Sizing a Pellet-Fueled Boiler

Bigger is NOT better

There are several types of boilers that can burn wood-based fuels. They include 2- stage wood-gasification boilers, wood chip boilers, and pellet-fueled boilers. Of these, pellet-fuel boilers come closest to mimicking the fully automatic operation of oil and gas- fired boilers. Modern pellet boilers can automatically load their fuel, ignite it, regulate the combustion process, maintain their internal surfaces free of sustained flue gas condensation, and even compress the small amount of ash they generate to lengthen time between ash removals.

Given these characteristics it’s easy to assume that using a pellet-fueled boiler is as simple as substituting it for a boiler that burns a conventional fuel. That implies that selecting a pellet-fueled boiler is as simple as determining the peak heating load of the building, and choosing a model with heat output capacity that at least equals the peak building load, and more likely exceeds that load by a generous safety factor.

Unfortunately, experience has shown that this is usually not the case.
Feedback from early adopters of pellet-fueled boilers, as well as conference presentations on operating experience with such boilers, indicates that short-cycling is a common characteristic in systems where a pellet-fueled boiler is selected the same way as a conventional boiler.

Snapshot: Most heating systems are sized based on a “snapshot” condition. That condition, commonly referred to as “design load,” is the theoretical rate of heat loss from a building when the indoor temperature is at an assumed comfort condition such as 70 oF, and the outdoor temperature is at a value called the 97.5% or 99% design temperature. The 97.5% design temperature for a given location is a value that the outdoor temperature is at or above 97.5% of the time during an average year. Viewed another way, the outdoor temperature is below the 97.5% design temperature only 2.5% of the time in an average year.

If one examines the “bin-temperature” data for a given location, it’s clear that the outdoor temperature is significantly warmer than the design temperature much of the year. Figure 1 (below) shows an example of bin temperature data for Syracuse, NY.

Fig 1

Imagine a building in Syracuse with a design load of 100,000 Btu/hr, based on 70 oF interior temperature and a 97.5% design temperature of 2 oF. If the bin temperature data is combined with this load information, another graph can be created that indicates the hours during which the heating load equals or exceeds the heat transfer rate shown on the vertical axis. An example of such a graph is shown in Figure 2 (below).

Fig 2

Now, consider a boiler that has been sized to provide only 50% of the design heating load.
The green area shown in Figure 3 (below) represents the portion of the total seasonal heating energy that would be supplied by such a boiler.

Fig 3

The green shaded area is 84.2% of the total area under the red curve. The implication is that this boiler, which most designers would say is only half as big a necessary, could still supply about 84.2% of the total heating energy required over the entire heating season.

If the boiler were sized for 60% of the design heating load, the energy it supplies would be represented by the orange area in Figure 4 (below). This boiler would supply 90.4% of the total seasonal heating energy requirement.

Fig 4

A similar analysis for a boiler sized to 75% of the design load indicates that it would supply 96.3% of the total heating energy requirement.

What’s the point? This type of partial load analysis shows that a boiler that’s sized significantly smaller than the design heating load could still supply a high percentage of the total heat energy needed. It also implies that a boiler sized to the design heating load (or that load plus a generous safety factor) will spend very little time operating at steady state conditions.

Automatically-controlled, pellet-fuel boilers can take anywhere from 10 to 30 minutes to reach steady state operating conditions following a cold start. The length of the warm up period depends on the water volume of the boiler relative to combustion rate, and how cool the boiler water is when a call for boiler operation occurs. Under partial load conditions, and without thermal storage, a boiler sized for design load will often turn off due to reaching a high limit water temperature before it achieves combustion conditions that yield the highest efficiency, and the lowest emissions.

Although opinions vary on design requirement, many of those who have worked extensively with pellet-fueled boilers, point to the following design “guidelines” for good pellet-fuel boiler performance: heating load (1000 Btu/hr)

1. Don’t size the pellet boiler for design load. Instead, size a single pellet boiler for 60 to at most 75% of design load. The higher end being for pellet boiler with modulation capabilities. This allows the pellet boiler to supply “base load” heating, while leaving only 5 to 10% of the total seasonal heating energy to be supplied by an auxiliary heat source. In most systems that auxiliary heat source is a second boiler fueled by propane or fuel oil.

