In northern New England, heat is required during winter months. Our house was built with very good insulation, but still uses over 700 gallons of fuel oil per year for heat and hot water. As we have an unlimited supply of firewood, we built with the intention of adding a wood-fired boiler at some point. However, wood boilers and stoves are not without their issues:
Resolving these issues involves a series of tradeoffs and decisions which really define the heart of this project. In our case, some of the decisions were made when we built the house nearly 20 years ago. The balance of this page assumes that wood heat will be used and explores these tradeoffs and our choices in each case.
For whole-house heating, these are the basic choices. Each has specific implications when considering wood heat.
Radiant: In addition to all the other benefits of radiant, it works extremely well with heat storage because you can use cooler water and still get useful heat out of it. Radiant is more complex from a plumbing point of view and difficult to retrofit. Excellent choice for new construction, especially for slab floors.
Baseboard: Inexpensive and unobtrusive. Output is very sensitive to water temperature, and does not work well at lower temperatures. Plan a bit extra if you're going to heat with wood, especially if you plan to use heat storage.
Radiators: Perhaps underappreciated, these work well with hydronic systems. They provide a warm heat source in the living space which can be a nice feature. There are two types - the old fashioned free standing cast iron units and radiant wall panels. Wall panels are designed to operate at lower temperatures than baseboards.
Hot Air: Often in place for existing oil or gas furnaces. Hot air wood furnaces exist, and water to air heat exchangers (similar to automotive radiators) can be placed into the ductwork to allow wood boilers to deliver heat into a hot air system.
Our choice: We already had baseboards, but we're adding a radiant zone. Radiant is a big benefit if you have storage, since it allows the use of water that's too cool to be effective with baseboards.
Wood boilers provide heat via baseboards, radiators, or radiant heat coils in the floor. They are typically placed in the basement or in an outbuilding. They can also be used to provide heat into a hot air system by means of an additional heat exchanger.
Wood Boiler Pros: Properly installed wood boilers typically have a long service life. They are larger and have higher heat output than wood stoves. Since they're not installed in the living quarters, the mess is kept away from the living area. They can be integrated very well with existing hot-water heating systems.
Wood Boiler Cons: Wood boilers are more expensive and more complex to install than wood stoves. They are large and quite heavy. Professional help is much more likely to be needed for installation. Beyond the boiler itself, the installation costs are much higher than for a wood stove.
Wood furnaces are designed for use with hot air ducting. Most of the comments about boilers apply to wood furnaces. Since we don't have hot air ducting, we didn't pursue this option.
Wood stoves are placed in the living area and provide direct heat to their surroundings.
Wood Stove Pros: Wood stoves are often beautifully made. They provide direct radiant heat, which is universally appreciated. They can heat a living area very quickly. They are relatively inexpensive and easy to install. There are a wide range of models with different sizes and prices. Some units burn wood pellets which are fed automatically from a hopper, providing many hours of automatic operation.
Wood Stove Cons: They take up space and bring wood debris into the living area. They can't evenly heat a large house. Since they idle much of the time, they often produce creosote deposits in the chimney which must be cleaned regularly.
Our choice: We had planned from the beginning to have a wood boiler. We built the house with an extra flue, and designed space for it into the boiler room in the basement, which has direct outside access for passing in firewood. Our house is heated with hot water baseboards. We also have no good spot in the living area for a wood stove, and the house is large with multiple levels. There's a pretty good argument to be made that a boiler and a wood stove would be a good combination. We miss the comfort of a hot stove to warm cold body parts.
This is really two different discussions. Boilers come in two flavors: indoor and outdoor. However, an indoor boiler can be installed in an outbuilding. Outdoor boilers are a different species. They are much larger, and usually come with a prebuilt metal enclosure. They are generally unpressurized (more later) and designed to accept very large fuel loads. This decision is about the boiler type, not the location.
Indoor Boiler Pros: Indoor boilers typically burn much cleaner and are much more efficient. although some outdoor boiler manufacturers are beginning to produce better designs. Indoor boilers are designed to be part of a pressurized system, which means that the boiler water jacket is part of the closed hydronic loop that heats the house. Closed systems typically have much longer lives, less maintenance, and fewer corrosion problems.
Outdoor Boiler Pros: The outdoor boiler design can be loaded up with an very large fuel load, and can burn untended for a long time. It comes preassembled with its own enclosure.
Our choice: Again, we had designed for an indoor boiler from the beginning so this was really not a question for us. In addition, the outdoor boilers have gotten an extremely bad reputation because some models will generate enormous amounts of noxious smoke when they're idled for long periods, especially if they're burning green wood.
Gasification boilers employ a two-stage process to first create flammable gases (primarily carbon monoxide and hydrogen) and then burn those gases at very high temperatures to achieve very high efficiency and relatively low emissions. There are many more models available now, and there's a gray area between boilers that achieve some level of secondary combustion and the true gasifiers, which have a secondary combustion nozzle where fresh air is mixed with the flammable gases produced in the primary chamber. Test results on true gasifiers show combustion efficiencies around 90%, which is close to the theoretical maximum. At this time, similar independent test data is not available for the other designs.
There are many cases of people switching from clean burning conventional boilers to gasification boilers and seeing a 40% reduction in wood consumption.
Gasification Pros: Very clean burn with virtually no smoke or odor. Dramatically lower wood consumption. No creosote in the chimney.
Gasification Cons: More expensive. More complex and finicky, with a longer learning curve. More sensitive to moisture content in the firewood.
Our choice: We went with the gasification design. At the time we built our system, residential gasification boilers were relatively unusual and hard to find. We found a model built in Eastern Europe (Orlan EKO) that was available at an attractive price at the time. The clean burn and creosote-free operation were important, and the reduced wood consumption is a nice bonus.
While an outdoor boiler can't be installed indoors, an indoor boiler can be installed outdoors as long as a suitable structure is provided.
Indoor Pros: Indoor installation provides the convenience of tending the boiler without braving the elements. It also simplifies the installation, since all the heating system components can be together in one place. Heat losses are reduced, and no outbuilding is required. Freezing is not a concern.
Indoor Cons: You have to bring firewood into the house. Any fire hazards such as creosote affect the house rather than an outbuilding.
Our choice: Since we had designed for indoors, this wasn't a question in our case. The gasifier eliminates the creosote hazard.
If you choose indoors, you want direct outside access to the boiler room for bringing in wood. Designing a nearby outbuilding for an indoor boiler with room for wood storage is also a very attractive option.