Wood-fired Central Heat
(Page 2 of 7)
February/March 2003
By Greg Pahl
HYDRONIC SYSTEMS
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A wood-fired boiler is an obvious match for a home with radiators or other hydronic systems (hot water moving through pipes). Most boilers feature heavy cast-iron firebox doors, automatic draft regulators and high-limit controls. The firebox is generally surrounded by a water jacket, which transfers the heat from the fire to the water. The heated water is then circulated through the hydronic heat-distribution system to your living space. In addition, the water jacket provides protection from overheating of the firebox, a design feature not available in a hot-air furnace. Most wood boilers offer domestic hot water coils and coal grates as options.
A hydronic, wood-fired central heating system offers many advantages. One of the most obvious is the ability to heat your domestic hot water along with your home from the same boiler. Another potential advantage is that a hydronic system usually can be designed to include extra heat-storage tanks that allow the boiler to be fired somewhat like a masonry heater, where one good hot fire a day may be all you need. Because the fire burns quickly at a high temperature, it also burns cleanly, with virtually no emissions up the stack. The additional water tank(s) store excess heat for later use. This type of storage arrangement is an excellent match with radiant hydronic floor heating, but baseboard heaters or radiators also will work.
COMBINATION SYSTEMS
One way to get more flexibility is to install a combination central-heating unit. These boilers or furnaces are designed to burn wood and either oil or gas if the wood fire dies down before you get home. This type of system allows you to burn wood when it's convenient to do so, and to rely on the backup fuel when wood is not convenient. Some systems are even equipped with an electric-backup heating coil. Some older combination boilers or furnaces use a single combustion chamber for both fuels. Unfortunately, this creates a design compromise, and these units will never operate at peak potential efficiency for either fuel.
It's also not unusual for the soot and fly ash from a wood-fired combination unit to coat the head of the oil burner, requiring frequent cleaning of the oil-burner head. This problem is eliminated when the combination boiler or furnace has two separate combustion chambers, a feature more typical of current technology. With this design, both combustion chambers can be engineered to maximize the efficient burning of their respective fuels. In a multi-fuel heater, the two fuels are not normally used simultaneously. Wood combination heaters can be either hot-air furnaces or hot-water boilers. The main disadvantage of most combination units is that they tend to be almost twice as expensive as a single-fuel heater.
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