What capacity (in kW) heat pump is required for a detached house measuring 100, 120 or even 150 square metres? The correct answer does not depend solely on the floor area: there can be a significant difference in the heating requirements of two houses of the same size, depending on the quality of their insulation, the type of windows and doors, and what type of heat distribution system they have, as well as whether the unit is used solely for heating and cooling or also for domestic hot water (DHW) production.
In this article, we use specific examples to illustrate the order of magnitude of the energy requirements that can be expected for different detached houses with different energy performance ratings.
How many kW heat pump is needed for a detached house?
For accurate sizing, you should consult an energy specialist or a building services engineer, who carries out the calculation of the building’s heating requirements. One of the most widely used methods for this is the method described in Appendix 1 of Decree No. 9/2023 (25 May) of the Ministry of Economy (which can be viewed here); building energy performance certificates for buildings are prepared on the basis of this method.
It is much simpler than that; one of the simplest methods, which is sufficient for a preliminary estimate, is to start with the building’s floor area and an estimated specific heat demand. The formula for the calculation is:
Required heating output (kW) ≈ heated floor area (m²) × estimated specific heat demand (W/m²) / 1000
For example, if we calculate in advance using a specific value of 50 W/m² for a 120 m² house: 120 × 50 / 1000 = 6 kW
In other words, the estimated heating power requirement is 6 kW.
This method is NOT suitable for calculating heat requirements, whilst it is advisable to carry out a precise sizing exercise on each occasion, it is, however, ideally suited to providing an initial indication of the power range worth considering, which can serve as a useful guide, for example, when drawing up a preliminary cost estimate.
The table below provides a guide. The actual heat loss must be calculated on the basis of the structure and other characteristics of the building in question.
Estimated kW requirement by insulation and floor area
| General condition of the building | Indicative specific heat demand | 80 m² | 100 m² | 120 m² | 150 m² |
|---|---|---|---|---|---|
| Modern, very well insulated, exceeding current standards | 20–35 W/m² | 1.6–2.8 kW | 2–3.5 kW | 2.4–4.2 kW | 3–5.2 kW |
| A well-insulated house that meets modern building standards | 35–50 W/m² | 2.8–4.0 kW | 3.5–5 kW | 4.2–6 kW | 5.2–7.5 kW |
| A well-insulated property which, a few years ago, still complied with building regulations (10–15 cm of insulation) | 50–70 W/m² | 4.0–5.6 kW | 5–7 kW | 6–8.4 kW | 7.5–10.5 kW |
| A poorly insulated building (5–10 cm of insulation) | 70–100 W/m² | 5.6–8 kW | 7–10 kW | 8.4–12 kW | 10–15 kW |
| An old house with absolutely no insulation | 100–150+ W/m² | 8–12+ kW | 10–15+ kW | 12–18+ kW | 15–22+ kW |
The figures above are merely indicative estimates suitable for estimating heat loss; however, the heat pump’s output is NOT the same as the heat loss!
The W/m² method is therefore a quick estimate that provides a good approximation of a building’s heat loss. However, it is not a sufficient method for selecting a heat pump.
What factors determine the sizing of a heat pump?
The heat pump is primarily sized on the basis of the building’s heat loss. It’s a bit of engineering jargon, but it’s worth bearing in mind. “Second law of thermodynamics: heat flows from a warmer place to a colder one” (the ‘two’ here refers to the second law), meaning that in winter, the internal, heated space continuously loses heat to the external, colder environment, and the heating system’s job is to replace this lost heat. Heat escapes, amongst other things, through the building envelope: walls, windows, doors, the roof or floor slab, and structures in contact with the ground; in addition, heat is lost through ventilation and air exchange, and the effect of thermal bridges must also be taken into account.
Thermal bridge: 3D heat flow, where the structure is not “uniform”, meaning that the heat flowing out does not leave the building uniformly or at right angles. In such cases, heat loss is always higher than if a completely uniform structure were assumed; this effect increases the theoretical heat loss of the structure by 5–30%.
In other words, the factors that determine heat loss in the first instance are the thermal insulation of the walls and floor slabs, the quality and size of the windows and doors, as well as how airtight the house is and the quality of the workmanship – because material quality is key, but the use of inappropriate technology can cause problems even with the highest-quality systems.
As well as floor area, the building’s mass, height and internal height are also important factors. A compact building with an average internal height and a building with a high internal height and an open gallery have completely different characteristics.
A modern building today has heat loss in the order of a few kW – a fraction of that seen in older buildings. They are also considerably more expensive to build than they were 20–30 years ago. “Thanks, EU” – one might say sarcastically, given that we have the EPBD Directive’s harmonisation of legislation to thank for all this, but behind this lie numerous models and simulations, where the aim was to minimise the costs of the building structure over a 30-year operational period, taking into account both capital and running costs.
Heating energy requirements have fallen significantly, often to such an extent that annual heating energy consumption is lower than the heat required to produce domestic hot water, and, due to the increasing prevalence of large window areas, it is often the case that peak cooling demand is higher than peak heating demand. So, if we wish to use the heat pump not only for heating but also for cooling or domestic hot water production, we can immediately see why we must not base our selection of heat pump capacity solely on the peak heating demand.
