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Boiler Efficiency Ratings Compared
Understand ErP ratings, SEDBUK, seasonal efficiency, modulation range and the difference between lab and real-world boiler performance in the UK.
Efficiency Rating Systems Compared
| Rating | ErP Class | Seasonal Efficiency | Boiler Type |
|---|---|---|---|
| A+++ | 150%+ | Heat pumps | ASHP/GSHP |
| A++ | 125-150% | Heat pumps | ASHP/GSHP |
| A+ | 98-125% | Solar + boiler | Package systems |
| A | 90-98% | Modern condensing | All modern boilers |
| B | 86-90% | Older condensing | Early condensing models |
| C | 82-86% | Non-condensing | Pre-2005 boilers |
| D-G | Below 82% | Old/inefficient | Should be replaced |
Modulation Range by Brand
| Boiler | Max Output | Min Output | Ratio |
|---|---|---|---|
| Worcester 8000 Life 30kW | 30 kW | 3.0 kW | 10:1 |
| Vaillant ecoTEC Exclusive 35kW | 35 kW | 3.3 kW | ~10:1 |
| Ideal Vogue Max 32kW | 32 kW | 5.0 kW | 6:1 |
| Baxi 800 30kW | 30 kW | 5.2 kW | ~6:1 |
Lab vs Real-World Efficiency
- Lab-rated 94% typically achieves 85-90% in real homes
- Condensing only occurs when return temperature is below 55°C
- Oversized boilers cycle more, reducing effective efficiency
- Low flow temperatures (55°C or less) maximise condensing operation
- Weather compensation controls can improve real-world efficiency by 3-8%
Related Calculators
Use the Boiler Size Calculator to correctly size a replacement boiler and avoid oversizing.
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How We Calculate This
Frequently Asked Questions
ErP (Energy related Products) is the EU/UK energy labelling system for boilers, rated A+++ (most efficient) to G (least efficient). All modern condensing gas boilers achieve ErP A (90%+ seasonal space heating efficiency). The rating considers seasonal performance across the heating season, not just peak efficiency. A-rated boilers range from 90-94% — the letter alone does not distinguish between good and excellent performers.
SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK) was the UK-specific efficiency rating, now largely replaced by ErP. SEDBUK 2009 ratings ranged from A (90%+) to G (below 70%). The SEDBUK database (pcdb.co.uk) still lists boilers with their seasonal efficiency percentages and is useful for SAP calculations. The percentage figure is more informative than the letter grade for comparing similar boilers.
Modulation range is how far a boiler can reduce its output below maximum. A 30 kW boiler with 10:1 modulation can operate down to 3 kW, while 5:1 modulation means a minimum of 6 kW. Better modulation means the boiler runs more efficiently at low loads (mild weather, small hot water demand) instead of cycling on and off. Cycling wastes energy through repeated ignition, purge cycles and standby losses.
Lab tests (EN 15502) measure efficiency under controlled conditions with steady-state operation, precise flow/return temperatures and no standby losses. Real-world boilers face: oversized installations running at part load, high return temperatures preventing condensing, short cycling, pump overruns, standby losses and pilot/ignition losses. A boiler rated 94% in the lab may achieve 85-90% in practice.
Key steps: lower the flow temperature to 55-60°C (or lower with weather compensation), ensure radiators are correctly sized to work at lower temperatures, add thermostatic radiator valves, insulate all pipework, service the boiler annually, descale the heat exchanger if needed, and fit a magnetic filter to prevent sludge. Lowering flow temperature by 10°C can improve real-world efficiency by 3-5%.
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Last updated: April 2026
Verified against UK standards · estimates only, confirm with your supplier.