New build BBC or passive: how to size a pellet boiler in Luxembourg from concept stage

What output for a new build BBC or passive project?

In a BBC or passive home, heat demand is very low — typically 15 to 30 W/m² depending on the envelope. An oversized boiler short-cycles, loses efficiency and burns pellets for nothing. For a new single-family BBC home, 6 to 12 kW usually covers the load.

Always size to the calculated peak from the energy study, never to outdated rules of thumb from uninsulated buildings.

• Design load: 15 to 30 W/m² depending on envelope quality
• Modulating output is a must for stable part-load operation
• Size to the calculated peak, not to outdated references
• Plan a tuning visit after the first heating season

Pellet silo and buffer tank: where to place them at concept stage?

The pellet silo is a technical room in its own right: blower-truck access within 30 m, dry and ventilated. The buffer tank (≥ 500 L for a house, 1 000 to 2 000 L for collective housing) keeps boiler cycles long and protects seasonal efficiency.

• Silo: at least 6 m² in a house, twice that in collective housing
• Truck-to-silo distance ≤ 30 m, filling port accessible from the street
• Buffer tank: 20 to 30 L per kW of boiler output
• Check ceiling height: 60 cm diameter for a 1 000 L tank

What financial support in 2026?

Klimabonus 2026 supports pellet boilers only — logs and wood chips are not eligible. +15% bonus with a buffer tank, +€1,000 with thermal solar.

• Up to €8,000 for a single-family home (€4,000 without replacing an old system)
• €6,000 per dwelling in collective housing (capped at €40,000)
• +15% bonus with buffer tank, +€1,000 with thermal solar
• Stackable with municipal grants, which vary by commune

What about seasonal efficiency and maintenance?

Seasonal efficiency (ηs) must reach ≥ 90% under EcoDesign UE 2015/1189. For the architect, that means specifying Class 5 EN 303-5 and planning access for the annual service: flue sweep, ash removal, silo check.

• EcoDesign UE 2015/1189: ηs ≥ 90%
• Class 5 EN 303-5: biomass boiler quality benchmark
• ENplus-certified pellets for steady efficiency
• One service visit/year: plan access and connections at concept stage