BMK 20-50kW Biomass Boilers

The Guntamatic BMK range of log-burning biomass boilers.

Guntamatic: The BMK Range

These highly efficient, modern log boilers have heat outputs from 20-50kW.

They are cased in stainless steel for a hard-wearing and sleek finish.

The BMK is MCS accredited and the whole range is RHI compliant.

Seriously Low Emissions

The BMK's emissions have been measured as significantly below the maximum emissions set out as a requirment for the RHI. The values of each model are in the table below and you can see the certificates here.

Boiler PM NOx

Maximum Emissions

30 g/GJ 150 g/GJ
Guntamatic BMK 20 Emissions 8 101
Guntamatic BMK 30 Emissions 14 85
Guntamatic BMK 40 Emissions 19 99
Guntamatic BMK 40 Emissions 16 109

Modulating Log Boiler

Thanks to innovations in the field of log gasification technology, the BMK is one of a new generation of ‘modulating’ log boilers. It features primary and secondary Servo air controls, achieving high standards in terms of reliability, flexibility and efficiency.

A Complete Burn

Your logs should have moisture content of below 20% and be free from contaminants and metal. The BMK has a large 580mm deep combustion chamber that offers controllable combustion so that the system can cope with a range of fuel quality. Load in a huge volume of logs (up to 500mm long) and you won’t need to fill it again for a long time.

And with just 0.5-1% ash to the volume of fuel, you won’t need to empty the ash bins too often, either.

Key features of a BMK boiler


The BMK features Guntamatic’s innovative self-cleaning grate, with moving turbulators that keep the heat exchanger tubes free from deposit buildup. This ensures your boiler’s efficiency is always at its maximum.

Fantastic Control

Touch screen control panels are supplied as standard with all BMK biomass boilers. They give you access to vital performance information such as efficiency and output, as well as allowing you to diagnose faults, keep on top of maintenance. You can even control the boiler from your smartphone (with the optional GSM module).

Automatic Hot Air Ignition

The modern and sophisticated design of the BMK range incorporates an automatic hot air ignition option, making it one of the most advanced designs on the market.

50 Years’ Experience Built In

Having recently hit their 50-year anniversary, it’s clear that Guntamatic are market leaders in renewable energy technology. Experience means that every boiler they make is created with huge industry expertise, and the BMK is no exception.

Parts Available for 20 Years

Guntamatic are unique amongst biomass boiler manufacturers as they make all of their own structural components. Every single part of the BMK is of the highest standards, offering you maximum return on your investment, low maintenance requirements and a long service life. You can get hold of spare parts easily as Guntamatic guarantees to make parts available for 20 years.

European Training Academy

They put an equal amount of effort into developing installers at their European Training Academy, so an authorised, highly trained and experienced engineer will fit the system for you.

BMK System Dimensions

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BMK Technical Data 20/30 40/50
Fuel Seasoned Logs, Max Length 500mm, Max 20% Moisture
Min-Max Output (kW) 15-30 20-50
Width (mm) Incl Insulation 973 973
Height (mm) Incl Insulation 1,440 1,640
Depth (mm) Incl Insulation 940 940
Width (mm) No Insulation 965 965
Height (mm) No Insulation 1,310 1,510
Depth (mm) No Insulation 795 795
Fuel Capacity (Ltrs) 166 215
Combustion Chamber Width (mm) 370 370
Combustion Chamber Depth (mm) 580 580
Shipping Weight (kg) 630 730
Water Contents (Ltrs) 125 175
Operating Pressure (Bar)    
Flow & Return Dia (Inches) 1 1/4 BSP 1 1/4 BSP
Flange Height (Flow) mm 1,047 1,247
Flange Height (Rtn) mm 568 568
Flue Connection (No Elbow) mm 1,334 1,534
Flue Connection Dia (mm) 150
Required Flue Draft (pa) 2
Flue Draught Fan 0.12
Power Supply 230V/ 13A




What is the calorific value of logs?

5.1 kWh per kg, depending on the moisture content of the logs.

What kind of logs should I burn?

Logs need to be sustainably sourced, as dry as possible and have no coatings or preservatives. Only use logs that have been seasoned for 1-2 years before use in your biomass boiler. The moisture content needs to be 20%, otherwise energy is wasted burning off moisture, meaning your boiler isn’t as efficient as it could be.

How long do logs take to dry?

Hard woods need longer to season - around two years. Soft woods need around one year. The ideal size is 500cm long and 12-15cm thick.

Where can I find a good log supplier?

We always recommend using the fantastic Biomass Suppliers’ List (BSL) for a comprehensive list of high quality suppliers across the UK.

How much should pay for logs?

Logs will cost you approximately £90 per tonne.