For those who want to burn cordwood, modern wood-gasification boilers offer mid-80% steady state combustion efficiencies and full-cycle efficiencies in the range of 70% to 75%. This is much higher — almost double — the average burn cycle efficiencies achieved by outdoor wood-fired furnaces using single-stage combustion.
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 steady state 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.
The upward trending and often unpredictable cost of No. 2 fuel oil and propane, which are commonly used in rural and semi-rural buildings, have stoked interest in high-efficiency, wood-based biomass boilers. This is especially true in the northeastern United States, where burning wood for home heating is ingrained in the culture and already used in tens of thousands of homes.
Beyond ‘box swapping’
The average heating contractor might assume that installing a high-efficiency biomass boiler is as simple as piping it into the system along side an existing oil-fired or propane-fired boiler. He assumes the biomass boiler supplies the load when it’s operating and automatically hands off the load to the conventional boiler if it runs out of fuel. Although this seems logical, and is partially true in some systems, it often overlooks the unique operating requirements of biomass boilers that must be respected if peak performance is to be realized.
To achieve high thermal efficiency, low emissions and unsurpassed comfort, all biomass boilers must 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 and piped 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 perform up to its potential. 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.
Here’s an example. Many state-of-the-art, pellet-fueled boilers are capable of modulating their heat production down to about one-third of their rated capacity. Although this improves their ability to match heat production with heat demand, it doesn’t eliminate the possibility of short-cycling under low-load conditions, especially if the pellet-fueled boiler is supplying a highly zoned distribution system with low thermal mass heat emitters. A homerun distribution system serving several panel radiators, each equipped with a thermostatic radiator valve, would be an example of such a distribution system.
To put this in perspective, have you ever watched a modern gas-fired mod/con boiler short-cycle when connected directly to a highly zoned distribution system, without a buffer tank? I often ask the contractors who attend my training seminars this question. Considering the responses I get, it seems short-cycling remains a rather pervasive issue in our industry.
Now, compare this situation to that of modulating pellet-fueled boilers. Most current generation gas-fired mod/con boilers have turndown ratios of at least 5:1. Most current-generation pellet-fueled boiler have turndown ratios of 3:1, with some managing 4:1. The smaller the turndown ratio, the more limited the boiler’s ability to match the heating load, especially if it’s oversized relative to design load. This implies a pellet-fueled boiler, with a lower turndown ratio, is even more likely to short-cycle relative to a gas-fired mod/con boiler, in systems without adequate thermal storage.
One way to avoid such problems is to install a thermal storage tank between the pellet-fueled boiler, as shown in Figure 1.
Notice the thermal storage tank is piped differently than in other systems using buffer tanks. The details of this piping are “complementary” to the operating characteristics of the pellet-fueled boiler. They also allow adequate buffering of the auxiliary boiler while not requiring that boiler to heat the entire buffer tank. These piping details enhance temperature stratification within the tank.
There also are three different temperature sensors, located at different positions in the tank. Their placement, and the control algorithm they facilitate, also are complementary to the operation of both boilers.
Getting optimal performance from a thermal storage tank supplied by a pellet-fueled boiler requires more than just calculating the tank’s volume and selecting a readily available product of the required volume. A tank with the wrong piping connections will not stratify properly. This reduces the usefulness of the tank when attempting to create long operating cycles for the pellet-fueled boiler.
I’ve seen this happen in several systems involving tanks that cost several thousands of dollars. One example is shown in Figure 2.
This suboptimal performance is both unfortunate and avoidable. The latter requires careful consideration about how the pellet-fired boiler and auxiliary boiler should operate, knowledge of the flow and temperature characteristics of the load, and how all this affects flow into and out of the tank. This information can then be used to determine the size and locations of piping connections that encourage rather than destroy temperature stratification.
Custom design
Modern hydronics technology provides a “media” that can be skillfully manipulated to create an ideal balance-of-system for all types of biomass boilers. Though 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 minimal electrical power. The heat is gently released into the space without the occupants even noticing. When necessary, an auxiliary boiler assumes some or all of 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. Designs optimized for wood-gasification boilers are not necessarily optimal for pellet-fueled boilers, or those burning higher-moisture wood chips.
Want to learn more? This spring I’ll be teaching an online course that drills down into the details of combining biomass boilers with modern hydronics technology — “Hydronic-Based Biomass Heating Systems.” It will be delivered over 10 weeks, through partnership with the HeatSpring Learning Institute and the Biomass Thermal Energy Council. This course explores the possibilities for using wood-gasification boilers, pellet-fuel boilers and boilers that burn wood chips. It introduces building blocks and subsystems, and goes on to merge them into several complete and fully documented systems.
It focuses on the nuts and bolts needed to achieve optimum results using readily available hardware. If you’re interested, consider taking the free “test drive” course at bit.ly/btecbiomass.
Biomass-fueled boilers and modern hydronics technology are made for each other. The boilers deliver heat from a renewable fuel source that is often locally sourced and substantially less expensive than fuel oil or propane. Modern hydronics technology moves heat when and where it’s needed and delivers it with unsurpassed comfort. I’m confident this complementary relationship has set the stage for substantial market growth as more heating professionals learn what’s available.
Are you ready to seize that opportunity?
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