2. Install thermal storage between the pellet-fueled boiler and load. That storage can range from 1 to 2 gallons of water per 1,000 Btu/hr of the boiler’s full rated capacity. Thus, a pellet boiler rated at 60,000 Btu/hr would be combined with 60 to 120 gallons of water storage. The upper end of this range (2 gallons / 1,000 Btu/hr) is appropriate when the boiler has limited modulation or no modulation of heat output, the distribution system is highly zoned, and has relatively low thermal mass. The lower end of the range (1 gallons per 1,000 Btu/hr) is appropriate for systems having boilers with a wider modulation range, a high mass distribution system such as a heated concrete slab operated with constant circulation, and minimal zoning.

Figure 5 (below) shows one method in which a pellet fired boiler could be combined with a conventional auxiliary boiler, and a thermal storage tank.

Fig 5

The pellet fuel boiler, as well as the auxiliary boiler are protected against sustained flue gas condensation by independently operated 3-way thermostatic mixing valves. During a cold start condition, the thermostatic mixing valve recirculates water leaving the pellet- fueled boiler back to the boiler’s return port. This allows the boiler temperature to rise quickly since no heat is “exiting” to the storage tank. As the boiler inlet temperature rises above a threshold where sustained flue gas condensation would occur, (typically 130 to 140 oF), the 3-way valve smoothly modulates to allow hot water flow to the thermal storage tank. The cool water returning from the lower portion of the tank is blended with hot water as necessary to maintain an acceptable boiler inlet temperature. The thermostatic mixing valve on the auxiliary boiler provides similar protection against lower entering water temperature.

Notice that the auxiliary boiler is piped so that it interacts with the upper 25% of the buffer tank. This detail, in combination with thermal stratification, allows the upper portion of the thermal storage tank to serve as a buffering mass that protects the on/off auxiliary boiler against short cycling. This is especially useful in systems with extensively zoned distribution systems. Temperature stratification within the tank, in combination with careful detailing such as horizontal piping connections carrying flow into the tank, minimize thermal interaction with the lower portions of the thermal storage tank. The mass in the lower portion of the tank is not needed to buffer the auxiliary boiler, but can be used by the pellet-fired boiler.

When piped as shown in figure 5, the thermal storage tank also serves as a hydraulic separator between the boiler circulators and the variable speed distribution circulator.

Restrain Yourself: As “primal” as it feels to size a boiler to the design heating load of building, and perhaps even include a generous safety factor, this should not be done with pellet-fueled boilers.

A good target is to size for 60% of design heating load, and use an auxiliary boiler as the “peaking” heat source when the system needs it on those exceptionally cold winter nights. An auxiliary boiler sized for 25 to 40% of design load also provides adequate capacity to handle the heating load under most conditions, if the pellet-fueled boiler is offline for maintenance.

This “hybrid” approach will reward you with reliable heat delivery, high combustion efficiency, and low particulate emissions. These are all vital to widespread acceptance of pellet-fueled boilers in North America.


© Copyright 2016, J. Siegenthaler, all other rights reserved.
No part of this article may be reproduced without written permission from the author.
All information presented is conceptual, and not meant to be used for the installation of any specific system.

Enroll in Hydronic Based Biomass Heating Systems with John Siegenthaler and learn…

  • How to design the most efficient biomass heating systems: including wood gasification, pellet-fueled, and wood chip boiler systems
  • Specific control techniques that allow the operating characteristics of a boiler to match the load profile of a building
  • Building blocks for modern systems: including high efficiency circulators, distribution efficiency, air & dirt separation, hydraulic separation, auxiliary boiler integration, expansion tanks, and more
  • Residential design example applications: including a combisystem using a wood gasification boiler, auxiliary boiler, unpressurized storage, and floor heating
  • Commercial/Municipal design example applications: including a multiple pellet-fired boiler system with low temperature convectors.
  • How to confidently design a quote system for installation of biomass heating systems


Person medium john siegenthaler About author John Siegenthaler – Principal, Appropriate Designs

John Siegenthaler, P.E., is a mechanical engineer and graduate of Rensselaer Polytechnic Institute, a licensed professional engineer, and Professor Emeritus of Engineering Technology at Mohawk Valley Community College. “Siggy” has over 32 years of experience in designing modern hydronic heating systems. He is a hall-of-fame member of the Radiant Professionals Alliance and a presenter at national and international conference on hydronic and radiant heating. John is principal of Appropriate Designs – a consulting engineering firm in Holland Patent, NY. The 3rd edition of his textbook – Modern Hydronic Heating – was released in January 2011. John currently writes about hydronic heating and solar thermal system design for several trade publications.

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