How Much Wood Will Your Outdoor Wood Boiler Burn? What You Need To Know

As an outdoor wood boiler business, we run into some questions a lot. This is one of them: "How much wood does an outdoor wood fired boiler burn? How much wood should I have prepared for a heating season?" Spoiler alert, we usually can't answer the question exactly, but in this post, I'll try to explain the factors at play which determine how much wood your outdoor furnace will burn. There are three main factors: wood boiler efficiency, wood quality, and heat load. Each of these factors is the result of a bunch of things which I'll try to break down.

The first factor we'll get into is how efficient your wood boiler is. Outdoor wood furnaces can vary tremendously both in actual efficiency and in what manufacturers claim is their efficiency. As a general rule, models that meet or exceed the Environmental Protection Agency's Phase II emission standards are much more efficient than standard model stoves. It is generally accepted that a boiler using wood gasification technology often burns as much as 50% less wood than less efficient traditional outdoor wood boilers. Generally, outdoor wood boilers range between 90% efficient at best to around 40% efficient. This means that of the energy available in the wood placed in the furnace, between 40% and 90% will be transfered into the water jacket to be circulated to your heating system. There are several ways to measure furnace efficiency, but you can get a pretty good idea if you search the furnace model name on the EPA's wood heater database.

The second factor affecting how much wood your outdoor furnace will burn is the quality of wood you feed it. The energy stored in various types of wood can vary significantly. In general, there are three classes of woods: heavy woods, medium woods, and light woods. According to a paper published by Purdue University, heavy woods like oak, ash, or hickory typically run in the vicinity of 16million BTUs per dry cord. Medium woods like elm, maple or sicamore typically run closer to 12 million BTUs per dry cord. Light woods like box elder, aspen, or willow often run as low as 9 million BTUs per dry cord. As you can see, this means you will burn a lot less wood using say oak rather than box elder. Another factor that affects the available heating BTUs in your wood is its moisture content. when you burn very wet wood, as much as 40% of the BTUs in the wood go to drying off the moisture. You can gain a lot in efficiency just by cutting your wood an extra year ahead so it has time to dry a few more percentage points.

Finally, we get to the third factor affecting how much wood your boiler will burn: heating load. Once again, this varies a lot. Several factors affect how large your heating load will be. The first and most obvious factor is how much are you heating? Some people are just heating a home, while others attach a radiator in their garage or a swimming pool heater. In general, the larger the building you are heating, the more wood you will burn. Insulation quality also plays a large role. A small but poorly insulated old farm house may well cause a much higher heat draw than a large well insulated home with a two car garage. How cold the climate is in your area is also a big factor. We Love Fire has a great BTU calculator you can use to get a rough estimate for how much heat your home will require. One last factor is the quality of your insulated underground line. There are various styles of insulated underground line to choose from, each with varying heat loss ratings. Generally, solid core foam filled lines are slightly more efficient than bubble wrapped options, but be warned, foam filled line is very stiff and difficult to work with.

In summary, there are just way to many factors affecting how much wood you will burn for me to give you even a ballpark figure, but I hope you found this interesting. One way to get a good idea how much wood you might need is to ask around on online wood boiler forums to find folks with a similar heating situation to yours and see how much wood they are burning. Some people enjoy cutting wood, so this is a non-issue, but if you are trying to cut as little wood as possible, the bottom line is this: You want the most efficient underground lines and the most efficient wood boiler possible. With that, we can help. Please feel free to contact us for a free installation estimate or just to bounce some questons off us.