What is an ErP rating for boilers?

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.

What is SEDBUK and is it still used?

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.

What is modulation range and why does it matter?

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.

Why is real-world efficiency lower than the lab rating?

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.

How can I maximise my boiler efficiency?

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%.

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.

How We Calculate This

Frequently Asked Questions

Last updated: April 2026

All calculations are estimates. Verify with your supplier.

Boiler Efficiency Ratings Compared

Efficiency Rating Systems Compared

Modulation Range by Brand

Lab vs Real-World Efficiency

Related Calculators

How We Calculate This

Frequently Asked Questions

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