How to choose a heat pump – what to focus on before buying
How to choose a heat pump – what to focus on before buying
A heat pump is one of the most discussed investments for a household today. Not because it is some kind of fashionable item, but because with the right choice, it can multiply every kilowatt-hour of electrical energy into three to five kilowatt-hours of heat. With a wrong choice, it becomes an overpriced piece of equipment that does not meet expectations, does not sufficiently heat the house, and unnecessarily burdens the electricity bill. The difference between these two scenarios almost exclusively lies in the preparation and decisions you make before you order anything specific.
In this article, I will guide you through the entire decision-making process as we handle it in practice – from the heat loss of the house through the selection of the type of heat pump to the accessories you may not even realize you will need. If you are new to the topic, you will find a solid foundation here. If you are more experienced, you may find an answer to a specific detail that is bothering you.
Why choosing a heat pump is different from choosing a boiler
When you replace a gas boiler, the process is relatively straightforward: you determine the power of the old boiler, buy the same or a similar power, the technician comes and replaces it. A heat pump does not work this way. It is not a device that simply "replaces" a heat source – it is a system that closely interacts with the distribution circuits, regulation, hot water preparation, and external conditions. An insufficient power, an unsuitable type, or a missing buffer tank can cause the heat pump to operate in short cycles (so-called short-cycling), which dramatically shortens its lifespan and increases consumption.
Therefore, choosing a heat pump is always a complex decision that includes at least four areas: power sizing, type selection (air-to-water, ground-to-water, water-to-water), compatibility with existing distribution systems, and long-term operating costs. We will go through each of them.
Step 1: Heat loss of the house – the foundation of everything
The first and absolutely essential thing you need to know is the heat loss of your house. Without this number, it makes no sense to compare performances or prices. Heat loss expresses how many kilowatts of heat you need to supply to the house on the coldest day to maintain the desired indoor temperature (usually 20–21 °C).
For new buildings constructed according to current standards, heat loss typically ranges around 30–50 W/m² of floor area. For older houses without insulation, it can be 80–150 W/m² or more. A house with an area of 150 m² can therefore have a heat loss anywhere from 4.5 kW (a low-energy new building) to 18–22 kW (an uninsulated brick house from the 70s). A heat pump sized for 8 kW would cover the entire need in the first case, but would be insufficient even for half the required power in the second case.
Heat loss is ideally calculated by an authorized designer according to the standard STN EN 12831. If you want to estimate it yourself, use simplified calculation tools, but be aware that they can be off by 20–30 %. More on the calculation can be found in the article What heat pump power do I need for my house in the Knowledge Center.
What affects the heat loss of a house
- Thickness and type of insulation of the perimeter walls and roof – the biggest influence
- Windows and doors – glazing (double glazing vs. triple glazing), their number and orientation
- Age and construction of the building – panel houses vs. masonry vs. wooden
- Ventilation – natural ventilation is a major source of heat loss; recuperation significantly reduces it
- Climatic region – Bratislava has a design outdoor temperature of –13 °C, mountainous areas of Slovakia –18 °C to –22 °C
Step 2: Choosing the type of heat pump
On the Slovak market, you will encounter three basic types of heat pumps. The choice depends on your conditions – plot, geology, access, building, and available electrical power.
Air-to-water heat pump
By far the most common type today. The outdoor unit draws in air and transfers heat to the heating circuit. Installation does not require any earthworks, drilling or permits. That is why it is the most affordable in terms of investment costs. The disadvantage is that COP decreases with the outside temperature – it works less efficiently at –15 °C than at 0 °C or 7 °C. Modern inverter pumps, however, operate fully effectively even at –20 °C, which is more than sufficient for the vast majority of locations in Slovakia.
For a family house with insulation and floor heating, the air–water type is in most cases the optimal solution in terms of the price/performance/return ratio. More details and a direct comparison with the ground–water type can be found in the article Air-to-water heat pump vs. ground-to-water – which one is more cost-effective.
Ground-to-water heat pump (brine / geothermal)
Energy is extracted from the ground via horizontal collectors (large garden area) or vertical boreholes (100–150 m deep). The ground has a stable temperature of 8–12 °C throughout the year, which ensures uniform and high COP regardless of the outside temperature. Investment costs are significantly higher – drilling can cost 3000–8000 € depending on depth and number. In addition, you need a hydrological assessment and in some cases an official permit.
