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Wood-based biomass boilers come in several “flavors” and a wide range of sizes. They burn fuels ranging from sawdust to pellets to wood chips to cordwood. Some use very simple design (e.g., a steel firebox with a door and vent opening surrounded by a water-filled jacket). Others use sophisticated controls to accurately regulate air/fuel ratios for optimum combustion efficiency. The latter also produce far less particulate emissions compared to the former.

Anytime a hydro-carbon fuel is burned water vapor is produced. The amount depends on the fuel and the air/fuel ratio. For wood-based fuels it also depends on the moisture content of the wood prior to its combustion.

The greater the amount of water vapor produced during combustion, the higher the dewpoint temperature of the flue gases. This is the temperature at which the water vapor present begins to condense from a vapor to a liquid. Other chemicals are also present in the condensate including sulfur and in some cases chlorine compounds. These cause the condensate to be acidic. As such, it can be very aggressive to metals, especially carbon steel and iron.


Take a look at the rear side of the large wood chip boiler in Figure 1. This boiler heats an entire school complex as well as its associated bus garage and swimming pool.

Figure 1

John Siegenthaler July column Figure 1

The plenum box near the top of the boiler collects flue gases from the internal fire tubes and routes them to a chimney. The rust stains seen on the rear plate are from condensate that has seeped down from where the plenum joins the rear boiler plate. Although this rear plate is “dry” most of the heating season, it’s obvious that flue gas condensation has occurred.

Interestingly, the controls that operate this boiler are set to allow a minimum temperature of 160° F. That’s well above a typical dewpoint temperature for wood chips (typically 130°-150° F depending on the moisture content of the chips and air/fuel ratio).

So why did condensation occur? The answer requires consideration of the entire system rather than just the boiler and its controls. That system, along with the boiler, contains about 8,000 gallons of water and antifreeze.

Consider the situation when the boiler is first fired up in the late fall. That 8,000 gallons of fluid, as well as the thermal mass of the steel in the boiler, distribution piping and terminal units is relatively cold. It could take several hours for the boiler to reach it normal operating temperature range. During that time, the thermal mass of the water and steel is keeping the combustion side of the boiler’s fire tubes well below dewpoint temperature. This is when condensate is forming and seeping down the boiler’s rear plate. The takeaway is that even boilers intended to operate above the dewpoint of their flue gases experience transient conditions that allow flue gas condensation.

Thermal clutches

Minimizing flue gas condensation requires a means of thermally uncoupling the boiler from the thermal mass of its distribution system.

Think of this uncoupling similar to how a clutch uncouples the engine from the drive chain of a vehicle. When the clutch pedal is fully depressed, there is no mechanical energy transfer from the engine to the drive chain. As the clutch pedal is slowly released, there is progressively more transfer of mechanical energy. When the pedal is fully released, the full power of the engine is available to the drivetrain.

There are several piping configurations that can serve as “thermal clutches.” All involve the use of hardware that can partially or fully decouple the boiler from the distribution system when necessary.

Figure 2 shows three hardware configurations than can provide the thermal clutch function.

Figure 2

John Siegenthaler July column Figure 2

All three configurations use some type of flow metering method between the boiler and distribution system. All three also measure the boiler’s inlet water temperature and respond based on that temperature.

All three assemblies also use a hydraulic separator to prevent interaction between the boiler circulator and other circulators in the system. The hydraulic separator also provides air and dirt separation for the system.

Injection valve

Figure 2A uses a 2-way motorized valve as the flow metering device. If this valve is fully closed, there is no flow between the boiler and the load. This condition, when present during a cold start, allows the boiler to increase the temperature as fast as possible based on its own thermal mass and firing rate.

As the boiler’s inlet temperature begins to rise toward some minimum temperature setting, the valve progressively opens to allow more flow between the boiler and the load. If and when the temperature entering the boiler is at or above the minimum temperature setting, the injection valve is fully open.

The “flow restrictor” valve in Figure 2A is typically a globe-type valve. It’s needed to create sufficient differential pressure to drive flow through the injection valve. It should be adjusted so that flow through the injection valve is sufficient to meet the design heating load when the injection valve is fully open.

Injection circulator

Figure 2B is another form of injection mixing. It uses a variable speed circulator as the flow metering device. The circulator, or an external controller regulating the circulator, monitors the boiler inlet temperature. When that temperature is low, the injection circulator is off. The closely spaced tees decouple the boiler circulator from the injection circulator. As such, there is virtually no flow between the boiler circuit and the load.

As the boiler’s inlet temperature rises, the injection circulator speed increases, which allows greater rates of flow from the boiler to the load. At design load, the injection circulator — if properly sized — should be operating at or close to full speed.

3-way mixing valve

The piping shown in Figure 2C uses a motorized 3-way mixing valve to regulate the rate of hot water passed to the distribution system. It also monitors the boiler inlet temperature, and when necessary to prevent flue gas condensation, reduces or stops hot water flow to the load.

You might be wondering why there needs to be a boiler circulator involved in all three of these piping configurations. Why not just connect the injection valve, injection circulator or 3-way valve directly across the boiler?

The boiler circulator is there to keep a reasonable flow rate through the boiler and prevent high-temperature gradients that cause thermal stresses within the boiler when flow rates are very low. Some boilers may be able to operate without this flow, but that should always be verified with the boiler manufacturer. In general, maintaining a reasonable flow through the boiler (perhaps corresponding to a nominal 20° F temperature rise at design load) will reduce internal stresses that could otherwise lead to warpage, cracks, or failed welds.

Double duty

Thus far, we’ve discussed the anti-condensation function of the piping assemblies in Figure 2. With the proper control, all three configurations can also control the supply water temperature to the load. This requires another sensor just downstream of the distribution circulator. The same controller, or BAS code, that controls the anti-condensation function can also regulate the supply water temperature based on outdoor reset.

Figure 3 shows the same piping assemblies with the added sensors for controlling supply water temperature based on outdoor reset.

Figure 3

John Siegenthaler July column Figure 3

Several controllers are available that can provide a 0-10 VDC or 4-20ma output to the injection valve or injection circulator. These same analog outputs, or a controller that provides a 3-wire floating control output can be used with actuators operating 2-way or 3-way valves.

Which is better?

All three piping details provide the necessary hydraulics. The choice of which to use will depend on cost, availability and the design flow rate required. In large systems with near boiler pipe sizes over 6 inches, the cost of valves may be higher than the cost of a variable speed circulator that can provide design flow rate. This is especially likely when the biomass boiler is supplying a low-temperature distribution system. In this case, the injection flow rates needed may only be a fraction of the distribution flow rates, and thus smaller less expensive hardware can be used.

Biomass boilers, as well as fossil fuel boilers that are not intended to operate with flue gas condensation need anti-condensation protection. Be sure it’s addressed in your system designs.