Biomass Boilers – Selection Criteria

What are biomass boilers?

Biomass boilers, also called wood-fuelled boilers, burn wood to generate heat. Unlike a wood-fuelled stove a biomass boiler connects to the radiators and a hot water cylinder to provide your entire home with heating and hot water.

For more details and a general overview of what these products are, please see our Product Guide. For solid fuel heaters designed to heat a single room (e.g. stoves) please see our pages for wood-fuelled stoves.

Introduction

This page details the selection criteria applied to wood-fuelled biomass boilers on toptenuk.org. These can often be called solid fuel boilers, however, the term biomass boiler is more common in the UK and will be used throughout this document. Only products that reach the minimum criteria specified below are displayed on the website. This guide only covers biomass boilers that provide heating for a central heat distribution system. For more details and a general overview of what these products are, please see our Product Guide. For solid fuel heaters designed to heat a single room (e.g. stoves) please see our pages for wood-fuelled stoves.

Selection Criteria

The minimum criteria to be shown on toptenuk.org is shown in Table 1 that covers both the energy efficiency of the system as well as air quality factors, a key consideration when burning wood.

Table 1: Current selection criteria for biomass boilers. This covers both energy efficiency and air quality factors.

Criteria	Pellets (automatic)	Logwood (manual)
Energy label	A+ or higher	A+ or higher
Particulate matter [mg/m3]	<15	<30
CO nominal load [mg/m3]	<30	<100
Organic Gaseous Compounds (OGCs) nominal load [mg/m3]	<10	<15
NOx [mg/m3]	<150	<150

Relevant Regulations

The following regulations were the basis for the above selection criteria for biomass boilers.

Energy Labelling (EU 2015/1187)

Since September 26^th 2019, biomass boilers should come with an energy efficiency label, to provide information on the brand, the model, the energy efficient class, the direct and if relevant the indirect rated heat output, as well as the additional electricity generation function in kW. It applies to appliances with rated output up to 70 kW. Table 2 shows the details of the information contained in an energy label.

Table 2: Energy Labelling

The Energy Efficiency Index (EEI) is defined using :

EEI = η_son × 100 × BLF - F(1) - F(2) × 100 + F(3) × 100

Where,

ηson	is the seasonal space heating energy efficiency in active mode
BLF	is the biomass label factor (1.45 for biomass, 1 for fossil fuel)
F(1)	adjusts the EEI due to contributions from the temperature controls
F(2)	adjusts the EEI due to auxillary electricity consumption
F(3)	adjusts the EEI due to the electrical efficiency of solid fuel cogeneration boilers

For more details on how these values are calculated, please refer to the regulation document: EU 2015/1187. Once calculated, individual products are categorised into energy efficiency classes using Table 3.

Table 3: Definition of the Energy Efficiency Class of Biomass Boilers

Energy efficiency class	Energy efficiency index (EEI)
A+++	EEI ≥ 150
A++	125 ≤ EEI < 150
A+	98 ≤ EEI < 125
A	90 ≤ EEI < 98
B	82 ≤ EEI < 90
C	75 ≤ EEI < 82
D	36 ≤ EEI < 75
E	34 ≤ EEI < 36
F	30 ≤ EEI < 34
G	EEI < 30

Ecodesign (EU 2015/1189)

This regulation applies to appliances up to 500 kW and covers both efficiency and air quality measures. The requirements are specified for the following topics, applicable from the years mentioned onwards:

• 2017: information requirements (product fiche, technical documentation, product label)
• 2019: product database (upload of technical product documentation required)
• 2020 Tier 1: MEPS (Minimum Energy Performance Standards) for energy efficiency, emission of particulate matter, organic gaseous compounds, carbon monoxide and nitrogen oxides.

Tier 1 products must meet the criteria detailed in Table 4.

Table 4: Ecodesign Tier 1 criteria for solid fuel boilers less than 500 kW

Criteria	Pellets (automatic)	Logwood (manual)
Efficiency at rated load (%)	>75% (<20kW), >77% (>20kW)
Dust [mg/m3]	<40	<60
CO nominal load [mg/m3]	<500	<700
C org nominal load [mg/m3]	<20	<30
NOx [mg/m3]	<200	<350

Future Changes

Since the Ecodesign regulations MEPS have only been mandatory since January 2020, there is some uncertainty around how these criteria may change in the future. However, a preliminary estimate of this is included in Table 5. A tightening of the minimum energy class from A++ to A+++ is also possible.

Table 5: Expected Top Ten selection criteria for biomass boilers in 2022

Criteria	Pellets (automatic)	Logwood (manual)
Energy label	A++	A++
Dust [mg/m3]	<10	<20
CO nominal load [mg/m3]	<20	<50
C org nominal load [mg/m3]	<3	<10
NOx [mg/m3]	<120	<120

Glossary

• Particulate matter nominal load (mg/Nm³): specifies the particulate matter in the exhaust air at nominal load of the stove. Particulate matter is the microscopic particles of solid or liquid matter suspended in the air, and high levels of particulate matter can be hazardous to our health.
• CO nominal load (mg/Nm³): specifies carbon monoxide in the exhaust air at nominal load of the stove.
• Organic gaseous compounds nominal load (mg/Nm³) : specifies the organic carbon in the exhaust air at nominal load of the stove.
• NOx (mg/Nm³): specifies nitrogen dioxide in the exhaust air at nominal load of the stove.
• EU regulation 2015/1187 with regard to energy labelling of solid fuel boilers and packages of a solid fuel boiler, supplementary heaters, temperature controls and solar devices; https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A02015R1187-20170307

Links

EU regulation 2015/1189 with regard to ecodesign requirements for solid fuel boilers; https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A32015R1189&qid=1603279712274