How should I take hot water production into account?
In order to take the preparation of the HMV into account, it is necessary that We estimate the expected hot water consumption in the building and base our sizing on users’ habits. It is best to build it ourselves, because then we can extrapolate by observing our own habits and consumption patterns; however, if we wish to use rules of thumb here as well, we can calculate the dimensions using the following formula:
Required hot water volume (litres/day) ≈ per capita consumption (litres/person·day) × number of residents (people)
Per capita consumption, calculated using domestic hot water at ~50 °C, is 30–45 litres per person per day for economical users, 45–60 litres per person per day on average, and for high consumption (e.g. based on a rain shower head), it is 60–80 litres per person per day; in other words, a family of four uses 120–240 litres of hot water. If the temperature is lower than this (and the heat pump will be grateful for every degree), it is advisable to size the system to account for a water volume that is 2.5% higher per degree.
Another important concept is the hysteresis, which specifies by how many degrees the set (let’s say) 50 °C hot water can cool down before DHW production switches on again. As we generally use hot water at 38–44 °C (depending on what each person finds comfortable), it is not worth setting the value too low; moreover, the further away a tap is from the DHW tank, the greater the heat loss through the water pipes; in a larger house (200 m²+), the DHW temperature at the tap may be as much as 2–3 degrees lower than in the storage tank. Typically, we do not allow the system to cool below 42 °C; therefore, assuming a flow temperature of 50 °C and a hysteresis of 8 K, it is advisable to apply the following logic.
Example:
The heat pump designed for the 120 m² detached house serves four people; we have fitted a 200-litre DHW tank to it. The 6 kW heat pump, sized to meet the heating requirements, produces the hot water. The family are away from home and are not using any water, but they have not switched off the tank so that there will be sufficient DHW when they return. The water in the tank, which has an initial temperature of 50 °C, cools evenly by approximately 3–5 degrees over 24 hours, which means that when the heat pump switches on (every two days with a hysteresis of 8 K, for the sake of simplicity) to recover the lost heat, it will
Time required [s] = Temperature difference (8) [K] × specific heat of water (4.2) [kJ/kgK] × storage volume (200) [litres] × density (1) [kg/litre] / Heat output (6) [kW] = 1120 seconds
It takes about 20 minutes to complete this process. This means that, as a rough estimate, there is no heating for 20 minutes every two days (plus a further 10 minutes, taking into account the time taken for the heat pump to switch from heating to DHW production mode and back to heating mode). This means that the heat pump’s average effective output decreases, but this only results in a reduction in output of ~1%.
However, if we assume that they are at home and that DHW production begins once 50% of the tank’s contents has been used up, but they consume 100% of the tank’s volume each day, the following formula applies. (Let us assume an inlet water temperature of 6 °C)
The water at the top of the tank has already cooled down a little, so let’s assume it’s not 50 but 48 degrees, whilst at the bottom it’s a consistent 6 degrees throughout. As the thermometer is positioned right in the middle, we can safely take the arithmetic mean of the two readings as the average temperature in the tank, meaning the average temperature in the tank is 27 degrees.
Time required [s] = Temperature difference (50–27 = 23) [K] × specific heat of water (4.2) [kJ/kgK] × storage volume (200) [litres] × density (1) [kg/litre] / Heat output (6) [kW] = 3220 seconds,
That is, roughly 54 minutes; allowing for switching over, approximately 1 hour. On the one hand, it takes about this long to reheat the tank (whilst, based on a flow rate of 10 litres per second, it takes 10 minutes to use up the remaining water). This is done twice a day, as we have only calculated with half a tank here; in other words, when factoring in usage, DHW production takes up 2 hours a day from the heating system, which reduces the time available for heating by nearly 10%, so it is actually worth sizing the system with a higher heat output to ensure the property can be heated properly.
After all, what impact would this power loss of 10% (~0.5 kW) have?
At an outside temperature of -15 °C and an inside temperature of 22 °C, the heat output required for heating increases in direct proportion to the temperature difference, that is to say, if 6 kW is required for a temperature difference of 37 K, then we can only generate a temperature difference of 34 K, meaning we will only be able to maintain an indoor temperature of 18 °C.
It does make a difference, doesn’t it?
Therefore, when using a hot water tank, depending on the size of the tank and usage patterns, it is advisable to allow for 10–20% of extra capacity.
How does the heat dissipation system affect the required output? How does this vary in the case of underfloor heating, fan coils or radiators?
In a thorough heat loss calculation, we dimension not only the heat loss but also the heat emission system, because it does make a difference whether we assume a floor temperature of 34, 36 or 38 °C when designing the heating system.
Heat pumps typically operate most efficiently at low flow water temperatures; they are therefore particularly well suited for use with underfloor heating or a suitably sized fan-coil system. A radiator system can also be operated using a heat pump, but it is necessary to check whether the radiators are capable of delivering the amount of heat required for each room at a lower water temperature.
We present a detailed overview of the operation and design considerations of various heat sinks in the Underfloor heating with a heat pump vs radiator heating: design and benefits in our article entitled ….
