Sizing a pellet boiler for a passive house in Luxembourg: 4 APS reflexes for architects
How to size a pellet boiler from the APS stage for a low-energy or passive project in Luxembourg: output, silo, buffer tank and Klimabonus 2026.
What output for a passive house in Luxembourg?
A 150 m² passive home in Luxembourg (≈15-30 kWh/m²/yr) only needs 2,250-4,500 kWh/yr for heating. A BBC low-energy building stays around 6,750-9,000 kWh/yr.
At ≈4.8 kWh/kg of pellets, that is 470-940 kg in a passive home, 1.4-1.9 t in a BBC one. The recommended boiler output is 4-8 kW (passive), 6-12 kW (BBC): oversized units see their efficiency collapse at part load.
- Passive 150 m²: 4-8 kW, ≈ 1 t pellets/yr
- BBC 150 m²: 6-12 kW, ≈ 1.5-2 t pellets/yr
- Modulation 30 %-100 %: essential during shoulder seasons
Silo, buffer tank and plant room: what follows from correct sizing
The silo must hold one full heating season plus a safety margin: 1 m³ ≈ 650 kg of pellets. For a passive project, 2-3 m³ is enough (2-3 years of autonomy); for a BBC one, 4-5 m³ is more comfortable.
Design at APS a filling port accessible from the road, within 30 m of the blower truck. Place the 500-1,000 L buffer tank within 5 m of the boiler to limit thermal losses.
- Silo ≥ 3 m³ (BBC), ≥ 2 m³ (passive), bulk density ≈ 650 kg/m³
- Filling port to truck distance: ≤ 30 m, otherwise long auger needed
- Dry, ventilated plant room, maintenance access ≥ 80 × 80 cm
Pellets vs wood chips: what does the Klimabonus really say?
In Luxembourg, the 2026 Klimabonus only funds pellet boilers. Log and wood-chip boilers are simply not eligible.
This regulatory choice rests on the standardised quality of ENplus pellets (moisture ≤ 10 %, stable calorific value) and on lower emissions under EU Regulation 2015/1189 (EcoDesign).
- Eligible: pellet boiler (Klimabonus 2026)
- Not eligible: logs, wood chips, agri-pellets
- Can stack: variable municipal grants (Klima-Agence simulator)
Seasonal efficiency and maintenance: what to write in the specs
Require EN 303-5 Class 5 and EcoDesign 2015/1189 compliance. The seasonal efficiency of a modern pellet boiler reaches 85-92 %.
Annual servicing (de-ashing, flue cleaning, burner check) takes 1-2 h and costs €150-250. Plan a maintenance contract from handover.
- EN 303-5 Class 5 + EcoDesign 2015/1189: minimum bar
- Seasonal efficiency target: ≥ 85 %
- Servicing: 1-2 visits/yr, plan plant room access
Sources
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