Ground–water is worthwhile if you have suitable geological conditions, sufficient garden space and are calculating on a long horizon of 20+ years. For older houses with higher heat loss and thus higher annual heat consumption, the return on the higher COP is better.
Water-to-water heat pump
It uses groundwater or surface water as a heat source. It has the highest COP of all types, but also the strictest requirements – you need a pumping and injection well or an accessible water flow, a permit from the relevant authority and water quality that does not damage the heat exchanger. In Slovakia, it is a less common solution, usually reserved for industrial applications or specific locations.
Step 3: Heating system – temperature levels and types of distribution
Heat pumps achieve the best COP with low-temperature heating. This means that the water temperature in the heating circuit should be as low as possible – ideally 35–45 °C. Underfloor heating operates precisely in this range. Panel radiators sized for 70/50 °C (old gas boiler) will become insufficient at 45 °C and the house will not be heated sufficiently.
If you have an older house with classic radiators, you have two options: either upgrade the radiators (replace them with larger ones so they can deliver the same performance at a lower temperature), or choose a heat pump with bivalent operation, where an electric booster or the original boiler will assist in extreme cold.
What is bivalent operation and when is it worth it
Bivalent operation means that the heat pump does not operate alone, but in combination with another heat source – most often an electric booster (built into the pump or an external tank), a gas boiler or a stove insert. The heat pump covers the normal seasonal demand, and when the outside temperature drops below the so-called bivalent temperature (usually –5 to –10 °C), the supplementary source is activated.
This approach is economically justified in cases where dimensioning the heat pump for 100 % performance in extreme conditions would require an disproportionately large and expensive unit. For a house with a heat loss of 12 kW, a heat pump of 8 kW is sufficient (covers 90 % of the annual heat demand), and the remaining 10 % will be covered by electric heating in the coldest 10–15 days of the year.
Step 4: Preparation of domestic hot water (DHW)
A heat pump is not only used for heating, but in most cases also for the preparation of domestic hot water. That is logical – you have invested in an efficient heat source, so why not use it to the full. When choosing a DHW solution, two things are important: the volume of the tank and the way it is connected.
For a four-bedroom house with four people, a 200–300 liter tank is recommended. Heat pumps of the ITEC-T, ATHENA-T, LEGEND, CALIBRA and ATLAS series have built-in support for hot water circulation. If you have a long DHW distribution system in the house and do not want to wait for hot water, it is worth installing Hot water circulation kit for these heat pump series, which is integrated directly into the control logic of the heat pump and intelligently controls the circulation pump – not continuously, but according to a time schedule or impulse.
Important note: heat pumps have a limited maximum output temperature for the heating circuit (55–65 °C depending on the model). For hygienic preparation of DHW above 60 °C (legionella), an electric booster in the tank or a special mode where the heat pump briefly increases the temperature is required. Do not forget this when selecting the tank and configuring it.
Step 5: Electrical power and electrical supply installation
A heat pump is an electrical appliance with a significant power consumption. A device with a thermal output of 10 kW and a COP of 4 has an electrical power consumption of 2.5 kW. This sounds low, but during the compressor start-up, the inrush currents can be several times higher. Moreover, most heat pumps require a three-phase supply of 400 V (3×16 A or 3×20 A), although some smaller models are also single-phase.
Before ordering the pump, check the following:
- Whether you have a three-phase connection available (400 V / 3-phase)
- The capacity of the main circuit breaker – whether it can handle the addition of a new appliance
- The distance between the distribution board and the installation location of the outdoor unit – long cable runs increase installation costs
- The possibility of switching to a special tariff for heat pumps (D5 or D6) with your distributor – this tariff significantly reduces the electricity price for HP in exchange for blocking the pump during peak hours (2–4 hours daily), which is usually not a problem with a properly dimensioned buffer tank
Step 6: Buffer tank – yes or no?
A buffer tank (puffer) is a reservoir that "dampens" the fluctuations between heat production and consumption. A heat pump prefers long continuous cycles over short on/off cycles. If the system operates in such a way that the pump turns on and off every 5–10 minutes (short cycling), this significantly shortens the life of the compressor and reduces efficiency.
A buffer tank with a volume of 100–200 liters is usually sufficient as a hydraulic link between the source and the distribution system. In some projects, a larger tank (500–1000 liters) is installed if the D6 tariff with blocking periods is used, and the heat pump produces the entire daily heat supply "overnight" outside of peak hours.
