Using Pellet Boilers with Thermal Storage

Scott Nichols, President
Tarm USA, Inc.

May 15, 2013

Why use thermal storage with pellet boilers?

Wood pellets are refined wood fuel. They are very dry, vary little from manufacturer to manufacturer, and burn consistently. However, the wood in wood pellets retains many characteristics of the raw wood they are comprised of. One of those characteristics is that wood does not light and extinguish cleanly and quickly. The simplest way to illustrate the smoke producing tendency of wood is by lighting a wooden match. There is a lot of smoke emitted when the match ignites and also when it is extinguished. However, while the match is burning, there is little smoke. Even though wood pellets are refined wood they exhibit the same tendency when burned. Modern pellet boilers take advantage of the refined nature of wood pellet fuel to the greatest extent possible by carefully metering air and fuel for the cleanest combustion. These boilers often have an ability to control combustion and modulate heat output, but will also turn on and off as needed. It is during these on/off cycles that the most smoke is produced.

Modulating pellet boilers operate relatively cleanly and efficiently between 30% and 100% of rated output. It is said that modulating output allows pellet boilers to closely match a heating load. However, practice has found that modulating pellet boilers will cycle on and off 1000-3000 times per heating season in a typical installation. Why are there so many on/off cycles with a modulating appliance? Primarily because heating loads are often not consistent. In addition to the inconsistent heating loads placed on boilers, there is a sizing conundrum. In order for a boiler to be able to provide heat during the coldest day of the year, it typically is oversized by 60-70% for the average heating day. Imagine 100,000 Btu output pellet boiler with an ability to reduce its output to 30% of maximum rated output, or down to 30,000 Btu/hr. Then imagine this boiler installed in an application with a 100,000 Btu design day load (coldest day) in New Hampshire. Finally, imagine that rather than being -20 degrees Fahrenheit (design temperature), the temperature is 30 degrees Fahrenheit and the pellet boiler is operating at 30% of its maximum output to try to match the relatively low heating load. If the heating load then disappears all together (night setback, zones switch off, the showering teenager finally shuts off the water), the boiler will soon be forced to shut down or risk over-heating. When there is no place for the heat from the fire to go and the boiler is operating at 30,000 Btu/hr. the water in the boiler will have to get hotter. At some point the boiler must switch off or it will make steam. A boiler having a 30 gallon water jacket capacity, which is a typical volume, will have a 20 degree Fahrenheit rise in water temperature in less than 10 minutes at 30,000 Btu/Hr. output if it is operating with no heating load. This means that absent an active and substantial heating load, even a modulating pellet boiler will have to shut down rather quickly or risk over-heating.

Fitting pellet boilers to heating loads (decoupling example):

In a typical Northern US location, 4800 heating hours per year are at 55 degrees Fahrenheit or lower. 3900 heating hours occur at 25 degrees Fahrenheit or higher. This means that 80% of all heating output is needed at 50% of less of peak heating load as can be seen in the below graph:

There is another way to explain on/off pellet boiler cycling with an automobile analogy. When a car is doing its job moving us from point A to point B, the engine is almost always running. We take it for granted, but even while a car is at a dead stop, the engine is turning at about 750 RPM, idling. The engine is capable of running while the wheels are stopped because it is decoupled from the wheels by the transmission. Without the transmission the engine would have to be shut off or it would stall. Why were decoupling transmissions created? Transmissions give our vehicles a broader range of operation and allow drivers to avoid constantly turning cars on and off. Can you imagine turning a gasoline engine car on and off each time it came to a stop light? The demand on the starter and battery would increase dramatically. Response to changing traffic lights would be dramatically slower. If the car would not restart, there could be real problems. Today automobiles utilize finely tuned electronic fuel injection, which makes starts and stops quite clean, but in the days of carbureted gasoline engines, engines more similar to wood burning boilers, turning engines on and off was comparatively dirty. (Of course hybrid automobiles do turn on and off cleanly, but these utilize advanced combustion controls and electric motors to start the car rolling again).

The thermal storage tank becomes the gear box.

Demands placed on a wood pellet boiler are similar to those placed on a carbureted car engine. With a pellet boiler there is a normal range of operation between idle (30%) and over-firing (<100%). There is also a level below idle (<30% boiler output in this analogy) when the boiler will have to be shut off or it will run improperly and produce a relatively smoky exhaust just like a car engine. With boiler demand (rather than stop lights), we have seasonal differences, diurnal differences, lifestyle differences (ex. are we having guests or not) and occupant control differences based on programmable thermostats, for instance. Just as a stopped and started car engine will need more turns of the starter, will use more fuel turning on and off, and will have more engine deposits, a boiler turned on and off is similar. A thermal storage tank de-couples the boiler output from the heating load like an automobile transmission decouples the engine from the wheels. By utilizing a buffer tank, a boiler facing a load less than 30% of its rated output will often be able to prolong its run time because the boiler will begin storing excess heat in the storage tank. When the heating load resumes, often the load will be greater than 30% of the boiler output. If this happens, some heat will come from the thermal storage tank and some will come from the boiler, which can continue to operate cleanly at just 30% output. Eventually the thermal storage tank may be depleted of heat and the boiler will begin modulating to a higher output. If there is a heating load, but it is lower than 30% of the boiler output and the thermal storage tank becomes fully charged with heat, the boiler will extinguish its fire. As a heating load resumes or continues, the boiler will only come back on when the thermal storage tank begins to reach its low temperature setting. The boiler will then have much longer off cycles and reduced needs for turning on and off. As with automobiles that don’t require shutting the engine off at the stop light, pellet boilers used with thermal storage tanks will also have faster response to demand because the immediate heating needs can come from stored energy in the thermal storage tank while the boiler is re-activating its fire. Reducing on/off cycles furthermore reduces condensation in venting components, reduces mechanical wear, reduces emissions, uses less electricity, and increases efficiency.

