Frequently Asked Questions About Heat Pumps
Frequently Asked Questions About Heat Pumps
A heat pump is today the most frequently recommended heating source for new builds and renovations alike in Slovakia, yet it is also the appliance surrounded by the most half-truths and myths. Homeowners ask whether a heat pump will really save them money, whether it will still work in frosts below minus ten degrees, whether it is noisy, how long it lasts, and whether it is worth it even in an older house with radiators. In this article we have collected the questions we are asked most often through our advice service, via forms and directly in the shop, and we answer them honestly - including the situations where a heat pump does not pay off, or where you need to reckon with a compromise. The heat pump category is currently temporarily out of stock with us, so you will not find specific models or product prices in this article - it is purely advisory content to help you form a clear picture before you approach a supplier or installation company.
How does a heat pump actually work
Before we get into the specific questions, it helps to understand the basic principle - most misunderstandings arise precisely because people imagine a heat pump as an electric boiler that simply looks different. That is not the case. A heat pump does not generate energy, it transfers it from one place to another - from the outdoor environment (air, ground, groundwater) into the home's heating system. It does this using a refrigeration circuit you also know from your fridge, only in reverse: a fridge takes heat from the inside and throws it outside, a heat pump takes heat from the outdoor environment and pushes it into the house.
The whole cycle has four basic steps, repeated endlessly:
- Evaporator - refrigerant with a very low boiling point picks up heat from the outdoor air, ground or water and evaporates even at low temperatures.
- Compressor - the gaseous refrigerant is compressed, which significantly raises its temperature (this is the only step that actually consumes electricity).
- Condenser - the hot refrigerant releases heat into the heating water (radiators, underfloor heating, DHW tank) and condenses back into a liquid.
- Expansion valve - the refrigerant pressure drops sharply, it cools down, and the whole cycle repeats from the start.
Precisely because electricity is only consumed to drive the compressor and not to directly produce heat, a heat pump can "produce" three to five kilowatt-hours of heat from one kilowatt-hour of electricity. This ratio is called the coefficient of performance (COP), and it is a key figure we will return to several times in this article.
What is the difference between air-water, ground-water and water-water
This is probably the most frequent question we get - we cover it in detail in a separate article Heat pump air-water vs. ground-water, but here let's summarize the key figures that matter when choosing.
Air pumps (air-water) draw heat directly from the surrounding air. They are the cheapest to buy, the simplest to install (no earthworks or drilling needed), and therefore also the most widespread - they make up the vast majority of installations in Slovak family houses. Their disadvantage is that their coefficient of performance drops as the outdoor temperature falls, which we cover in detail below.
Ground pumps (ground-water) draw heat from the soil using a horizontal collector laid at a depth of about 1.2 to 1.5 metres, or via vertical boreholes 80 to 150 metres deep. Ground temperature is far more stable throughout the year than air temperature (typically 8 to 12 °C even in winter), so these pumps achieve a higher and more stable coefficient of performance. The downside is high upfront costs for earthworks or drilling, and the need for a sufficiently large plot for a horizontal collector.
Water-water pumps draw heat from groundwater using an intake and an infiltration well. They generally achieve the highest and most stable coefficient of performance of all three types, because groundwater temperature stays almost constant throughout the year (7 to 12 °C), but they require sufficient water yield and quality and a permit from the relevant authority.
Indicative average annual coefficient of performance (SCOP) under normal operation with underfloor heating is roughly as follows: air-water 3.2 to 3.8, ground-water 4.3 to 4.8, water-water 4.5 to 5.2. These differences translate directly into operating costs - the higher the coefficient of performance, the less electricity the pump uses for the same amount of heat.
Which type is right for me?
If you have an ordinary plot in a built-up area without a large garden and want a quick installation without earthworks, an air pump is practically the only realistic choice and makes economic sense for most households. If you have a larger plot, are planning a new build with underfloor heating, and want the lowest possible running costs, it is worth considering a ground collector or borehole - the higher upfront investment will pay for itself within 8 to 12 years through lower electricity consumption. You can find a detailed selection guide in the article How to choose a heat pump, and for calculating the required output see What heat pump output do I need.
Does a heat pump work in severe frost?
This is the question that puts off the most people from buying - and it is entirely justified, because physically it is true that as the outdoor temperature drops, so does the output and efficiency of an air-source heat pump. However, modern pumps with what is called refrigerant injection (EVI technology or two-stage compression) commonly work today even at temperatures around minus twenty to minus twenty-five degrees Celsius, just with lower efficiency.
