Outdoor Wood Furnace Pros and Cons

A long discussion of wood-burning furnaces. May 18, 2010

From the Staff at WOODWEB
We've found there are a number of benefits to the use of outdoor wood furnaces. Safety is the most recognizable; since these units are placed outdoors, there is less concern regarding fire and smoke hazards. Another benefit is cleaner building interiors. With the unit outside, smoke, soot, and ash are eliminated, and dryness is reduced in the buildings. Outdoor wood furnaces also have a thermostat control that keeps the heat at a constant level, which is sometimes difficult to achieve with a woodstove or fireplace. Minimal loading is a convenience as some outdoor furnaces only require loading once or twice a day.

There are less desirable characteristics of wood furnaces that also need to be considered. These include wood heat's impact on the environment, emissions, local disruption, and health problems.

There has been much debate concerning outdoor wood furnaces and their cost, efficiency, and environmental impact. When comparing outdoor wood furnaces to other wood heating methods, outdoor wood furnaces appear to be more efficient and reduce environmental impact, and proponents of outdoor wood furnaces claim a cleaner, safer, and more efficient heating system that will help everyone, not just the operator.

What has been your experience with outdoor wood furnaces?

Forum Responses
(Sawing and Drying Forum)
From contributor S:
I have been using outside woodstoves since 1995. My home has one. With it I also heat my domestic hot water (never a cold shower). Before switching over, my gas bill was $3,500 per year at 60/gal. My wood costs $1600 per year. New stoves all setup run around $8-10,000. The nice thing is, if you set the temp at 70, it stays there.

I also have one for my shop - a lot bigger stove heating a space of 8500 sq.ft. with 13 ft. ceilings. We burn is wood scraps all day. Fill it with round wood for the night. We run 4 modine heaters with it. I live in northern Wisconsin - 30 f is not uncommon. Open the big overhead door to move wood around, the shop cools right down - but that's okay, as we just burn more scraps, not dollar bills. Anyone thinking about doing it should check it out!

From contributor G:

Air pollution can be a big issue. Some designs are just too dirty but are still being sold. Other units are not used properly or installed correctly and they pollute for those reasons. Only a few states have insisted that the OWB (outdoor water boiler) burn cleanly.

The water boiler solution that appears to be popular because it is more efficient is called a gasifier. When used properly, this boiler burns very hot and cleanly. A significant savings in fuel is realized. Gasification is the process of wood decomposition from a solid to a flammable gas; it requires the stove to reach and maintain a high temperature and then to route this "wood gas" mixed with fresh air to create a secondary combustion, and this step is what greatly increases the overall efficiency. The heat exchanger tends to be quite good in these boilers with nearly a 2000 F difference between the combustion temperature and the chimney exhaust temperature.

The EPA has asked for the builders of OWB to voluntarily join with the EPA to monitor and migrate boiler designs to meet the same standards as the indoor woodstove. A similar product is a hog fuel boiler. Many countries have these and they burn chips, not pellets, although the concept is the same. Not sold in USA.

From contributor M:
I was just reading about this issue. People frequently burn trash, garbage and pressure treated wood in them. I would not want to be downwind from a furnace burning toxic substances.

From Professor Gene Wengert, forum technical advisor:
I wonder about the control of such furnaces, compared to wood stoves (including pellet), by city zoning and other regulations in a city.

From contributor S:
I for one burn only wood and wood scraps. My stoves, when burning, don't even show smoke. It is a concern - people burning stuff other than wood. But a few shouldn't give the stoves a black eye. Isn't wood smoke (pollution) still better than fossil fuel pollution? I don't know, only hope so.

From contributor Z:
We have an H.S. Tarm gasification boiler that heats our building and hot water. It is amazingly clean-burning, at least from the look of the smoke. We cut 24-25 standard cords of firewood per year and burn year round. We've got over 100 acres of woodlots plus hedgerows and such, so there is plenty of supply. We have to burn well dried hardwood in the boiler, so we send our softwood slabs and firewood to Unity Acres to burn in their conventional boiler, which will burn anything.

From contributor P:
Maybe they have gotten better, but the existing wood furnaces seem to burn about twice as much wood as using regular wood stoves, and emit lots more smoke. The smoke is so bad that NH passed some laws about how close they can be installed to property lines and how high their stack heights have to be. Certainly the typical low stack height contributes to the pollution problem, as the smoke drifts downwind directly into open windows. Of course, many are marketed for their ability to burn green, unsplit wood, so the poor fuel used may also contribute to the problem.

From contributor G:
Yep, there are some real polluting pigs out there. Just the opposite of what is good for the environment; they need to burn a lot of wood to generate the heat needed, putting out 100 times the pollution of the same home with an indoor wood burning stove.

I did talk with one gasifier manufacturer about competing with those stoves. It was their hope that the EPA would outlaw the polluters. But that did not happen. Sadly in every case those stoves sell for less; folks always want a low price, and unless they learn about the pollution problems and low efficiency, they will likely buy one of these.

From contributor F:
In Canada all insurance companies ask for an inspection certificate for all wood burning heating systems from the local fire department. They look for an EPA or CSA approval sticker. This covers both safety and pollution. Anyone installing a homemade or unapproved system has no insurance.

