The market for high-efficiency boilers that burn cordwood, wood pellets or wood chips is growing in North America. Much of this is happening in geographic areas that are traditional bastions of hydronic heating. The Northeastern states in particular are becoming the epicenter for state-of-the-art applications of wood-fueled biomass heating.


Modern hydronics technology can leverage the inherent benefits of wood-based biomass boilers. It can be used to create a balance of system that respects and even enhances the unique operating characteristics of these systems, while also delivering the unsurpassed comfort and control long associated with well-designed hydronic systems.

Similar to conventional steel boilers that burn natural gas, propane or fuel oil, biomass boilers must be protected against sustained flue gas condensation. This can be done using one of several hardware configurations.

One approach uses a variable-speed circulator to “shuttle” heat from the boiler to the thermal storage tank or heating load, while keeping the boiler’s inlet water temperature high enough to prevent sustained flue gas condensation. Figure 1 shows how this technique can be used with a cordwood-gasification boiler supplying heat to a thermal storage tank.

The boiler loop circulator (P1) is turned on when the boiler operator kindles a fire. It creates flow through the short piping loop, connecting the boiler outlet to its inlet. This flow remains contained within this loop as the boiler’s internal temperature rises to the point where sustained flue gas condensation will not occur within the boiler’s heat exchanger. This can take several minutes for boilers starting from room temperature conditions.

The boiler protection circulator (P2) contains electronics that let it measure and respond to the temperature at the boiler inlet. Even though it receives power at the same time as the boiler loop circulator, the boiler protection circulator doesn’t spin its impeller until the boiler inlet temperature has reached 130° F (or whatever temperature it is set for). This eventually creates mixing at point A where hot water from the boiler loop meets up with cooler flow returning from the lower portion of the thermal storage tank. The temperature of the water entering the boiler is determined by the proportions of hot and cool water mixing at point A.

The boiler protection circulator increases its speed as the boiler inlet temperature rises above 130°, eventually reaching full speed when the temperature has climbed to several degrees above this setpoint. If the inlet temperature drops back toward the setpoint, the circulator slows down and eventually stops, if necessary, to prevent conditions that would otherwise allow sustained flue gas condensation.

This control action provides a full range “thermal clutch” between the boiler and the thermal storage tank. It allows the boiler to transfer its full heat output to the tank, while at the same time it prevents conditions that would cause sustained flue gas condensation.

The normally open valve (V1) near circulator (P2) is a full-port ball valve with a spring return 120 VAC actuator that opens the valve if a power failure occurs. This creates an uninhibited flow path between the boiler and thermal storage tank, through which thermosiphon flow can develop. This allows residual heat within the boiler to pass into the thermal storage tank.

The operation of this valve should be tested at the beginning of each heating season, and a couple of times during each heating season, to ensure that it opens when power is removed. I suggest using a valve with low flow resistance in this location.

Along with the above-mentioned controls, be sure the wood-gasification boiler is equipped with all required safety devices such as a manual reset high limit controller and a low water cut-off. Mount and wire these controls as required by the boiler manufacturer and any local codes.

Injection coupling

The next design task is establishing a way to extract heat from the thermal storage tank and pass it to a heating load. Again, several options are available. One of the most common is the use of a three-way motorized mixing valve, regulated based on outdoor reset control, to mediate between what may be very hot water in the thermal storage tank and a low-temperature distribution system. The latter is always preferred because it allows the thermal storage tank to cycle over a wide temperature range, thus providing a high Btu/gal. storage capacity.

Although three-way motorized mixing valves can function well in this application, they are not the only viable approach to mixing. We just demonstrated this in how the boiler was protected against sustained flue gas condensation. The same method can be used to couple the upper storage tank header to a distribution system, as shown in Figure 2.

