Pellet boiler in new passive/BBC builds: what architects must lock in at preliminary design
kW sizing, pellet silo, buffer tank, seasonal efficiency and Klimabonus 2026 for architects and developers on new passive or BBC projects in Luxembourg.
How many kW does a passive or BBC new build need in Luxembourg?
A very well-insulated house (≤ 15 kWh/m².yr) requires only 12–18 W/m² of heating capacity. For 150 m² of heated floor, that is 1.8–2.7 kW — but the boiler must cover peaks, domestic hot water and restart periods.
Practical rule: nominal output = 100–120% of total demand (heating + DHW), capped at 4× the mid-season load. A 10 kW boiler with 1:4 modulation runs steadily between 2.5 and 10 kW — ideal for a BBC home with underfloor heating or low-temperature radiators.
- ≤ 120 m² BBC: 8–12 kW
- 120–200 m² BBC/passive: 10–15 kW
- ≥ 200 m² or collective: project-specific calculation, typically 15–25 kW
Pellet silo and buffer tank: what to reserve at preliminary design?
The pellet silo is not a last-minute add-on. For one-season autonomy: 4–6 tonnes (~3 m³), minimum height 2.20 m for auger feeding. The buffer tank (500–1,000 L for ≤ 15 kW) smooths cycling and raises seasonal efficiency.
The plant room needs truck access for bulk delivery (6–8 m turning circle), a flue route (Ø 150–200 mm) and a separate ash storage area. Lock in these volumes and floor loads before the concrete pour.
- Silo: 2.5 × 2.5 m footprint, 2.20–2.50 m height
- Buffer: 60–80 cm diameter, 1.8–2.2 m tall — not in a cold corner
- Klimabonus: +15% grant with buffer tank, +€1,000 flat rate with solar thermal
Seasonal efficiency, EN 303-5 and maintenance: what the architect must plan for
EcoDesign (Regulation 2015/1189) and EN 303-5 Class 5 are the market benchmarks in Luxembourg. Current pellet boilers reach 92–95% seasonal efficiency with ENplus A1 pellets — provided there is a buffer tank, compliant flue design and annual servicing.
The architect provides access for the annual service (burner pot, combustion chamber, flue) and ash storage (1–2 bags/week in peak season). Without this access, efficiency drops and the Klimabonus application is at risk.
- Annual professional service: mandatory for warranty and grant documentation
- Pellets: ENplus A1 certification, moisture < 10%
- Flue: designed to EN 303-5, no 90° bends in the hot zone
New collective housing: pellets or wood chips?
On a Luxembourg new-build, the choice is also financial: Klimabonus 2026 subsidises pellet boilers only (€6,000 per dwelling, €40,000 project cap). Wood chips and logs are not eligible.
Pellets enable automated silo delivery, compact storage and hands-off operation — critical for property managers and landlords. Wood chips need 2–3× more storage space, strict moisture control and a bunker with reclaim auger.
- Pellets: 10–30 t silo for 4–12 dwellings, truck access mandatory
- Chips: only if local supplier and storage area are secured long-term
- Municipal top-ups: vary by commune, stackable with Klimabonus — use the simulator at aides.klima-agence.lu
Sources
Need personalised advice?
Our team supports you with selection, sizing and subsidy paperwork.