The maximum heat output (and, in particular, the coefficient of performance (COP)) that heat pump systems can generate is not constant; it depends on a number of factors. The nominal output specified on the data sheet is generally the A7/W35 value, i.e. the heat output measured at an outdoor temperature of 7 °C and a heating water temperature of 35 °C. In this case, the temperature difference between the heat source and the heat transfer medium (water) is 22 K. However, much greater temperature differences can also occur. The greater the temperature difference between the heat source (e.g. outside air) and the heat transfer medium (outgoing heating water), the lower the COP and the maximum output.
| Reference value | Underfloor heating (35 °C) | Fan coil (45 °C) | Radiator (55 °C) |
|---|---|---|---|
| Maximum heating capacity of the ERGA06 + EHVX08 heat pump (at an outside temperature of -2 °C) | 6.2 kW | 6.2 kW | 5.3 kW |
| The heat pump’s maximum capacity at an outside temperature of -10 °C | 6.0 kW | 6.0 kW | 4.9 kW |
| The same heat pump has a maximum capacity at an outside temperature of -15 °C | 5.6 kW | 5.5 kW | 4.9 kW |
| (Ibid. Heat pump COP at an outside temperature of -2 °C) | 3.15 kW | 2.49 kW | 1.76 kW |
| Ibid. (heat pump COP at an outside temperature of –10 °C) | 2.51 kW | 2.04 kW | 1.39 kW |
| Heat pump COP at an outside temperature of –15 °C | 2.14 kW | 1.80 kW | 1.39 kW |
It is quite clear that Both the maximum capacity and the coefficient of performance of the heat pump system decrease as the outside temperature falls, and as the flow temperature rises. This is why we recommend that heat pump heating systems are best installed for surface heating.
At an outdoor temperature of –15 °C – which forms the basis for the sizing – it is in fact no longer capable of delivering the designed 6 kW power requirement, let alone the power output required for DHW production, which was designed with a 10-20% margin. Therefore, if we plan to meet the heating demand using the heat pump alone, the planned heat pump’s nominal output of 6 kW is insufficient. The actual output that results from the calculation of the heating requirement, based on the specified design flow temperature and design outdoor temperature, must be taken into account.
What seems to be the logical conclusion?
Let’s choose a larger heat pump, as this will cover both the heat demand for DHW production and the heating demand.
However, sizing a heat pump in this way does not necessarily yield the best results. Inverter-driven heat pumps are capable of regulating their output, but below a minimum output level (typically around 30% of the rated output), they are no longer able to modulate further, from this point onwards, the system will operate by switching on and off – a bit like not being able to drive a car at a steady speed below 25 km/h; so if we wanted to go at 20, we would accelerate to 25, then brake the car to a standstill, then set off again and accelerate to 25, repeating this cycle so that the average speed ends up being 20 km/h. This driving style is less efficient in terms of both fuel consumption and the car’s service life.
Typically, in today’s energy-efficient houses, the heating needs to be switched on only when the temperature remains consistently below 12 °C; until then, internal heat gain compensates for the house’s heat loss. During the heating season, temperatures are most often within the range of 5–12 °C; therefore, it is important to avoid oversizing the system as much as possible to minimise the number of on-off cycles. For correctly sized heat pumps, the ratio of switch-ons to operating hours falls between 0.5 and 2.
So what is the proposal?
In this situation, we usually make the following recommendation: size the heat pump to provide full heating and DHW supply at an outside temperature of -7 °C. For periods when it is colder than this, use the heating element built into the heat pump to provide additional electric heating (with a COP of 1) to the system.
At an outside temperature of -7 °C, the heating requirement is 20% lower than at -15 °C, which means that, for the property in question, a unit with a rated output of 6 kW is a suitable choice, and by connecting a 2–6 kW heating element, we can heat the property even on the coldest days.
What additional costs and tasks does this involve?
A 1×25 A supply is not sufficient for the heat pump; the heating element requires an additional 1–2–3×10 A (2–4–6 kW)
What are the risks associated with an undersized heat pump?
The heating element will run for too long, resulting in high running costs; in extreme cases, it will not even heat the property.
What are the risks associated with an oversized heat pump?
Frequent switching on and off of the heat pump on autumn and spring days makes it difficult to regulate the indoor temperature accurately and comfortably, leading to higher energy consumption and, in all likelihood, premature failure.
It is therefore very important that the equipment is not designed and built on a whim, but in collaboration with experts.
The one that is tailored to individual needs
When sizing a heat pump, it is worth distinguishing between three interrelated stages. Using the floor area and a guide value in W/m², we can quickly estimate the order of magnitude of the required capacity. However, the actual design heat demand is determined by the building’s heat loss calculation. The specific heat pump can then be selected by taking into account the manufacturer’s performance data, the required flow water temperature and the entire mechanical system.
A well-designed heat pump system treats the building’s heating requirements, heat sinks, domestic hot water demand, control systems and the operation of the equipment itself as a single unit. If you’d like to find the type of heat pump that truly suits the needs of your family home, get in touch with the SMARTAIR team!