The variable-speed injection circulator (P3) regulates the flow of hot water from the upper header of the thermal storage tank into the distribution system. The greater the injection flow rate, the greater the rate of heat transfer to the distribution system. When the injection circulator is off, there is no heat transfer into the distribution system. A temperature sensor (S1) mounted downstream of the distribution circulator (P4) provides feedback to the injection mixing controller operating the injection circulator.

The injection circulator (P3) should be sized so that at full speed it provides design load flow to the distribution system when the temperature difference between points B and C equals the design load temperature drop across the distribution system.

The injection mixing controller also can measure the outdoor temperature and use this value, along with its other settings, to determine the target supply water temperature to the distribution system based on outdoor reset. This allows the distribution system to operate with the lowest possible water temperature that can still meet the heating load. The lower the supply water temperature requirement of the distribution system, the lower the temperature to which the storage tank can be thermally drained before the boiler must be refired.

The closely spaced tees that couple the injection riser piping to the distribution system provide hydraulic separation between the variable-speed injection circulator (P3) and the distribution circulator (P4).

The proximity of the injection risers to the thermal storage tank, in combination with the short and generously sized headers, allow the tank to provide hydraulic separation between the injection circulator (P3) and the previously mentioned circulators (P1) and (P2).

This two-pipe configuration of the thermal storage tank allows the injection circulator to draw heated water from either the wood-fired boiler, the thermal storage tank or both. Where the hot water comes from depends on the operating status of the boiler. If it’s off, all flow will come from the thermal storage tank. If the boiler is operating and has reached a sufficient temperature to prevent sustained flue gas condensation, some flow into (P3) will come directly from the boiler and remaining flow will come from thermal storage. This allows for rapid heat transfer into the distribution system during times when the system is warming up.

If the flow rate from the wood-fired boiler exceeds the flow through the injection circulator (P3), the difference passes into the thermal storage tank. This will usually reduce the flow velocity into the tank — compared to tanks piped in a four-pipe configuration. Lower entering flow velocities enhance temperature stratification within the storage tank, which is always desireable.

Booster boiler

Many customers who opt for a manually fired, wood-fueled boiler want an auxiliary boiler that can automatically turn and supply the heating load if there is no fire, or an insufficient fire, in the wood-fueled boiler.

Figure 3shows how a modulating/condensing boiler can be easily integrated into a distribution system that also is supplied by a wood-gasification boiler and uses the previously discussed piping details.

The heat emitter in this system is assumed to be a single–zone, heated-floor slab. It provides sufficient thermal mass to buffer operation of the auxiliary boiler. Thus, there is no need to couple the auxiliary boiler to the thermal storage tank.

However, not all distribution systems have these high thermal mass and minimal zoning characteristics. A highly zoned distribution system that uses low thermal mass heat emitters requires a modified approach. Figure 4 shows how the system from Figure 3 can be altered to handle this situation.

Only the upper portion of the schematic has changed. A 25-gal. buffer tank has been added to prevent the auxiliary boiler from short cycling under partial load conditions. The injection circulator (P3) transfers heat from the wood-gasification boiler or thermal storage tank into this small buffer tank. The small buffer tank also provides hydraulic separation between the injection circulator (P3), the auxiliary boiler circulator (P5), and the variable-speed, pressure-regulated distribution circulator (P4).

The systems shown in Figures 3 and 4 are carefully arranged combinations of readily available hardware. They accomplish several necessary tasks such as boiler protection, overheat protection, stratified thermal storage and supply water temperature control, while remaining relatively simple, cost-effective, scalable and adaptable to variations in heat emitters or zoning methods.

These piping methods also could be adapted to systems using pellet-fired boilers, wood-chip boilers or multiple biomass boiler arrays.

Every elegant hydronic piping assembly deserves an equally elegant control system. We’ll get into that in part two. You’ll see it’s relatively easy and inexpensive to provide the brains that make this system operate smoothly and efficiently.

This article was originally titled “Cooperative effort, part one” in the May 2015 print edition of Plumbing & Mechanical.