Wood Heat for Greenhouses

Fuel wood, waste wood and biomass are potential sources of heat for greenhouses. An adequate supply at a low cost is needed to pay for the additional cost of the equipment and operation as compared to conventional fossil fuel units.

Combustion basics

Combustion of wood has three requirements - fuel, air and heat. If any of these is removed, burning ceases. When all three are available in the correct proportion, combustion is self-sustaining, because the wood releases more than enough heat to initiate further burning.

The rate at which wood bums is controlled by the amount of air. A lack of air causes wood to smolder and produce pollutants. Too much air will cool the fire and waste heat.

Another important aspect of combustion is the energy content of the fuel. This is normally expressed in British thermal units (Btu's). Energy content is greatly affected by the moisture content and weight of the wood. For example, hardwood and softwood at 50% moisture will have about 4,700 Btu/lb whereas the same wood at 20% moisture will contain about 6,200 Btu/lb. Hardwood has about twice the weight as softwood and therefore twice the heat content. The same is true with wood chips - 4,000 Btu/lb green (50% m.c.) and 7,400 Btu/lb dry (10%).

In the burning process, wood goes through three stages. Stage 1, the wood is heated to evaporate and drive off the moisture. The heat generated does not provide heat for the greenhouse. Stage 2, starting at about 500°F the wood starts to break down chemically and volatile mater is vaporized. The vapors contain between 50 and 60% of the heat value of the wood. These vapors have to be heated to about 1100°F to bum. If not, smoke is generated which can coat heat exchange surfaces and chimneys with creosote. Stage 3', once the volatile gases are released, the remaining material (charcoal) burns at temperatures above 1500°F. All three stages can be present at the same time.

Fuel quantity

The amount of fuel needed depends on many factors including the heat required by the greenhouse, furnace efficiency, fuel type and moisture content. If you know your present consumption, you can estimate the firewood or chips you will need from the following table:

Above values are based on 75% heating system efficiency for fossil fuels and 70% for solid fuels.

Equipment selection

The wood-fired heating system is a major investment that should be selected to give efficient operation for many years. It pays to spend a little more on the initial investment to get a unit that will reduce handling, increase efficiency and provide a safer operation. Consider the following:

• Size of system. The unit should be sized to offset the heat losses. Too large a unit may create inefficiencies in fuel usage and excess smoke and pollution. The installation of modular units will allow for expansion of the growing area and greater fuel efficiency during mild weather.
• Furnace (hot air) or boiler (hot water). Most units are boilers as it is easier to get the heat where you need it with a hot water system. Water can be modulated for root zone heating.
• Firewood or chips. There is little savings from solid fuel if you have to pay the homeowner rate. Solid wood may be available for landscapers or arborists at little or no cost but requires time to get it sized to fit the firebox. Larger firewood units require handling several hundred pounds of wood a day. Chips and sawdust are delivered in bulk and are automatically fed to the firebox. A good, long-term supply source is needed.
• Indoor or outdoor location. Location inside the greenhouse or headhouse results in shorter supply piping.An outdoor installation can be located close to the wood storage. It also keeps the smoke away from the greenhouse.
• Lined or unlined firebox. A firebrick lined firebox will usually burn hotter, create less smoke and be more efficient than an unlined one especially if it has a water jacket. 
• Gasification. In these units, the volatiles are driven off in an oxygen deprived chamber and then moved through a burner nozzle where they are superheated and mixed with air for complete and even combustion. The increased efficiency of this two-stage process produces greater economic benefits and shorter payback.
• Natural or forced draft. The chimney on a natural draft unit needs to be tall to get adequate draw on the fire. A forced draft maintains a hotter, more efficient fire and decreases smoke, creosote and ashes. This reduces the need for a water jacket with a large capacity as temperature recovery time is reduced.
• Primary and secondary air supplies. Choose a unit that has both primary and secondary air supplies.Many new designs have electronic controls that regulate the rate of firing, draft inducers that provide the right amount of air, heat storage that absorbs extra heat and heat reclaimers to capture the heat of combustion before it escapes up the chimney.
• Duel fuel capability. Some units are available with fossil fuel burners for starting the wood and also providing backup if the solid fuel fire goes out.
• Will the unit meet local and state codes? Larger units usually have to meet emission codes for particulate matter, carbon dioxide and other pollutants. In some states such as Connecticut, outdoor wood furnace installation and operation are regulated.
• Solid fuels offer a heat alternative for many growers throughout the U.S. Their availability, low cost and high heat value can replace expensive fossil fuels. Care in selection and installation is important.

John W. Bartok, Jr., Extension Professor Emeritus
University of Connecticut , Storrs CT 06269-4087
Natural Resources Mgt. & Engr. Dept.
University of Connecticut , Storrs CT

2007