Heat Pump Sizing in Luxembourg: Calculating the Right Power Output
Sizing is the most critical step in any heat pump project: an undersized heat pump cannot maintain comfort, while an oversized one wastes energy and wears out prematurely. In Luxembourg, two specific parameters make a difference compared to French or Belgian methods: the design outdoor temperature of −10 °C for Luxembourg City (altitude 300–400 m, moderate continental climate) and the high proportion of older buildings with significant heat losses. This page explains, with formulas, how to calculate the exact power you need — and why only a professional can produce a sizing report compliant with EN 12831.
Why sizing is the most important step
Choosing a heat pump means choosing a heating capacity, expressed in kilowatts (kW). This figure determines whether your home will be heated comfortably even on the coldest nights of the Luxembourg winter — and whether your investment will be profitable over 15 to 20 years.
Incorrect sizing creates two opposite but equally costly risks:
⬇ Undersized heat pump
- Backup electric resistance running constantly
- Set temperature not reached in severe cold
- Compressor running non-stop → premature wear
- Electricity bill far higher than expected
- Klimabonus refused if power doesn’t cover calculated heat losses
⬆ Oversized heat pump
- Short-cycling (frequent on/off)
- Actual COP well below catalogue COP
- Accelerated compressor wear (non-Inverter models)
- Unnecessarily high purchase cost
- Thermal discomfort from heat surges
Luxembourg’s design outdoor temperature: −10 °C
The design outdoor temperature is the key climatic input for any sizing calculation. It represents the lowest temperature statistically recorded for at least 5 consecutive days per year in a given area — the extreme condition the system must handle without backup.
In Luxembourg, classified in zone H1 under EN 12831, the reference design temperature is −10 °C for Luxembourg City and surroundings (altitude 300–400 m). In the Oesling (Luxembourgish Ardennes, 400–560 m), it can reach −12 °C. In the Moselle valley (Grevenmacher, Remich, 130–150 m), −8 °C applies.
Luxembourg City’s annual average temperature is around 10 °C and winters are milder than 40 years ago (heating degree-days dropped from ~3,000 to ~2,000 between 1985 and today). However, the design temperature for sizing remains −10 °C to guarantee comfort during exceptional cold snaps — which still occur regularly. Do not confuse average winter temperature with design temperature.
| Region | Approx. altitude | Indicative design temp. | ΔT (setpoint 20 °C) |
|---|---|---|---|
| Oesling (north, Ardennes) | 400–560 m | −12 °C | 32 K |
| Luxembourg City and centre | 300–400 m | −10 °C | 30 K |
| Gutland (centre-south) | 200–350 m | −10 °C | 30 K |
| Moselle (Grevenmacher, Remich) | 130–200 m | −8 °C | 28 K |
Heat loss calculation formula
Heat pump sizing is based on calculating the building’s heat losses at the design temperature. The reference method is the European standard EN 12831, which every certified professional must apply. A simplified method is also useful for preliminary estimates.
Simplified method (indicative estimate)
The formula is:
G = volumetric heat loss coefficient of the building (W/m³·°C)
V = heated volume (habitable floor area in m² × ceiling height in m)
ΔT = temperature difference = desired indoor temperature − design outdoor temperature
For Luxembourg City, with an indoor setpoint of 20 °C and a design temperature of −10 °C:
G insulation coefficients by construction era
The G coefficient (W/m³·°C) is the key to the simplified method. It reflects the building’s overall thermal quality: the higher G is, the more energy the building consumes. In Luxembourg, a significant proportion of the housing stock was built before 1970, with stone or brick walls without insulation — buildings where G can exceed 1.6 W/m³·°C.
