So you want to heat with wood? Here are a few things to think about.
Economics of Woodburning
The economics of wood burning consists of several elements including initial cost, system life, recurring cost, and the cost of alternatives. Each of these can be highly variable, so anyone considering wood must evaluate their specific situation.
Initial cost depends on the size and complexity of the chosen system as well as the amount of third party work involved in installation. Local codes can add significantly to initial costs, as can site-specific issues such as the availability of a satisfactory chimney flue. Initial costs can range from a few hundred dollars for a simple wood stove to $20,000 or more for a complete wood gasification system with heat storage and associated controls.
System life is a consideration for the more expensive systems. A typical high quality indoor boiler can be expected to last 20 years or more. Outdoor boilers may have a shorter useful life for a variety of reasons. It's reasonable to divide the system cost by the useful life to obtain an annual system cost.
Recurring cost for a wood heat system is primarily fuel, although chimney cleaning should be included as appropriate. Some designs require much more frequent chimney cleaning. If you have a wood lot and are willing and able to harvest the wood yourself, your fuel costs could be quite low. Otherwise, get quotes for wood deliveries. If you don't already have the equipment, you can spend a lot getting started. Chainsaw(s), axe, splitting maul, woodsplitter, carts and other tools can add up quickly. The system design section of this site contains tools to help estimate the amount of wood that you will need per season. A rough rule of thumb is that a cord of wood equals 100 gallons of fuel oil. There's quite a bit of variation between wood burning equipment, though. Some of the most efficient boilers will burn barely half that, while the least efficient may burn twice that amount. If you're burning 1000 gallons of oil per year and you plan to reduce that by 60%, you could expect to need about 6 cords of wood.
Cost of alternatives is the cost of heating your house with oil, gas, electricity. The options and costs vary by location, and there's no way to tell what they will be in the future. Probably safe to assume that they're going up, though. Unfortunately, most wood heat installations will require some form of backup heat, if only to ensure that the house doesn't freeze if you go away on vacation.
The US Department Of Energy has a spreadsheet at http://www.eia.doe.gov/neic/experts/heatcalc.xls that compares energy costs for a wide variety of energy sources. If that link expires, there is a copy here.
In our case, we calculate that it costs us about $200 in tools, fuel, and equipment to harvest 4 cords of wood. We also assume a conservative 70% efficiency with our gasification boiler. That translates to a cost of $3.25 per million BTUs. Oil at $4.50 (the current price) burned at a typical 78% efficiency translates to $41.60 per million BTUs. Since we need about 60 million BTU per year, that equates to $2500 in fuel oil vs. $200 in wood.
Our wood boiler, storage, controls, and plumbing cost about $7000, and we expect a 20 year useful life. That works out to $350 per year for the equipment.
Despite arguments to the contrary, burning wood is carbon neutral as long as the wood is not harvested in a destructive way. The carbon in the wood will return to the atmosphere when the tree dies and decomposes. At worst, wood burning causes that carbon to be released a few years earlier. However, the removal of the tree allows new trees to grow in its place sooner than would have happened otherwise. Proper woodlot management is focused on removing damaged trees and relieving overcrowding so that the remaining trees will grow faster and larger.
If the trees are harvested in such a way that the forest is transformed to grassland or desert, then wood burning is not carbon neutral. Otherwise, it's fine.
A related topic is sustainable harvesting levels. In New England, studies have shown that sustainable yields range from 1/3 to 1/2 cord per acre per year. If you burn four cords per year, that means you need a woodlot of 8 to 12 acres. Careful management can increase the yields, but that's a good starting point.
All wood burning appliances emit some level of particulates and complex hydrocarbons. There's room for debate about how harmful these emissions are, but there's no doubt that at high levels they constitute a nuisance to neighbors. The simplest way to reduce noxious emissions is to burn clean dry wood. Any woodburning appliance will burn more efficiently and cleaner with proper fuel. That means either buying dry wood or planning ahead and providing space to stack and store wood at least a full year ahead to allow time for drying.
Beyond using proper fuel, there are dramatic differences in technology. For instance, gasification boilers generate less than 1% as much particulate emissions per BTU as some of the dirtier designs.
Comfort and convenience
Let's face it - no wood burning appliance will ever be more convenient than a typical conventional heating system where you set the thermostat and the fuel delivery guy comes automatically. Wood heat requires more effort - typically, much more effort. Wood has to be cut and split and stacked, hauled and restacked. Fires have to be built and ashes removed. Wood and ashes leave messes that must be cleaned up. Most systems require a fair amount of tending, and even so you can expect indoor temperatures to fluctuate more than with a conventional heating system. Wood heat requires a degree of commitment and effort. If you enjoy the physical work and get satisfaction from providing for your own comfort, then it can be a rewarding experience.