Practical advice from practice: with a properly dimensioned underfloor heating system with sufficient thermal mass (concrete slab), a buffer tank is not always necessary. The concrete floor itself functions as a buffer. With radiators that have low thermal inertia, a buffer is almost always recommended.
Step 7: Accessories and extensions – what you will need
A heat pump is not an isolated appliance – it is the center of a system. Several add-ons stand around it, which are easily overlooked during planning, but are essential in practice.
Connection kit and hydraulic connection
For the CALIBRA DUO and ATLAS DUO series models, a special connection kit for CALIBRA DUO and ATLAS DUO heat pumps is available. This is original equipment specifically designed for these heat pumps, ensuring a tight and correct hydraulic connection between the outdoor and indoor units or the buffer tank circuit. Using non-original fittings can lead to leaks or incorrect flow, so avoid this solution.
Self-regulating heating cable
The outdoor unit of the heat pump must have its condensate drains and possibly the connecting pipe block protected from freezing. For the ATEC and ITEC series, a self-regulating heating cable 2 m is available. This cable automatically adjusts its power output according to ambient temperature – it provides more heat in strong frost and almost none in mild weather. It is a small but important detail that is not forgotten during winter installation, but is easily overlooked during summer installation.
Relay set for a mixing circuit
If your system includes a mixing circuit (e.g. underfloor + radiator heating combined via a three-way valve controlled by a pump), and the heat pump outputs an Optimum 3–10 V signal, you need a converter of this signal to a classic on/off signal. This is provided by the Relay set – signal for pump with Optimum 3-10V function converted to on/off. Without it, the mixing control would not function correctly, and some circuits could overheat or remain cold.
Expansion module
If you need to connect additional heating zones, solar collectors, an external TÚV tank, or other external devices, there is an Expansion module for ATEC and ITEC heat pumps available, which expands the communication and control capabilities of the heat pump. When exactly you need it and how to connect it is described in a separate article in the Knowledge Center Expansion module for heat pumps – when and why you need it.
Step 8: Noise of the outdoor unit
This aspect is often underestimated. The air-to-water heat pump outdoor unit contains a fan and a compressor – both generate noise. Values range from 45 dB(A) for quiet premium models to 65 dB(A) for cheaper products. 10 dB represents roughly double the perceived noise, so the difference between 45 and 55 dB is significant.
Practical consequences:
- Neighbor's house at 3 m distance: at 55 dB(A) noise at the property boundary may exceed permitted night-time limits (in Slovakia it is 40 dB in night-time hours in residential zones)
- Placing it under a bedroom window is not a good idea even with quiet models
- Inverter heat pumps are quieter at partial load (normal operation) than at full load
- Noise barriers or special covers can reduce noise by 5–10 dB, but they worsen air flow, so professional assessment is needed
Step 9: Economic return and subsidies
A heat pump is an investment ranging from 5,000 € (simple air-to-water solutions, installation, no construction work) to 20,000–30,000 € for a complex renovation with ground boreholes, storage tanks, new piping and underfloor heating. Return on investment depends on:
- Savings compared to the original heat source – gas vs. electricity is a key ratio; at current prices, a heat pump is economically self-sufficient compared to gas only when gas prices exceed a certain threshold
- Annual volume of consumed energy – the larger the house and the higher the heat loss, the greater the absolute savings
- Electricity tariff (D5/D6) – a cheaper tariff can reduce operating costs by 20–30 %
- Subsidies – in Slovakia, schemes are available from Envirofond, Obnovme si domov or EU structural funds; the amount of contribution depends on the current program period and the conditions of the specific scheme
Approximate calculation for a family house with an annual need of 15,000 kWh of heat: gas (efficiency 90%) at 0.07 €/kWh costs about 1,167 € annually. A heat pump with SCOP 3.8 at electricity price 0.16 €/kWh (tariff D6) costs about 630 € annually. You save 537 € annually. With a net investment of 10,000 € (after subsidy), the payback period is 18–19 years. With a smaller subsidy and gas at 0.10 €/kWh, the payback period is shorter – about 10–12 years. These figures are approximate and must be verified by a specific project.