| Era / Construction type | G coefficient | Characteristics |
|---|---|---|
| Pre-1945 — uninsulated old house | 1.8 — 2.5 | Stone/brick walls without insulation, single glazing, uninsulated loft |
| 1945–1975 — post-war construction | 1.5 — 1.8 | Uninsulated concrete, cavity walls, little or no roof insulation |
| 1976–1995 — first regulations | 1.1 — 1.5 | Partial insulation, early double glazing, MVHR rare |
| 1996–2005 — standard insulated building | 0.8 — 1.1 | Wall + roof insulation, double glazing, single-flow MVHR |
| 2006–2017 — enhanced thermal regulation (LU: RRE 2007) | 0.5 — 0.8 | Good insulation, double/triple glazing, dual-flow MVHR possible |
| 2018–2025 — low-energy building (Energy Passport C–D) | 0.3 — 0.5 | Triple glazing, enhanced insulation, dual-flow MVHR, thermal bridges managed |
| Passive house / NZEB (Energy Passport A–B) | 0.15 — 0.3 | Ultra-insulation, airtightness ≤ 0.6 ACH, high-efficiency dual-flow MVHR |
Want to find out the ideal power for your home and get quotes from certified installers in Luxembourg?
Get my personalised sizing →3 concrete Luxembourg sizing examples
Here are three representative cases from the Luxembourg housing stock, calculated using the simplified method (G × V × ΔT) and a design temperature of −10 °C. These examples illustrate the decisive impact of insulation on required power.
Example 1 — Old townhouse in Luxembourg City (pre-1950)
Characteristics: 130 m² living area, 2.6 m
ceiling height, uninsulated stone walls, partial single glazing, loft
later insulated
→ Heated volume: 130 × 2.6 = 338 m³
→ Estimated G coefficient: 1.6 W/m³·°C
→ ΔT = 20 − (−10) = 30 K
→ Heat losses = 1.6 × 338 × 30 = 16,224 W ≈ 16.2
kW
Recommended heat pump: 13–16 kW (bivalent
mode, 80% coverage at design temperature)
Priority: insulate before
installing
Example 2 — 1990s detached house in Bertrange
Characteristics: 160 m² living area, 2.5 m
ceiling height, partial insulation, double glazing, single-flow
MVHR
→ Heated volume: 160 × 2.5 = 400 m³
→ Estimated G coefficient: 1.0 W/m³·°C
→ ΔT = 20 − (−10) = 30 K
→ Heat losses = 1.0 × 400 × 30 = 12,000 W = 12
kW
Recommended heat pump: 10–12 kW (monovalent
mode possible with Inverter)
Good candidate for direct
air-water heat pump
Example 3 — New low-energy house in Strassen (2022)
Characteristics: 180 m² living area, 2.5 m
ceiling height, triple glazing, enhanced insulation (energy passport C),
dual-flow MVHR
→ Heated volume: 180 × 2.5 = 450 m³
→ Estimated G coefficient: 0.35 W/m³·°C
→ ΔT = 20 − (−10) = 30 K
→ Heat losses = 0.35 × 450 × 30 = 4,725 W ≈ 4.7
kW
Recommended heat pump: 5–7 kW (monovalent
mode, underfloor heating 35 °C, SCOP 4.4–4.7)
Ideal candidate for geothermal or
high-performance air-water heat pump
The same floor area (around 150–180 m²) may require between 5 kW (passive house) and 16 kW (uninsulated old building) — a 1-to-3 ratio. Building insulation is by far the most decisive factor in heat pump sizing in Luxembourg.
The 80–100% rule: monovalent or bivalent operation
A heat pump does not need to cover 100% of heat losses at the design temperature — in fact, this is rarely economically optimal. EN 12831 defines two operating modes:
Monovalent mode (heat pump alone, 100% of heat losses)
The heat pump covers all heating needs, even during extreme cold peaks. This mode applies mainly to well-insulated buildings (energy passport C or better), new builds, and homes with underfloor heating. The heat pump is sized at 100% of calculated heat losses.
Bivalent mode (heat pump + backup, 80% of heat losses)
The heat pump is sized to cover 80% of heat losses at the design temperature. An electric backup (integrated resistance) or existing boiler takes over on the 5 to 15 coldest days of the year. The heat pump + backup combination must cover 120% of heat losses.
Geothermal heat pump sizing specifics
Geothermal (water-to-water) heat pump sizing involves two inseparable aspects: the heat pump power itself (calculated from building heat losses, as for an air-water heat pump) and the size of the geothermal collectors (area or length), which determines how much energy can be extracted from the ground.
Horizontal collectors (available land)
Pipes are buried at 60–120 cm depth over an area typically 1.5 to 2 times the habitable floor area. Extractable power depends on soil type (20–50 W/m² depending on moisture). For a 10 kW heat pump: 200–500 m² of collector area is typically required.
