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.
Radiant / baseboard / radiator / hot air?
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 Boiler, furnace, or stove?
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.
So you're going with a boiler. Indoor or outdoor type?
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.
So you're going with an indoor boiler. Gasification or conventional?
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.
Indoors or outdoors installation?
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.
If you've been reading all of this, it's time for a break. Get a cup of coffee. Stretch. Go chop some wood for a while.
Storage or no storage?
One of the problems with wood heat is that it's tough to match the output of the boiler with the heat demands of the house. Without storage, there are three choices: Build lots of small fires, let the house temperature fluctuate a lot, or let the boiler idle a lot.
This gets at the heart of a major issue with wood heat. Building fires is a lot of work, and nobody likes to be cold. Many users take the third option and keep their fires going 24/7, running them at a small percentage of the boiler's rated capacity. This is convenient and helps maintain an even temperature, but idling is not ideal. This type of operation typically produces smoke, creosote, and reduced efficiency. Outdoor wood boilers represent perhaps the extreme of this approach.
Heat storage provides a way to capture the excess heat produced by the boiler and use it when the boiler is not burning. This allows a series of short, hot, efficient fires with time between fires where the heat comes from storage.
Storage Pros: More efficient fires, and flexibility about when to build fires. Reduced temperature fluctuations in the house. Near-infinite domestic hot water.
Storage Cons: Storage involves additional cost and complexity. Planning and designing storage is a big task, and qualified third parties may be hard to find. Both plumbing and controls become more complex. Storage requires space.
Our choice: We started without storage and added it after the first season. The big benefit to us is the ability to space out fires. We can often skip one or more days, especially early and late in the heating season. The house temperature remains constant, and we never run out of hot water.
Pressurized or unpressurized storage?
Storage tank systems can be broadly categorized into two classes: pressurized and unpressurized. Pressurized systems are closed tanks where the water in the tank circulates through the boiler. In unpressurized systems the water in the tank is isolated from the water in the boiler, and heat transfer is accomplished via heat exchangers either in or out of the tank. 500 gallon propane tanks are often used as pressurized storage tanks. Unpressurized systems are very often open top containers, and may take many forms, including cisterns, 'above ground pool' type construction, and metal tanks of various configurations.
Pressurized pros: Because the water in the tank is being used directly, there is no temperature drop effect. If the water at the top of the tank is 180 degrees, then the water going to the heat zones is 180 degrees. With proper attention to plumbing, a higher degree of stratification can be maintained. There is no need for and no expense involved in a heat exchanger for the tank.
Pressurized cons: Pressurized tanks have to be designed and constructed to withstand the highest possible system pressures (typically 30 psi). The increased volume of water means that a much larger expansion tank is required - often in the 60 to 80 gallon range. The additional volume of water means that there is more dissolved oxygen to cause corrosion of metal components. It's difficult or impossible to add heat exchangers in the tank for things like domestic hot water preheat or solar panels.
Our choice: We went with unpressurized storage, mostly because we need heat exchangers for DHW and solar. We also found an unpressurized stainless tank that fit our available space perfectly.
Circulator based or valve based zone control?
There are a near-infinite variety of plumbing approaches, but one useful distinction is pump-based vs. valve-based zone control. In a pump based system, each heating zone has its own circulator pump which runs when there is a demand from that zone. In a valve based system, there is one circulator pump at the boiler and each heating zone has a zone valve which opens when there is demand from that zone. If there's more than one heat source, each source typically has its own circulator.
In a pump based system, each zone thermostat goes to a controller which controls the pumps.
In a valve based system, each thermostat is wired directly to the corresponding zone valve. Contacts in the zone valves provide a demand signal to the boiler when the valve is open.
Circulator pumps and zone valves cost about the same, so there is no significant cost difference. Both are reliable and easily repaired.
New pump technologies are producing pumps with much lower energy consumption as well as variable speed pumps with smart controllers. At the present, these pumps are not widely available in the USA.
Some pump-based systems are quite complex - see the section below on primary / secondary loops.
Pump pros: Pumps can be sized to the flow requirements of each zone. Flow rates can be better controlled. Newer technology pumps offer the possibility of very precise matching of flow to heat loads in the future.
Pump cons: Pumps consume more power than valves. Pumps do not provide positive isolation - water will flow forward through a pump even if it has a check valve. It can be more difficult to understand what's happening in a system with multiple pumps.
Our choice: We went with a valve based system, in part because that's what our plumber had recommended for our initial oil boiler installation. The lower power consumption is also a factor, and our installation is designed so that there is only one active heat source and one pump running at any time. That make it easy to understand what's happening with the system. We are upgrading to a multiple speed pump for the wood boiler to provide better control of flow rates.
Series or parallel wood / backup installation?
If you're adding a wood boiler as a heat source, there's likely a gas or oil boiler in place that will act as a backup heat source. In some cases, the oil or gas boiler has an internal coil that provides hot water. There are two basic options for plumbing the new boiler: It can be plumbed in series with the existing boiler, or it can be plumbed in parallel.
A series installation routes the outlet of the wood boiler to the inlet of the oil/gas boiler, often using only a single circulator. A parallel installation has each boiler drawing from a common return manifold and pumping to a common supply manifold. Each boiler has its own circulator with a check valve to prevent reverse flow when the other boiler is operating.
Parallel Pros: The idle boiler is not heated, so there is no standby loss to the room or up the chimney from the idle boiler. Since the idle boiler does not have to be heated, the system comes up to temperature more quickly. Either boiler can be isolated and repaired or replaced without losing the ability to heat the house.
Cons: If the oil/gas boiler has an internal coil that provides domestic hot water, a parallel installation will require additional work and expense to provide hot water. Parallel installations involve a bit more plumbing and require an additional circulator.
Our choice: We chose parallel, and that's probably the best solution in most cases. Losing heat up the oil boiler chimney 24/7 was not appealing.
Primary/secondary loop or supply/return manifolds?
Ideally, every heat load and heat source has a flow rate that's well matched to its capacity. Ideally, the plumbing is simple and straightforward with a minimum of components and low power consumption. These objectives are in conflict to some degree. For larger or more complex systems, the primary/secondary loop approach is often used.
In this technique, there is a primary loop that all heat sources and heat loads connect to. The primary loop has a relatively large circulator that runs whenever there is any active heat transfer. Each heat source and heat load has its own loop with its own circulator that runs whenever that loop is active. Connections are made using closely spaced tees so that there is no flow in any loop if that loop circulator is not running.
In supply/return systems, there is a supply (hot) manifold and a return (cold) manifold. Each heat source and heat load is connected between the two manifolds. Heat loads may be managed with either circulators or zone valves (see section on circulator vs. zone valve control above). Each heat source typically has its own circulator.
It's also important to realize that there are many possible variations on these approaches. For instance, you could have two heat sources in parallel as part of a single secondary loop, or the heat sources and storage could form the primary loop as shown below.
Advantages of Primary / Secondary: It's possible to closely match the flow in each loop to the needs of the heat load or source. Multiple heat sources can be active at the same time. Adding sources and loads causes minimal disruption.
Advantages of Supply / Return: Conceptually simpler and easier to understand with fewer components. Typically only one pump is running, so power consumption is lower.
Our choice: We went with a supply / return configuration. My sense is that if pressurized storage is an option, the hybrid system shown above would be a very good solution for many residential applications.