Sizing a pellet boiler for a new low-energy or passive build in Luxembourg
A practical method for architects in Luxembourg: boiler output, pellet silo, buffer tank, efficiency and Klimabonus 2026 — everything to plan from day one.
What pellet boiler output for a low-energy or passive home?
Oversizing hurts both the budget and the efficiency of a new build. A low-energy (BBC) envelope consumes 30 to 50 kWh/m²/yr; a passive home drops to 15 kWh/m²/yr. For a 120 m² house, useful output typically ranges from 6 to 12 kW in a BBC project, and from 4 to 8 kW in a passive one. Since pellet boilers struggle to modulate below 30 % of rated output, a buffer tank is not optional — it is essential.
- BBC (≤ 50 kWh/m²/yr) → 6 to 12 kW for 120 m²
- Passive (≤ 15 kWh/m²/yr) → 4 to 8 kW for 120 m²
- Buffer tank: 30 to 50 L/kW to absorb short cycles
- In Luxembourg, only PELLET boilers qualify for the Klimabonus — not wood chips
Silo and buffer tank: how to integrate them from day one?
Size the silo on annual consumption: roughly 1 to 2 m³ per year for a BBC single-family home, at 650 kg/m³ pellet density. Keep it within 30 m of the boiler to limit vacuum losses, and provide a 100 mm fill port on the façade or pavement for the delivery truck.
- Silo volume: 1 to 2 m³/year in a single-family home; 5 to 15 m³ in small collective housing
- Silo-to-boiler distance ≤ 30 m, ideally ≤ 15 m
- 100 mm fill port on façade, accessible 365 days/year for the blower truck
- Vertical buffer tank 500 to 1 000 L: about 1 m² of floor space
What efficiency and maintenance should you plan for?
A modern pellet boiler reaches a seasonal efficiency of 88 to 94 % — provided it is well controlled and fed dry pellets. EU Regulation 2015/1189 sets the EcoDesign requirements; EN 303-5 Class 5 remains the quality benchmark. Insist on ENplus A1 pellets to limit heat-exchanger fouling.
- Seasonal efficiency ≥ 90 % on EcoDesign 2015/1189-compliant models
- Chimney sweep 1 to 2 times/year; full annual service: roughly €250 to €400
- ENplus A1 pellets: moisture ≤ 10 %, ash ≤ 0.7 %
- Service contract strongly recommended — often required to validate the Klimabonus
How much can the Klimabonus 2026 actually save?
The grant covers the installation, not the fuel. For a single-family home, you can claim up to €8 000 if you replace an old system, or €4 000 otherwise. For multi-family buildings, the grant reaches €6 000 per dwelling, capped at €40 000 per project. A buffer tank unlocks a +15 % bonus, and adding solar thermal brings an extra €1 000.
- Single-family: up to €8 000 (with replacement of an old system); €4 000 without
- Multi-family: €6 000/dwelling, capped at €40 000
- +15 % bonus with a buffer tank; +€1 000 with solar thermal
- Municipal grants: variable by commune, stackable with the Klimabonus — check the Klima-Agence simulator
Sources
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