Step 10: Brand selection, warranty and service
You buy a heat pump for 15–25 years. Brand selection is therefore not only a matter of price, but also of availability of service, spare parts and warranty. Before purchase, find out:
- How many years of warranty the manufacturer offers on the compressor (key component) – standard is 2 years, premium brands offer 5–7 years on the compressor
- Whether there is an authorized service network in Slovakia – not only the seller, but also real service technical facilities
- Availability of spare parts – for less known brands it may be problematic
- Whether the company offers a service contract and what exactly it includes
- What are the conditions for commissioning – most manufacturers require the first start-up by an authorized service technician, otherwise warranty conditions may not apply
On what you can do yourself and what must be done by a service technician, you will find more in the article Maintenance and service of heat pumps – what you can do yourself and what not.
Installation – what you need to know in advance
A heat pump is installed by a professional refrigeration technician (for the refrigerant part, an F-gas certificate is required) and a heating company (for the water circuit), when it is properly designed. Installation typically takes 1–3 days depending on complexity. More about the procedure, requirements for the base under the unit, installation height, condensate drain and most common errors can be found in the article Heat pump installation – procedure, requirements and common errors. Details on the installation of accessories are also described in Connecting and installing heat pump accessories step by step.
Most frequently asked questions (FAQ)
Does an air-to-water heat pump work at temperatures below –15 °C?
Yes, modern inverter models function down to –20 °C and more, although COP at such temperatures drops to 1.5–2.0. In practice, extreme frosts occur only a few days a year in most of Slovakia, so a heat pump covers 90–95 % of the annual heat demand even without the help of a backup source. For extreme mountain locations, the ground-to-water type or a bivalent configuration is more suitable.
Do I have to insulate my house before installing a heat pump?
It is not a mandatory condition, but it significantly affects economics. If a house has a heat loss of 20 kW, you need a large and expensive pump. After insulation to 10 kW, a half-size model is sufficient. Investment in insulation often "pays back" through a cheaper heat pump as well, not only through lower operating costs. Ideal sequence: first insulation and window replacement, then sizing of the heat pump for the new condition.
What is SCOP and why is it more important than COP?
COP (coefficient of performance) is an instantaneous value under specific measurement conditions (e.g. outside air 7 °C, water output 35 °C). SCOP (seasonal COP) expresses average annual efficiency taking into account the entire range of outside temperatures and operation at partial load. It is a much more realistic indicator of actual efficiency. For the climatic zone of Central Europe, SCOP values are typically 3.5–4.5 for air-to-water and 4.0–5.0 for ground-to-water.
Can I connect a heat pump to an existing radiator heating system?
Yes, but with conditions. Existing radiators must be verified by calculation whether they provide sufficient output at a lower temperature level (45–55 °C instead of 70 °C). If not, they need to be oversized (replaced with larger ones). In practice, this mainly applies to older apartment buildings. Modern panel radiators from the last 10–15 years are usually designed more generously and can handle the transition to 55 °C without replacement.
What impact does a heat pump have on electricity – can a standard household connection handle it?
A standard household connection 3×25 A (17 kW) usually suffices for a heat pump up to 12 kW thermal output, if you do not have other large appliances (electric stove, water heater, EV charger) turned on at the same time. For larger installations or a combination of multiple appliances, an assessment by an electrician and possibly increasing the main circuit breaker is needed. This needs to be coordinated with the electricity distributor in advance, not during installation.
How long does it take for a heat pump to pay back?
Return on investment depends on many factors: house size, original heat source, available subsidies, electricity tariff, energy prices. Realistically, when replacing gas, the return on a net investment (after subsidy) ranges from 10–18 years, when replacing electric direct heating or oil it ranges from 6–12 years. The lifespan of a heat pump is 20–25 years with proper maintenance, so even with a longer payback period, it is an economically sensible investment.
Conclusion: Invest time in preparation, not just in the equipment
Everyone who installed a heat pump hastily – without a thermal balance, without verifying the piping, without thinking about the tariff and accessories – regretted it somehow. Either the pump is insufficient during frosts, or it pays back only at a low electricity price, which they have no access to, or they lack TÚV circulation and legionella grows in the tank, or they lack an expansion module and cannot connect solar panels they later bought.
A good choice of a heat pump is primarily answering five questions correctly: What is the heat loss of the house? What type of heat pump is suitable for my conditions? What do the piping need to do? What accessories will I need? And who will you entrust with the installation? If you know the answers to these questions before you sign the order, you have a great chance of satisfactory operation for the next two decades.
For deeper study of specific aspects, use other articles in the Knowledge Center: Integration of hot water circulation with a heat pump – how to do it, Common heat pump faults and how to recognize them or Common questions about heat pumps. The complete range of accessories and heat pumps can be found on the category page Heat Pumps.
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