Do pellet boiler burner and/or technology have a large effect on on/off cycling?

Typically, a residential pellet boiler runs about 1800 hours/year and has the same number of starts without a buffer tank. It turns out that boiler burner and technology has little influence on the number of on/off cycles. In side by side testing of pellet boilers with and without thermal storage, a major pellet boiler manufacturer using a drop-in fuel burner found on/off cycling to be reduced by about 60% in a calendar year with thermal storage. As a comparison another major manufacturer has indicated that a typical boiler using a bottom feed burner operating without thermal storage had 3000 on of cycles in one year. Of those on/off cycles, 1000 on cycles required the use of the electric igniter. Consequently, even though there are different pellet boiler operating platforms, testimony about on/off cycles appears consistent. Different boilers with completely different fuel feed and combustion control technologies face similar operating conditions and react similarly. There does not appear to be a special combustion or control technology that allows one boiler to operate markedly better than another below 30% heating load rates.

There is a publicly available report titled “Staged Combustion Biomass Boilers: Linking High-Efficiency Combustion Technology to Regulatory Test Methods”. It was produced by The New York State Energy Research and Development Authority and dated August 2010. This report was the product of a daylong industry round table meeting. The report accurately reflects the thinking of several major manufacturers of pellet burning appliances including Ökofen, Fröling, Köb/Viessmann, Evotherm, and Advanced Climate Technologies (ACT) that thermal storage is preferred for use with pellet burning boilers.

The report states,

“All participants in this working group voiced the opinion that storage is a very good if not essential part of a proper system installation.”

When might it be more acceptable to use a pellet boiler without thermal storage?

If pellet boilers could anticipate changing heating loads, thermal storage would not be as necessary. For instance, many pellet boilers in Europe control the heating systems in buildings. These heating systems typically use constant circulating low temperature distribution based on outdoor temperatures. In these types of systems, there are relatively few abrupt output changes demanded of the boiler. Any changes required of the boiler can also be anticipated because the boiler is controlling the distribution. In Austrian and German markets, some pellet boilers, about 30-40% of new units, are sold without thermal storage. In other European markets a higher percentage of thermal storage is used. These non-storage-integrated boilers are normally being used in more sophisticated distribution, modulating not only pellet feed rate, but also supply water temperature to match the load. This is a very different situation than what is typically found in the United States. In the United States it is typical that due to the installation costs of any boiler, very little heating distribution is updated. Most existing and even many new hydronic heating systems in the United States use simple on/off distribution, which is the hardest system for pellet boilers without thermal storage to work with. For this reason, it is considered best practice to utilize thermal storage with pellet boilers.

When is thermal storage an absolute must or of double benefit?

Using thermal storage with cascaded pellet boilers provides such great advantages that it should be considered a must. Where 2, 3, or 4 boilers are being operated as one unit there is dramatic heat output flexibility. Having all boilers react in concert with a single or bank of thermal storage tanks allows these boilers to react immediately to changing load conditions as an entire group, yet each individual boiler is able to achieve the longest run times possible. Pellet boilers combined with solar hot water systems also benefit by sharing common buffer tanks. Because the cost can be shared between both systems, the financial benefits can be substantial.

What are other advantages?

With a buffer tank, boiler operating hours in a household installation can be reduced by a third, to roughly 1200 hours/year on average.

Why isn’t thermal storage more widely accepted for use with pellet boilers?

Despite general agreement that using thermal storage is best practice, many U.S. based pellet boiler companies do not integrate pellet boilers with thermal storage. Because thermal storage has been a foreign concept, many pellet boilers sold in the United States are not built with controls that allow them to be installed easily and to operate smoothly with thermal storage. Thermal storage is also relatively expensive. Lastly, there is a space consideration, as thermal storage tanks generally contain 100-400 gallons of water and are relatively large. These conditions have led to much confusion about how best to sell and install pellet boilers.

Should thermal storage be made a mandatory for all pellet boiler installations?

While it is clear that using pellet boilers with thermal storage is best practice and should always be recommended, it is possible to install a modulating pellet boiler without thermal storage while keeping on/off cycling to a minimum. As stated above, heat distribution systems that create constant loads or slowly changing loads are beneficial. Pellet boilers that are sized for average loads rather than peak loads will also tend to have relatively few on/off cycles. Even modulating pellet boilers used without thermal storage offer a major exhaust emissions advantages over cord wood appliances. From a smoke and efficiency standpoint, a pellet boiler used without thermal storage will outperform most cord wood boilers. However, there are even some cord wood boilers that have extremely low emissions (some even challenge pellet boilers) when they are used with adequate thermal storage. Just as high efficiency condensing gas boilers are often installed in distribution systems that do not allow them to condensate and actually operate far below their stated condensing efficiencies, there should not be laws that require pellet boilers to be connected to thermal storage. However, as an inducement to encourage best practice in some markets, it could be a requirement that pellet boilers be used with thermal storage in order to take advantage of incentive offerings. At this time in North American markets, policy makers should take care to encourage best practice while generally encouraging high efficiency automatic pellet boilers, which present a very promising alternative to conventional fuel boilers.