Specific indicative coefficient of performance (COP) values for a typical air pump depending on outdoor temperature look roughly like this: at plus seven degrees, COP around 4.0; at zero degrees, COP around 3.2; at minus seven degrees, COP around 2.4; and at minus fifteen degrees, COP drops to around 1.8. Even at a COP of 1.8, the pump is still more efficient than direct electric heating (where the ratio is always 1:1), but the difference compared to summer months is noticeable.
This is exactly why every design always accounts for the so-called bivalent point - the outdoor temperature below which the pump alone can no longer cover the entire heat loss of the house, and a backup source kicks in (most often a built-in electric heating element inside the pump). In a correctly designed system, under our climatic conditions this backup source only switches on for a few dozen hours a year, usually during the hardest frosts, and it has practically no effect on total annual consumption.
How much do I really save compared to gas or direct electricity
We cover the detailed cost comparison in depth in the article How much a heat pump saves compared to gas and electricity; here we give at least an indicative overview for a typical family house with a heat loss corresponding to an annual heat consumption of roughly 12 to 15 megawatt-hours (a typical insulated house of around 120 to 150 m² of floor area).
At this level of consumption, annual heating costs run roughly as follows: natural gas (including standing charge and boiler efficiency) approximately 1,050 euro per year, direct electric heating (storage heaters or direct-heating panels) approximately 2,000 euro per year, a heat pump with an average SCOP around 3.8 and using a discounted electricity tariff for heat pumps approximately 550 euro per year. These figures are, of course, only indicative - actual consumption depends on the quality of the house's insulation, the size of the radiators or underfloor heating, the temperature settings in individual rooms, and current energy prices, which change over time.
It is important not to forget that a heat pump also involves a higher upfront investment compared to a gas boiler (the unit itself, installation, and possibly adapting the heating system). The payback period for this additional investment when switching from gas to a heat pump at current energy prices typically runs between 6 and 10 years; when switching from direct electric heating, payback is considerably faster, usually within 4 to 6 years.
Is a heat pump noisy?
The outdoor unit of an air-source heat pump is essentially a fan and compressor in a housing, and it does always produce some noise - physically there is no way around it. However, modern units designed for residential zones typically operate at a sound pressure level of around 35 to 45 decibels measured at a distance of three metres, comparable to a quiet conversation or the hum of a fridge. At full output during the harshest frosts, noise can briefly rise to 50 to 55 decibels.
In practice, correct placement of the unit matters far more than the technical noise figure alone. Common mistakes we come across: the unit placed right under a bedroom window, the unit installed in a corner between two walls (sound reflects and amplifies), or the unit mounted directly on the facade without anti-vibration pads, transmitting noise into the structure of the house so it is audible inside as well. When designing an installation, we always recommend keeping a distance of at least two to three metres from the windows of living rooms and from the boundary of the neighbouring property, ideally also consulting a noise study if the unit is close to a neighbour.
How long does a heat pump last and what does servicing involve
The lifespan of a quality heat pump with regular maintenance typically runs between 15 and 20 years; the compressor, as the most heavily stressed component, usually reaches a similar age as the whole unit when properly built and correctly sized. A shorter lifespan is usually the result of incorrect sizing (a pump too small or too large relative to the house's heat loss, causing frequent switching on and off) or neglected maintenance.
Routine annual maintenance includes checking refrigerant pressure, cleaning the evaporator and outdoor heat exchanger of dirt and leaves, checking electrical connections and safety devices, and, for pumps with a ground collector, also checking the antifreeze mixture in the primary circuit. You can find a detailed overview in the separate article Heat pump maintenance and servicing. As for fault rates, we summarize the most common problems we encounter in practice in the article Most common heat pump faults - typically these involve the outdoor unit icing up due to an incorrectly set defrost cycle, a drop in refrigerant pressure due to a leak, or a fault in the circulation pump inside the indoor unit.
Can a heat pump be used with old radiators?
This is the question that divides both experts and the general public the most. The short answer is: yes, but with reservations. A heat pump works most efficiently at a low flow water temperature (typically 30 to 40 °C), whereas old radiators were designed for an operating temperature of 70/55 °C or even 90/70 °C in older systems. If a heat pump is connected to original, undersized radiators without any modification, it will either be unable to achieve the required thermal comfort during harder frosts, or it will have to run at a higher flow temperature, which significantly reduces its coefficient of performance and therefore its savings.
In practice there are three solutions: replacing some radiators with larger or low-temperature ones (with a larger heat-exchange surface), adding fan coil units in the most demanding rooms, or combining radiators with partial underfloor heating in newly added or renovated spaces. We cover this topic in detail in the article Heat pump with underfloor heating or radiators. In general, the better insulated the house and the larger the heating surface (underfloor heating, oversized radiators), the lower the water temperature the pump needs, and the higher the efficiency it achieves.