From Professor Gene Wengert, forum technical advisor:
Smoke (unburned wood, commonly called creosote) results when there is not enough oxygen (probably because there was plenty of wood fuel, but not a need for heat, so the oxygen supply was cut back to reduce the heat (burning rate)). The user often puts in enough wood for 24 hours; the slow burn means limited oxygen and more smoke. So, smoke is an operator problem more than an equipment problem, unless the equipment does not adequately provide oxygen to the burning wood. To provide good oxygen, often the air enters from under the fire.

This smoke issue is handled much better with a pellet stove, as the pellets are metered to avoid excessive fuel when the heat is not needed.

From contributor Z:
Some, if not the majority, of boilers around here have thermostatically controlled draft fans. In our case, when the water in the boiler reaches 200 degrees, the fan shuts off and the fire stops burning till it comes on again. The fire can sit for at least 12 hours in my experience and still begin burning again when the air supply is resumed.

From contributor C:

I am in the process of building my house, and I have done extensive research into wood boilers. If you do not want smoke, you need lots of air. Too much air, too much heat. The solution to this problem is a heat storage tank. Just plumb a large insulated tank of water/glycol into the loop. Run the furnace on high output (this is the most efficient part of the burn). This will heat the tank to just below boiling, then shut it down. When your house calls for heat, another pump circulates the water/glycol through the house. Most of the wood gas boiler manufacturers are using this setup, as it greatly improves efficiency of the boilers.

From contributor G:
The seller of the wood fired units may not explain the smoke problem and the solutions, which means more installed cost, to the customer. Some brands may talk about the fact that their stove will stop burning when the demand is zero, but will stay hot enough to restart the fire when a demand becomes present.

Without a reservoir the chances are that the boiler will smoke, which can be caused by leaky gaskets, cross winds, heat loads, cycle rate, the way the fuel is loaded in the stove, and not properly prepared fuel.

As stated above, the more expensive solution is to have a reservoir so that the stove can burn a load of wood efficiently and for the stove to be properly maintain and operated. There are many successful users; the bad ones are the ones folks see and smell.

From contributor Z:
With ours there is no smoke and little heat when the fan shuts off. If you open the firebox door when the fan has been off for a little while, there is no flame at first, since the boiler is quite airtight. When the fan comes on it takes a few minutes for the fire to wake back up. We've had to replace the door gaskets a couple of times, but they're pretty cheap. We looked at getting a reservoir tank but decided that the expense and the question of where to put it made it impractical for us.

From contributor C:
A heat storage tank can be built rather easily for relatively cheap (especially for someone with a sawmill). Just build a strong frame with lumber, line with planks/plywood, then line with poly that is rated for higher temperatures. Then just place all lines over the side, and put a cover on it. My brother in-law made a cistern like this. It held almost 1500 gallons, and lasted 15 years with no problems.

From contributor J:
Undecided on the purchase, due to 5-8 year cost recovery. High efficiency or not, that's a lot of material. However, due to a waste stream and real winter, an outdoor boiler remains attractive. All insurance companies in Canada do not get a fire inspection (I am an inspector), but here in Manitoba the insurer has minimum requirements, such as 50 feet from any structure. Some jurisdictions also require same, thereby limiting installations by lot size. Tuesday I might be at the factory or on the phone for an available unit.

From contributor N:
The wood gasification boilers are worth the purchase price. I have had a Tarm for four years and have never been sorry for the purchase. My wood usage was cut in half compared to a conventional woodstove for heating my entire house. Yearly cleaning of my flue shows only dust and no creosote buildup. There is no visible smoke while burning. The manufacturers of these types of boilers claim emissions are below the proposed EPA guidelines for wood burning devices. I use a heat storage tank which is easy to build with simple construction skills. I would recommend rubber roofing for the liner which is typically rated for 180 to 190 degrees F. I have heated my home and domestic hot water averaging less than five cords per year. Two friends have installed them in their homes and both are extremely satisfied.

From contributor T:
I purchased a Central Classic 6048 Boiler. Total cost was about 10,000. I set it up and did all the excavation and plumbing. It is probably the best thing I have ever spent money on. I would normally spend about 4,000-5,000 on oil for my home. If the wife is cold, just crank the dial on the thermostat - I could care less.

Now I spend nothing on oil - not a drop of oil in several years. My boiler also heats my 40x60 barn and now is being used to heat my firewood kiln. I run it year round and burn sawdust and waste chips from my firewood processor. I typically burn junk wood in my boiler. Anything that I can sell or comes from one of the landings we are working on during the year.

Smoke is really not an issue. Softwood does smoke more than hardwood, same for green wood vs. dry. Typically I get all the pine blocking I could ever want during the year. I don't mind putting out a bit more smoke and paying nothing for the wood. Anything I can sell I do, and the less desirable wood I keep for me.

If you drive by an old farmhouse with a wood stove running on a dreary fall morning with little to no wind, they will be putting out 10X the smoke my wood boiler emits. And no one is telling farmer Joe to regulate the trash he is burning in his stove!

From contributor B:
Because wood and other biomass fuel produces ashes and soot, regular cleaning of a wood furnace is important to keep it in proper working order and to prevent the risk of fire or other malfunctions. The ventilation holes that allow air into the firebox should be cleaned so that the fire can get enough air to burn, and the smoke stack should also be cleaned so as to prevent buildup that can lead to a backup of smoke or other hazardous gasses. The furnace features a thermometer that allows you to monitor the temperature of the fire inside the firebox; this thermometer should be checked periodically to make sure that it is still functioning properly.