Biomass boiler problems

..... and how to fix them

6 minute read

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Biomass boiler problems

Like any bit of machinery a biomass boiler can have issues, not operate correctly and even breakdown. Over the past 15 years and 800 installs, we have come across a number of issues and have learnt many valuable lessons along the way. In the vast majority of cases we find the root cause of the problem is not actually down to the boiler, though we'd be lying if we were to say they were never to blame. Below we'll run through the main issues that we come across and the likely reason for the fault.

Before we get into things we cannot stress enough the importance of having your boiler serviced annually, or even multiple times a year for process heat where the usage is high. Not only does this keep your system running efficiently, but it is one of your ongoing obligations of being on the RHI to have your boiler serviced as per the manufacturers requirements. When Ofgem audit sites service records are one of the first things they request to see along with records of your fuel purchases, so do make sure you keep your paperwork in order otherwise your RHI account can be suspended. More information on the benefits of servicing can be found on our biomass boiler servicing page.

This page only provides a brief overview of the most common issues, so is certainly not a silver bullet to resolve all issues. Do feel free to contact us to discuss the issues you are having and we will see what we can do to help you resolve them. We do offer biomass boiler consultancy services which includes fault finding and efficiency reports, which you can read more about here.

Biomass boiler not working

If your boiler is on but not operating, one of the first things to check is that there is fuel in the store. Believe it or not we've lost count of the amount of times we have attended a site that has a reported issue, only to find there is no fuel in the store. Depending on the type of fuel feed delivery system, quite often there will be some residual fuel in the store that cannot be taken up to the boiler, so do check to make sure that the chip / pellet is covering the auger or suction cups and can be conveyed to the boiler.

Most modern biomass boilers will have a control panel on it that displays fault codes. These can be cross referenced against the boiler manual which will describe the fault and what simple actions can be done to rectify.

Biomass boiler with poor combustion

Poor combustion will be clearly evident by the quality of the ash after combustion. If everything is operating correctly then you should be left with a fine dry ash that will brush easily off your fingers and not stick to them. If however you find incomplete combustion where there are unburnt bits of wood, or there is clinker, then there is a combustion issue.

A low fuel quality could be the cause of poor combustion. With wood chip this can be down to the moisture content being too high, or large variations in the sizes of the chip. For logs the moisture content can also cause poor combustion if it’s too high, or if the logs have not been processed to the right size and stacked in the combustion chamber correctly. The main cause in pellet boilers is if the fuel is too dusty, which can be the result of the pellets being delivered to the fuel store at too high a velocity by the fuel delivery company.

Getting the correct flue draught through the biomass boiler is fundamental to achieving good combustion. If it is too high or too low then you will get poor combustion. This is something that should be tested at the commissioning of the boiler, and if found to be outside the manufacturers requirements it should be addressed by re-designing the flue before the system his fully handed over to the customer.

If the boiler has not been setup correctly for your fuel type then this can result in poor combustion for a number of reasons, such as getting the primary and secondary air mix wrong.

Ensuring that the fuel quality is correct is certainly something most customers can do themselves. When it comes down to checking / ensuring the correct flue draught is achieved or that the boiler is setup correctly, these are far more specialist tasks that we would recommend you get a biomass engineer in to check.

Biomass boiler with bad efficiency

There are multiple things that can result in lower levels of efficiency. First and foremost if you don’t have your boiler serviced at least annually then it won’t run as efficiently as it should. Deposits can build up on the boiler which reduce the transfer of heat from the combusted fuel to the water that then circulates around the system. As part of any annual service a deep clean of the boiler is undertaken to ensure the heat from combustion can be transferred though the system efficiently.

Poor combustion can be one of the main causes of poor levels of efficiency as you will not get as many kWh out of the fuel purchased that you should be getting. The main causes of this are covered in the section above.

Biomass boilers operate differently to fossil fuel systems and should be sized appropriately. They can take time to get up to temperature, not relatively instantaneous like fossil fuel, and the biomass boilers like to work hard. Due to the way the RHI was setup, many people went for very oversized boilers that did not meet their heating requirements in an attempt to generate more RHI payments. This however will have resulted in systems running inefficiently.

All of the required ancillary items may not have been included, or could have not been sized correctly. The vast majority of biomass boilers are designed to work with thermal stores, though in a bid to trim costs these are sometimes omitted. The thermal stores also need to be sized appropriately for the chosen biomass boiler to ensure they run efficiently, as boilers of the same output can require different sized thermal stores e.g. a 100kW chip boiler would require around a 3,000 litre thermal store, whereas a 100kW log boiler would require at least a 5,000 litre store.   

During our consultancy visits to other sites for efficiency issues, one of the things we do come across regularly is that the system is constantly circulating when it should not. Here when there is no / minimal requirement for heat, the hot water is still constantly circulating, but instead of the energy been stripped off and used for heating in buildings, it just results in high thermal losses through the heating system. This can be exacerbated further if poor quality heat main has been used which has higher thermal losses, or if it has not been buried at the correct depth in the ground.

Altogether there are a number of factors that can result in poor levels of efficiency. It could be that just one of these is the cause, or there could be multiple factors all contributing. This can be quite a complex issue and we would certainly recommend having a profession undertake a detailed analysis of the system. This will pay for itself many times over during the lifetime of the system.

Biomass boiler not providing sufficient heating

If the biomass boiler is not sufficiently sized to cover the required heat load, then it does not matter how efficiently it is running, it will never provide a sufficient amount of heating. Typically though this is not an issue we come across as part of our consultancy service, though it is something to be mindful of.

The main cause we find for this issue when we visit sites for the first time, is that the system has just not be been designed correctly. There are wide ranging factors that can come into play here, but largely it will be down to components being incorrectly sized and specified. Previously we have simply rectified this issue by installing a larger duty pump, though in other cases a complete redesign is required with a number of different components and emitters upsized.

If this is an issue you are having then we would highly recommend you contact a heating engineer to review the system and issue a report with recommended actions.


As with many things in life, there is no one straight answer for all the above issues, and it could be one of many factors or a combination of them that cause the problem. There are some simple checks that most customers can initially do to at least narrow down the potential causes of the issues. When these are exhausted and things become more technical, then we would recommend you contact a reputable biomass boiler company so a qualified engineer can come to fully review the system. This consultancy service is something we can offer, and to save you money we can combine it with an annual service. For more information feel free to contact us.

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