Winter 2017 saw several instances of clinkering in biomass boilers in the UK. And yet the biomass boilers that experienced this problem were burning ENplus A1 wood pellets – the industry’s premier standard

Clinkering happens in a biomass boiler combustion chamber when ash melts and fuses into a hard, glassy slag. Ash is the powdery non-combustible residue left over after the wood pellets have been burnt.

So, why did this clinkering occur?

Biomass heating installers and engineers, and biomass boiler manufacturers, often blame the wood pellet fuel whenever a boiler breaks down. In clinkering cases, they might have said, ‘you must have received poor quality wood pellets in your last delivery.’

But what if your engineer was wrong? It’s like getting an incorrect diagnosis from a doctor, who then goes on to prescribe a medicine that has little chance of working. The only way to eradicate a problem is to identify and remove the cause.

Here are some common factors, or symptoms, of clinker cases:

  • Clinkering happened when a biomass boiler burned ‘too hot’
  • The problem cropped up with certain makes of boiler more than others
  • Top quality boilers, like the ETA boilers that we install, have not caused problems
  • It tended to happen in times of severe cold weather
  • It happened with a small number of batches of ENplus wood pellets

In other words, there are several factors which can be involved. While further research is needed to come up definitive answers, we can explore some of these factors in greater depth.

Let’s start by looking at boilers, because that’s where clinkering occurs.


What appears to have happened in the ‘clinker cases’ is that the temperature in the grates of the boilers burnt at a much higher temperature than normal.

Biomass boilers should be designed to ensure that the wood pellets combust at a temperature well below 1200⁰C. For example, the top of the range ETA boilers, which we recommend and install, burn at substantially lower temperatures. We did not come across any reports of ETA boilers with clinker problems.

However, some other makes of boilers were tested after clinkering and were found to have been burning at temperatures above 1,480⁰C. The combustion temperatures should be controlled by a biomass boiler itself, with a Lambda probe playing an important part. It should be sensing the combustion chamber temperatures and adjusting the fuel feed and air flow levels automatically. If the fuel feed or air supply are wrong, the boiler will overheat and melt the ash, causing the clinker to form.

Boilers that claim to be able to handle ENplus A1 wood pellets should be designed on the assumption that wood pellet ash may start to melt at 1,200⁰C. They should self-regulate to ensure that combustion takes place well below that temperature.

Clearly, some boilers were not up to the task – at least in the very cold temperatures of winter 2017 and with certain batches of ENplus pellets.

ENplus A1 wood pellets

ENplus is a quality standard for wood pellet fuel that was developed in 2010. The aim was to ensure that owners of biomass systems knew they were buying a good quality of wood pellets. In theory, it also enabled manufacturers of biomass boilers to design their equipment to burn all pellets that met the ENplus quality standards.

Wood pellets are manufactured from timber milled down to fine particles that are then forced through a 6mm die at great pressure to produce the correct size. The heat generated in this process melts the natural lignin in the wood, so binding the particles together. Because of the natural variability of the raw material, the wood pellets that come off the various production lines cannot be identical. It’s not like producing metal car parts.

Therefore, the ENplus A1 standard allows a range of specifications. It sets upper and lower limits for various characteristics such as pellet dimensions, moisture content, durability and calorific value. This means that different batches of pellets will vary in both their combustion temperature and their ash-fusion temperature.

The ash-fusion temperature of the pellets must be at least 1,200⁰C to comply with the ENplus A1 standard. This means that if the boiler burns at, say 950⁰C, you will not see clinkering.

The combustion temperature of a batch of wood pellets will vary according to their energy density. This energy density, which results from the calorific value of the pellets and their bulk density, determines how much energy is fed to the boiler. The more energy that is fed to the boiler, the hotter it gets.

Other factors

The severe cold weather in winter 2017 put more strain on boilers. They were called on to burn a greater amount of wood pellets than in milder weather. Also, there is less chance to shut down boilers to service them in cold weather. If it had been some time since the boilers had been serviced and the hoppers cleared out, there could have been a build-up of wood pellet fines and this might have been a factor in raising combustion temperatures.

Once the pellets were cleared out of the ‘clinker boilers’ and a fresh consignment of pellets was delivered, the clinker problems usually disappeared. It’s not clear whether this was because the new pellets were of lower energy density, or because the biomass systems worked more efficiently once they had been serviced.

Yet another possibility is the biomass boiler installer, or heating engineer, had not adjusted the settings of certain boilers to their optimum levels. In certain makes of boiler, this could have been an issue, particularly where there was no automatic callibration linked to a Lambda probe.


So, further research is needed. What is clear though is that, it’s important not to jump to the easy answer that the wood pellets are at fault.

Also, if you are looking at having biomass installed, it makes sense to go with a top-quality boiler like the ETA, which is much less likely to run into problems.

The other point about the ETA boiler is that we can install remote monitoring equipment on to your boiler, so we can monitor its performance from our office. Nine out of ten issues can be fixed by our specialist engineers remotely, without the need for a site visit. Where a site visit is needed, our engineers will already have diagnosed the issue and will bring any replacement parts with them.

Boiler Replacement

If your boiler isn’t working as effectively as you’d like, it might be worth replacing it with an ETA or other top quality make. We’d be happy to run the figures for you, so you can assess whether this is a viable option.

Also, please ensure to have your boiler serviced at least once a year. Biomass systems with a heavy workload should be serviced several times a year. If you’d like us to service your boilers, please give us a call on 01437 761320.

Call our Biomass Helpline 01437 761320 or complete a contact form

If your boiler hits problems, whether related to clinker or anything else, we’re happy to try and help you – whether you are one of our customers or not.