Vertical boreholes (restricted land)
Vertical boreholes reach 50–150 m depth. Extractable power per metre varies from 30 W/ml (dry rock) to 85 W/ml (water-bearing rock). For a 10 kW heat pump with COP 4.0: ground extraction power = 7.5 kW, requiring approximately 90–250 ml of borehole depending on geology.
In Luxembourg, installing vertical geothermal boreholes requires administrative authorisation from the Ministry of the Environment. A geotechnical study must be carried out beforehand, incorporating the Grand Duchy’s geological map. Final sizing uses simulation software compliant with EN 17522:2022.
5 common sizing mistakes in Luxembourg
Gas or oil boilers are oversized by 30 to 100% versus actual needs. Replacing a 24 kW boiler with a 24 kW heat pump is almost always a mistake.
Some online tools use French design temperatures (−7 °C for zone H1). Luxembourg’s design temperature is −10 °C for most of the territory, increasing ΔT by 3 K and raising required power by ~10%.
A heat pump sized for underfloor heating (35 °C flow) cannot be used on high-temperature radiators (70–80 °C) without major COP degradation. The emitter type determines required water temperature, which determines actual COP.
Installing a 16 kW heat pump in a house that will later be insulated (reducing losses to 8 kW) results in massive oversizing. Best practice: insulate first, then size the heat pump on post-insulation heat losses.
Manufacturers typically display power at +7 °C (A7/W35). At −10 °C, an air-water heat pump loses 20–40% of rated power. A model advertised at 12 kW may only deliver 7–9 kW at −10 °C. Always check the power curve at Luxembourg’s design temperature.
The EN 12831 sizing report: mandatory for Klimabonus
The simplified method above is useful for estimating your project. But for the actual installation and to obtain the Klimabonus, your certified installer must produce a sizing report compliant with European standard EN 12831. This document is both a technical requirement and the key piece of your subsidy application.
What the EN 12831 report contains
- Room-by-room calculation of heat losses by transmission (walls, windows, roof, floor) and ventilation
- Accounting for thermal bridges and air permeability
- Design outdoor temperature for the exact municipality
- Required nominal heating power at design temperature
- Justified heat pump selection (power at design temperature from manufacturer curves)
- Operating mode (monovalent or bivalent) with backup power
- Calculated flow temperature for each emitter
Learn more about your heat pump project
Frequently asked questions about heat pump sizing in Luxembourg
What heat pump power is recommended for a 100 m² home in Luxembourg?
It depends entirely on insulation. For a 100 m² home with 2.5 m ceilings (volume 250 m³): uninsulated old building (G = 1.8) → approx. 13.5 kW; 1990s building (G = 1.0) → approx. 7.5 kW; recent well-insulated home (G = 0.4) → approx. 3 kW. Only a professional EN 12831 thermal assessment gives the exact value.
Does a heat pump’s power really drop in very cold weather?
Yes — it’s a physical characteristic of all air-water heat pumps. As outdoor temperature drops, the heat pump extracts less energy from the air. At −10 °C, a heat pump may deliver 20–40% less than at +7 °C. Always check the manufacturer’s power curve at −10 °C (Luxembourg’s design temperature), not just at A7/W35. Geothermal heat pumps are unaffected as ground temperature stays stable.
Do I need a thermal assessment before buying a heat pump?
Absolutely. It’s not only best technical practice, but also an implicit Klimabonus condition. If the installed heat pump is clearly over- or under-sized relative to the building’s heat losses, the Klima-Agence may challenge the application. Any serious installer carries out this assessment before quoting.
What power heat pump to replace a 24 kW gas boiler in Luxembourg?
In most cases, an 8–14 kW heat pump is sufficient. Gas boilers rated at 24 kW are typically 50–100% oversized. A thermal assessment usually reveals actual heat losses of 8–14 kW for a standard Luxembourg home from the 1990s–2005s. Never replace a boiler with a heat pump of the same power without a prior thermal assessment.
Get the ideal power calculated for your home
Our certified installers in Luxembourg carry out the EN 12831 thermal assessment and provide a sizing report included in your quote. Compare multiple offers for free and optimise your Klimabonus.