Buyers focus on the SCOP in the catalogue, but forget that the real value in their house depends on what water temperature the pump will actually work with - high-temperature radiators noticeably reduce real efficiency compared to the catalogue value measured at low temperature.

Underestimating outdoor unit noise. Placement too close to a bedroom (your own or a neighbour's) can cause a long-term problem that's difficult and expensive to fix afterwards (noise enclosures, relocating the unit).

Forgetting about the bivalent source and its control. If the bivalent source (usually an electric rod in the pump) switches on too early or too often due to incorrect control settings, electricity consumption can rise significantly above expectations, even if the pump itself is correctly sized.

Underestimating hydraulic balancing of the system. A heat pump is sensitive to correct water flow through the heat exchanger and to balancing individual heating circuits - neglected hydraulics can reduce the system's real efficiency regardless of how good the pump itself is.

Summary - what to focus on when choosing

If you take just a few points away from this article, let them be these: first, have the house's real heat loss calculated, not estimated from its area. Second, choose the pump type based on the available plot, budget, and payback time horizon - air-water for most typical houses, ground-water if you have the plot and are planning long-term. Third, look at the SCOP at a realistic heating water temperature for your house, not just the best catalogue number. Fourth, don't underestimate hydraulic balancing and the placement of the outdoor unit with regard to noise. And finally, keep in mind that real savings depend on a combination of correctly chosen output, the type of heating system, and current energy prices - no number from the internet replaces an individual calculation for a specific house.

Frequently Asked Questions

Is a heat pump worth it even in an older, uninsulated house?

Yes, but with reservations. An uninsulated house has higher heat loss, so it will need a more powerful (and therefore more expensive) pump, and its real SCOP will be lower if the house is heated with high-temperature radiators. In many cases it's worth at least partially insulating (roof, windows) before or at the same time as installing the pump - the investment in insulation pays back in the form of a smaller (cheaper) pump and higher operating efficiency.

Can a heat pump be combined with an existing gas boiler?

Yes, such a setup is called a hybrid system - the heat pump covers most of the heating season, while the older gas boiler serves as a backup or peak source during the hardest frosts instead of an electric heating rod. This solution can be attractive especially where the boiler is still functional and there's no wish to invest in immediately replacing it.

How long does a heat pump last, and what wears out the most?

The typical lifespan of a quality heat pump runs around 15-20 years, longer with regular maintenance. The most heavily stressed component is the compressor, whose lifespan is directly linked to the number of starts and stops (cycling) - which is why correctly sizing the output is so important, not just for efficiency but also for long-term reliability.

Do I need a building permit for a heat pump?

For an air-water pump, a notification of construction modifications is generally sufficient, or the installation is treated as maintenance work, since it doesn't significantly affect the house's load-bearing structure or the plot on a large scale - however, the exact procedure varies by municipality, and it's worth checking in advance with the building authority. For a ground borehole, the situation is more complex, since it counts as mining activity subject to notification to the mining authority, and in some cases also to water-law proceedings if the borehole comes into contact with groundwater.

Can a heat pump completely replace my current boiler without any modifications to the heating system?

Not automatically. If the house is heated with high-temperature radiators designed for temperatures around 70-90 °C (typical for older installations with an original gas or solid-fuel boiler), you need to allow for either lower pump efficiency, or replacing/supplementing the radiators with low-temperature variants with a larger surface area. A professional technical assessment before replacing the source is therefore always advisable.

Is a heat pump noisy inside the house too?

The indoor unit (if separate from the outdoor one) mainly contains the circulation pump and electronics; its noise level is usually comparable to, or lower than, an ordinary washing machine, and with correct placement (a utility room, not a bedroom) it's practically unnoticeable. The main source of noise is almost always the outdoor unit with its fan.

Have a question on this topic?

Not sure what to decide, or dealing with a specific situation in your home? Write to us - we're happy to help.