Vermont Heat Research

An Experimental Wood Chip Furnace

Flame photo

A view through a glass port into the green wood chip furnace. Notice the blue flames. 
This is a single chamber stratified combustor running with no secondary air.
Chips: 50 lbs. of 47% moisture content green hemlock boughs, including  foliage. 2 hour burn.

During the winter of 2006 we heated our Vermont home with wood chips. We tried both home grown chips and chips purchased from a sawmill. Our own chips were cut from brush growing on our 67 acre property -- a mix of black birch, red maple, and cherry saplings, hemlock boughs and assorted other species, burned green, straight from the chipper, with an average moisture content of 43-47% (wet basis).

Not only did we burn green wood chips, but our experimental furnace ran at high efficiency, and under normal operating conditions, smoke-free. We would fill it with four buckets of chips and in two hours producesan estimated  175,000 Btus of hot water in a heat storage tank to feed our home baseboard heating system.

Our experimental furnace was located in a small cinder block building which formerly housed a HAHSA II outdoor wood furnace. (That furnace was removed due to pollution concerns and poor performance.)

Thanks to a single chamber stratified batch combustion method, we sometimes even burned chips mixed with snow and well composted chips, again, cleanly and efficiently. The only noticeable difference with these unlikely fuels is a partial reduction in total heat output.

Why use wood?

Wood is still an abundant self-renewing material throughout the United States. If it is harvested through managed forestry, woody waste conversion, and active fuel wood cultivation it can remain a self-sustaining resource and reduce our dependence on petroleum.

While it's difficult to use wood for powering engines wood can provide a boost in petroleum and gas supplies by replacing these important fluid fuels in many heating applications. Simple heating does not require a sophisticated fuel like petroleum..

In my small state of  Vermont alone, it has been estimated that low grade timber could provide 1.5 million harvestable tons of green wood per year on a self renewing basis. This is the yearly eqivalent of about 92 million gallons of home heating oil which costs at present prices, a quarter of a billion dollars.

The amount of wood available for whole-tree chip harvesting has been conservatively estimated at one million green tons per year in Northern Vermont alone.” Burlington Vt. Electric Department

The accelerating cost of fossil fuels now provides good reasons to develop clean wood heat as a lower cost alternative to petroleum.  What is needed is an available clean-burning efficient method of converting wood into heat energy on a domestic scale. 

Why wood chips?

There are three kinds of common wood fuels: wood pellets, cord wood, and wood chips.  

Pellets: Wood pellets come from sawdust that is mixed with water, extruded and dried with heat. They are then bagged in plastic, and stored dry to be transported by truck and train from major lumber producing areas in the country. 

Pellets burn cleanly and efficiently. They are compact, and easy to handle. They are a uniform product. 

Unfortunately, pellets are energy-intensive to make, store and transport, and would be unaffordable if they weren't made from lumber mill sawdust and waste heat.This is becoming clear now that many people have bought pellet stoves. 

Wood pellets used to be priced competitively at $60 per ton a few years back. However, last winter wood pellet prices reached $320 per ton and  there were shortages. This was because the supply of cheap lumber mill sawdust has been depleted by the demand for pellets. On a heat-for-heat basis, the cost of pellets is now on a par with petroleum products as a fuel.

It's hard to make more pellets, or lower their cost. While sawdust can be ground entirely from logs, rather than gathered as a byproduct of lumber mill operations, it is even more costly to do this. Converting a whole tree to sawdust and then gluing it back into pellets is not a particularly efficient use of wood as an energy resource.

Cord Wood: Cord (log) wood has been burned since prehistoric people first made fires. Everyone loves a log fire in a fireplace. Unfortunately a conventional wood fire is not very efficient and creates air pollution to a greater or lesser extent. Half of the gasified fuel products and heat of conventional wood fires is lost up the chimney.

With the increased population in the United States, concerns over pollution has forced the EPA to take action to limit the adverse health effects of  wood stove pollution by testing and requiring emissions standards for manufacturers. This is making it more difficult to provide affordable wood heat.

In response manufacturers  have begun to use add-on catalytic converters or dual chamber gasification methods to try to reclaim some of the heat, creosote, unburned carbon monoxide and pollutants that go up the chimney as smoke in a typical wood fire. Unfortunately, gasification furnaces can be very complex and expensive and there can be operational and maintenance issues due to creosote buildup in the primary gasifying chamber.

Outdoor wood furnaces, which have so far been immune to EPA emissions requirements, are frequently being criticized as serious pollution offenders. (See:

Legislation has now been  passed in some areas to ban outdoor wood furnaces because of the high level of smoke pollution exhibited by the worst of them. The degree of pollution depends to a great extent on both design and operator practice, but in general they produce higher levels of pollution and lower levels of efficiency than indoor wood stoves.  

(The VTHR experimental chip burning furnace is an exception to this rule, but it was really an indoor type furnace housed in a dedicated masonry shed rather than a standalone outdoor furnace.)

In other pollution sensitive areas of the country, legislation against even fireplaces has also been introduced. 

Another consideration with traditional wood burning is that generally the most suitable, efficient fuel wood is good hardwood from a tree  from 6 to 18 inches in diameter. Such trees take up to a half century to grow and a large portion of the tree -- the limbs and branches -- is wasted when the tree is cut.

