This article is a guest post by John Siegenthaler, Principal at Appropriate Designs and expert instructor with HeatSpring. John’s 10-week online course, Hydronic-Based Biomass Heating Systems, teaches engineers, heating contractors, and installers how to design and install highly-efficient biomass heating systems.


In this article, John explains why it is wrong to assume that using a high-efficiency biomass boiler is as simple as piping it into the system alongside an existing oil-fired or propane-fired boiler. He explains that all biomass boilers must be combined with an appropriately designed “balance of system” to achieve unsurpassed comfort and optimum efficiency, some of the major components included in the balance of system, and what happens when there isn’t a “balance of system.”


  • HEATING CONTRACTORS: If you are a heating contractor and want to be better than your competition and install solid biomass heating systems, read this. John discusses why you must respect the unique operating characteristics of biomass boilers if you want them to operate at peak efficiency.
  • HOMEOWNERS: If you are considering installing a boiler system, read this article to get a better understanding of how modern biomass boilers work vs. earlier generation wood-burning boilers.
  • ENGINEERS: If you are an engineer hoping to learn more about the design of biomass boiler heating systems, this article will  serve as a good introduction to designing “balance of system” and includes additional resources for continued learning at the end.


Modern biomass boilers are as different from earlier generation wood-burning boilers as smartphones are from their rotary dial predecessors. For example, modern pellet fired boilers can operate unattended for several weeks. They automatically convey pellets from bulk storage to their combustion chamber, ignite them, and modulate the combustion process based on water temperature and the oxygen content of the exhaust gases. Some models even compress the small amount of ash produced, which lengthens the time between cleanings. Several of the pellet-fired boilers now available in North America can operate at combustion efficiencies of at least 85%. Models currently used in Europe can ramp this up another 5 to 7% by condensing flue gases before they are vented away from the boiler.

For those who want to burn cordwood, modern wood-gasification boilers offer mid-80 percent steady state combustion efficiencies, and full cycle efficiencies in the range of 70 to 75 percent. This is much higher – almost double – the average burn cycle efficiencies achieved by outdoor wood-fired furnaces using single stage combustion.

With increasing prices on the traditional fossil fuels that are commonly used in rural and semi-rural homes (e.g., #2 fuel oil, and propane), wood-based biomass boilers are getting lots of second looks these days, especially in the Northeastern US where burning wood for home heating is ingrained in the culture and already used in tens of thousands of homes.


The average heating contractor may assume that using a high efficiency biomass boiler is as simple as piping it into the system alongside an existing oil-fired or propane-fired boiler. Their thinking is that the biomass boiler handles the heating load when it’s operating, and that load is automatically handed off to the conventional boiler if the biomass boiler runs out of fuel. Although this seems logical, it ignores the unique operating characteristics of biomass boilers that must be respected if they are to operate at peak efficiency.

To achieve unsurpassed comfort, as well as optimum efficiency, all biomass boilers much be combined with an appropriately designed “balance of system.” The latter refers to all aspects of the complete heating system other than the biomass boiler. Some of the major components in the balance of system include a properly sized thermal storage tank, a properly configured control system, high efficiency circulators, mixing valves, and low temperature heat emitters. If any of these components or subsystems is improperly selected or installed, it’s likely the biomass boiler will not operate as intended. This will be reflected through increased fuel consumption, higher emissions, and compromised comfort. Although the boiler usually gets the blame, it’s typically not the root cause of most operational problems.


Modern hydronics technology provides a “media” that can be skillfully manipulated to create an ideal balance of system for all types of biomass boilers. Through proper selection and sizing of hydronic system components, the biomass boiler is encouraged to operate over long burn cycles where it achieves optimal efficiency and minimal emissions. The heat it produces is stored in a properly stratified thermal storage tank, awaiting a need for space heating or domestic water heating. When that need is present, the heat is silently conveyed to where it’s needed using less electrical power than an average light bulb. The heat is gently released into the space without the occupants even noticing. Comfort is maintained under all operating conditions. When necessary, an auxiliary boiler assumes the heating load with a seamless transition, and no compromise in comfort.


There are many ways to create balance of system designs depending on the exact type and size of biomass boiler that will supply the heat. In my “Hydronic-Based Biomass Heating Systems” course, we explore the possibilities for wood-gasification boilers, pellet-fuel boilers, and boilers that burn wood chips. We review the “building blocks” and subsystems and go on to merge them into several complete and fully documented example systems. My course focuses on the nuts and bolts needed to achieve optimum results using readily-available hardware. After completing this course, designers will know how to craft balance of systems that are as elegant as the biomass boilers that connect to them.


  1. Continue the Conversation – Hydronic-Based Biomass Heating LinkedIn Group: Join a community of contractors, engineers, installers, policy makers, and more
  2. Free 60-Minute Lecture: Temperature Stacking in Thermal Storage for Biomass Heating Systems: In this lecture, John Siegenthaler describes a unique method for managing the operation of biomass-fueled as well as auxiliary boilers for optimum system performance. He goes on to describe how temperature stacking is accomplished using multiple temperature sensors mounted in different vertical locations within a thermal storage tank and off-the-shelf controllers. He explains how to use the temperature stacking technique in systems using multiple biomass boilers as well as systems that combine a biomass boiler with an auxiliary boiler.
  3. Free 60-Minute Lecture: Low Temperature Heat Emitter Options in Hydronic Systems: The future of hydronic heating is low water temperature. This is necessary to optimize the performance of modern heat sources such as condensing boilers, heat pumps, and solar thermal collectors. With proper design, you can create systems that require supply water temperatures no higher than 120°F under design load conditions. In this lecture, you will learn why low operating temperatures are important in modern hydronic system, the differences between high-mass and low-mass heat emitters, details of the operation of micro-fan equipped panel radiators, how to describe the construction of radiant wall and ceiling panels, the best distribution piping system to use with low-temperature emitters, and more.
  4. Free Course: High Performance Building and HVAC 101: This is the most comprehensive free course on building science and high performance HVAC systems available on the internet It’s designed for engineers, architects, builders, trade contractors, and energy raters.
  5. Read 5 Ways the Biomass Industry is Laying the Foundation for Explosive Growth

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