What are the installation requirements and what needs to be arranged
Installing a heat pump is not just about setting up the outdoor unit - it involves work on the entire heating system and often on the house's electrical connection too. We cover the complete procedure step by step in the article Heat pump installation - what you need to know; in short, the steps are: calculating the house's heat loss and designing the pump output, choosing the location of the outdoor unit with regard to noise and air access, possibly upgrading the electrical connection (many pumps need a three-phase supply and their own circuit breaker), connecting to the heating system and the domestic hot water tank, commissioning and setting the weather compensation curve, and finally an inspection of the electrical installation.
For larger installations (especially ground boreholes), you also need to reckon with notifying the authorities of geological works, and in some cases with an opinion from the water management authority. For water-water pumps, a permit is required for abstracting and discharging groundwater. We recommend dealing with these steps before ordering the equipment, not after, to avoid unnecessary delays.
Is there a subsidy or grant for a heat pump?
Yes, various subsidy programmes for replacing a heating source with a heat pump are opened periodically in Slovakia, especially as part of programmes to reduce household energy intensity. However, the conditions, grant amounts, and when the calls are actually open change fairly often, so we keep a clear, continuously updated overview of the current situation and exact conditions in a separate article, Subsidies and support for heat pumps. We recommend checking the current conditions directly on the website of the relevant ministry or implementation agency before submitting an application, since calls usually have limited capacity and strict deadlines.
The most common myths about heat pumps
To close this section, let's summarize a few widespread misconceptions we repeatedly encounter in our advice service.
Myth: A heat pump doesn't work below freezing. Not true - as shown above, modern pumps work even at minus twenty degrees and beyond, just with a lower coefficient of performance.
Myth: The bigger the pump's output, the better. On the contrary - an oversized pump will switch on and off more frequently (so-called cycling), which shortens its lifespan and increases consumption. Correct sizing to match the exact heat loss of the house is key - more in the article on choosing output.
Myth: A heat pump is maintenance-free. Like any technical appliance, it needs regular annual inspection - neglected maintenance is the most common cause of premature faults and reduced efficiency.
Myth: A heat pump pays off for everyone. With very low heat consumption, an excellently insulated passive house with low gas costs, or conversely where an expensive overhaul of the entire heating system is unavoidable, the payback period can be disproportionately long. It always pays to make a real calculation for the specific house, rather than relying on general figures from the internet.
Frequently Asked Questions
Do I also need to replace the hot water tank with a heat pump?
In most cases yes, or it is at least strongly recommended. A heat pump needs a tank with a larger heat-exchange surface (a bigger or double coil), because it heats water at a lower flow temperature than a gas boiler. An original small tank connected to a gas boiler usually cannot efficiently use the lower temperature from the pump, and heating the hot water would take disproportionately long or would be insufficient.
Can a heat pump be combined with an existing gas boiler?
Yes, such a setup is called bivalent or hybrid operation. The heat pump covers heating for most of the year, when outdoor temperatures are milder and its coefficient of performance is high, and during extreme frosts or peak hot-water demand, the original gas boiler takes over part of the load. This solution is especially worthwhile for renovations where you don't want to replace the entire heating system at once.
How long does heat pump installation take?
For a simpler installation of an air pump onto an existing heating system, it usually takes two to four working days including electrical work and commissioning. For ground boreholes, the time is extended by an additional one to two weeks for the geological works themselves, which proceed independently of the installation of the unit itself.
Do I need a building permit for a heat pump?
For a standard replacement of the heating source in an existing family house, a simple notification to the building authority (a minor construction or building modification) is usually sufficient, not a full building permit. For ground boreholes, notification of geological works is also required. Exact requirements can vary by municipality, so we recommend checking directly with the local building authority before starting work.
Can a heat pump also handle cooling in summer?
Most modern air-water heat pumps can also operate in reverse mode, and combined with underfloor heating or fan coils, they can provide mild cooling of the interior in summer. This is not full-fledged air conditioning with intense cooling like a split unit, but a pleasant temperature reduction of a few degrees, which is entirely sufficient for many households.
What happens if the heat pump breaks down during frost?
Quality pumps have a built-in backup in the form of an electric heating element, which automatically takes over heating in the event of a fault or extreme frost, so the house doesn't go without heat. In bivalent operation with a gas boiler, the boiler takes over the backup function. In any case, we recommend having a functional backup set up in the system from the start, not dealing with it only after the first breakdown.
Related topics
- How to choose a heat pump
- What heat pump output do I need
- Heat pump air-water vs. ground-water
- How much a heat pump saves compared to gas and electricity
- Heat pump installation - what you need to know
- Subsidies and support for heat pumps
- Most common heat pump faults
- Heat pump with underfloor heating or radiators
- Heat pump maintenance and servicing
- Heat pumps
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.