Using wood-fired heat sources in conjunction with conventional fuel.

Interest in wood-fired hydronic heating systems continues to rise along with the price of conventional fuel. Wood-fired hydronic systems can be designed to leverage concepts like hydraulic separation, outdoor reset control and variable-speed pumping to enhance the high thermal efficiency of heat sources such as wood gasification boilers.

One common design scenario is using a wood-fired heat source along with an “auxiliary” boiler to supply a hydronic distribution system. The objective is to use thermal energy from a wood-fired heat source whenever possible. When the wood fire wanes, or goes out, the auxiliary boiler automatically takes over until the wood-fired heat source is refueled. This transition from wood to conventional fuel should occur seamlessly with no affect on interior comfort.

There are a number of possible approaches, which depend on the characteristics of the wood-fired heat source, as well as those of the existing forced-air system. For example, if the heat source is an outdoor wood-fired furnace, most of which contain 200 to 500 gallons of water, there is no need for an interior buffer tank.

Our discussion will focus on the use of a wood gasification boiler. Several suppliers currently offer such state-of-the-art wood burners in North America. When fired with dry firewood (20 percent or less moisture content), these boilers can approach 90 percent thermal efficiency.

The key to high efficiency is to burn wood hot and fast. Wood gasification boilers are designed for this. Once a stable fire is burning in its combustion chamber, a blower turns on to redirect the pyrolitic gases emitted by the heated wood in a downward direction. These hot gases pass through a slot in a ceramic plate, and are mixed with a forced jet of secondary combustion air. The resulting combustion resembles a blow torch and can reach temperatures over 2,000 degrees F (see Figure 1). Such combustion produces high efficiency and very little ash.

The implication of burning wood hot and fast is that heat production is often far greater than required by the load. The only way to tame the beast in this situation is to add storage. Figure 2 shows one way to do so.

The piping between the boiler and storage tank contains a bypass pipe and a boiler protection module. The latter contains a thermostatic mixing valve and circulator that operate whenever the boiler is firing. Their purpose is to keep the inlet water temperature to the wood-fired boiler high enough to prevent sustained flue gas condensation within the boiler and its venting. The exact temperature where such condensation forms depends on the moisture content of the wood being burned and how much excess air is supplied to the combustion chamber. A boiler inlet temperature of about 140 degrees F is generally considered safe for typical combustion conditions.

The boiler protection module also contains a special flapper check valve that opens to allow natural convective flow between the boiler and storage tank during a power failure. Several manufacturers, mostly European, now offer such modules through North American suppliers.

Delivering The Warmth

The schematic in Figure 3 shows how Figure 1 can be extended to form a complete hydronic space-heating system, including an auxiliary boiler.

Upon a demand for space heating from any zone, an outdoor reset controller is powered on to measure the temperature at the top of the storage tank and compare it to the “target” supply water temperature. If the tank is deemed sufficiently warm to supply the zones, the circulator (P2) is turned on to deliver heated water to the closely spaced tees that hydraulically separate the distribution system from the heat sources.

If the storage temperature is too low to supply the load, the auxiliary boiler is turned on along with the circulator (P1). Circulator (P2) is turned off. Again, heated water is delivered to the closely spaced tees.

Figure 4 shows an example of the logic executed by the outdoor reset controller.

For example, at an outdoor temperature of 25 degrees F, the “target” temperature calculated by the reset controller represented by Figure 3 is 95 degrees F (green vertical and horizontal lines on graph). This condition is represented by the yellow dot. The red and blue dots above and below the yellow dot represent a 10-degree F temperature differential that is centered on the target temperature. If the water temperature in the buffer tank is less than the target temperature minus half the differential (95 - [10/2] = 90 degrees F), the auxiliary boiler comes on when there is a call for space heating.

Assuming the call for heating remains active, the auxiliary boiler remains on unless the tank temperature climbs to the target temperature plus half the differential (95 + [10/2] = 100 degrees F). The latter condition would likely occur if the wood-fired boiler was refueled. The slope of the reset line and the differential temperature range are adjustable on most currently available reset controllers.

This strategy allows heat to be extracted from the storage tank to the lowest possible temperature “useable” by the distribution system. This lengthens the time between firing cycles of the wood-fired boiler.

The three-way motorized mixing valve in the distribution system also is turned on whenever there is a demand for heat. It operates based on outdoor reset control to deliver the appropriate water temperature to the heat emitters. It protects low-temperature distribution systems from potentially high water temperatures in the storage tank.

None of the heat produced by the auxiliary boiler is sent to the storage tank. To prevent short cycling, the extent of zoning used in the system should match the modulation range of the auxiliary boiler.

Adding DHW

Most residential hydronic systems include the ability to heat domestic water. Figure 5 shows one way to extend the system shown in Figure 3 to leverage the thermal mass of the storage tank for preheating (or fully heating) domestic water as it is used.

A flow switch detects whenever domestic water is required at a flow rate at or above 0.5 gpm. Under this condition, it turns on a small circulator that moves heated water from the storage tank through the primary side of a stainless-steel, brazed-plate heat exchanger. Cold water is instantaneously preheated (or fully heated depending on the tank temperature) as it passes through the other side of the heat exchanger.

An electric tankless water heater provides any necessary boost in domestic hot water delivery temperature. An anti-scald-rated thermostatic mixing valve protects against high domestic water temperatures when the buffer tank is at an elevated temperature. For the fastest possible response, the piping between the buffer tank and heat exchanger should be short and fully insulated.

This configuration allows for domestic water preheating whenever the buffer tank is above the temperature of the entering cold domestic water. It is beneficial when the tank is “coasting” down to a lower temperature several hours after the last boiler firing, or perhaps when the boiler is fired only once every couple of days in warmer weather. It also is beneficial if other low-temperature heat sources such as solar thermal collectors are used to add heat to the storage tank.

There is yet another modification of the system shown in Figures 3 and 5 that allows even more heat to be extracted from the storage tank during space heating. It involves a comparison between the tank temperature and the temperature of water returning from the heating distribution system. We’ll get into the details next month. In the meantime, go check that your firewood is drying properly...

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