Burning cord wood therefore presents some challenges for the future. In addition, as a fuel, cord wood isn't necessarily easy to  use or cost free. Cord wood must be split, stacked and dried for a year to burn efficiently in almost all wood stoves and furnaces. Dry hardwood is particularly important for the expensive high-tech dual-chamber gasifiers. Split dry hardwood prices are approaching petroleum prices for equivalent heat.

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Wood Chips: Wood chips can be made from waste wood, brush, saplings, limbs, tree slash from logging operations, and from forestry and roadside maintenance operations. Wood chips do not need to be made from  old standing timber. Other advantages are:

  • Wood chip prices are relatively stable between $15 and $30 per ton, and are often available free from tree services working in the neighborhood..
  • The energy required to chip wood is a fraction of the energy required to make pellets. A wood chipper uses about the same amount of energy per pound of product as a small wood splitter. 
  • Green chips can be used immediately without drying in stacks for a year (like cord wood) and without using process energy (like pellets)..
  • Chips can be transported and unloaded by dump truck. Because they are generally available locally, long distance haulage, packaging, and energy consumption can be reduced.
  • The supply of chips is not limited by another industry's waste output.
  • Fuel growing methods, such as brush and coppice farming can produce ideal wood for chipping on a sustainable basis with a very high yield per acre, and a short cycle. Coppicing can be mixed with conventional timber forestry to maintain an ecological balance   

Why don't we use chips to heat our homes?

One reason wood chips haven't been used for home heating furnaces is that there just hasn't been a long history of burning wood chips. Conventional methods of small wood fire building obviously don't work with wood chips.

Modern chip furnaces generally require an industrial size burn chamber (eg. a blown or “fluidized bed” combustor) and large scale fuel feed designs, with chip storage buildings and automated augers and conveyors.


This complex and expensive equipment  is considered necessary because of the uneven size of chipped wood – often mixed with twigs and sawdust. This mixture tends to jam in small feed mechanisms.
You can't run chips through a pellet stove's 2" auger.

The methods now used to handle chips duplicates old petroleum fluid fuel handling practices by creating very large auger and belt “conduits” compared to the much smaller “fluid” chip particles to minimize jamming. 

The usual burning method for wood chips also imitates fluid fuel practice by using powerful blowers to circulate fuel particles in a big combustion chamber. 

As a result of fluid fuel ideas, wood chips have been used mainly for commercial and municipal scale operations, in schools, and in power plants. Settings where the size of the chip is small by comparison to the operational equipment.

Another problem sometimes mentioned when discussing wood chip heat is the mistaken idea that it is necessary to dry the fuel to a low moisture content, as is done with pellets and cord wood. This is a holdover of  our prehistoric experience with wood fires, our belief that a green log fire is nearly impossible to start or maintain, and when burned emits lots of smoke, and little heat. 

Do everything wrong 

My original aim in this experiment was to explore a reversed approach to modern commercial practices and conventional wisdom. Namely trying to directly burn wet green (50% moisture content) chips in a self-sustained reaction in a small solidly packed single combustion chamber with little or no secondary air admission. I wondered if doing this in a batch-burn high-output hot water furnace would  provide enough heat for a Vermont home. 

By definition a batch process doesn't need continuous feed, or throttling, and so eliminates large augers or combustion chambers. As a result, the VTHR experimental furnace was about the size and shape of a household hot water heater.

Because the furnace uses a high output one-shot process, the fire can't be modulated during the burn. The excess liberated heat must be captured, stored, and later more slowly distributed to the home where a thermostat can control the demand for heat. 

The experimental furnace stored heat in 2500 lbs of water in an insulated tank. A small circulation pump deliverd this to the house. One advantage of this kind of system is that other heat sources, such as solar collectors, co generators, or even a conventional-fueled boiler can be added in parallel, while baseboard heating units, radiators, blown hot air exchangers, or domestic hot water tank exchanger can draw heat from it. 

This allows a very flexible system. It also becomes easier to make experimental changes in configuration, and monitor actual heat output, losses, and usage.

How do you burn wood well?

The problems of burning wood are the result of 5 simple things.
  1. Wood gives off fuel gas when heated. 
  2. Gas needs to be well mixed with air to burn well
  3. Wood is thick (massive).
  4. Embers fall
  5. Hot air rises
So when we look at a fire in a fireplace on a grate what we really see is some unburned wood in the middle, hot embers heating that wood below, flames and smoke rising above.

What's happening is:
  1. The hottest temperature in the fire is in the embers below and around the relatively massive wood, heating it, and forcing it to emit cooler fuel gases from within. 
  2. Air from the room goes to the embers first where it is depleted of oxygen. 
  3. The heated and depleted air, mostly inert nitrogen, rises and mixes with the wood fuel gases without burning them.
  4. Air also enters the upper portion of the fireplace, but too little and too late -- the pressure and velocity of the hot gases is higher than the air pressure so mixing is poor, there's lots of nitrogen, and temperature is actually dropping fast. The yellow flames we call fire are actually cool flames with deficient oxygen and lots of incandescent soot particles.
  5. Smoke containing fuel gases and creosote exiting above the flames is a result of too little oxygen and too cool a temperature to support combustion at all.

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(c) Copyright 2006, Stephen Redmond, all rights reserved