Wood Oil combination boiler (C) Daniel FriedmanCombination Wood-Oil or Wood Burning or Coal-burning Boilers, Furnaces & Woodstoves
Installation, inspection, troubleshooting

  • WOOD-OIL COMBINATION HEATERS - CONTENTS: How do we inspect and diagnose problems on combination wood and oil-fired heating boilers and furnaces? How are wood-burning furnaces controlled? How are wood-oil combination fuel boielrs and furnaces controlled? What special opeating concerns are faced by combination-fuel heating devicves?
  • POST a QUESTION or READ FAQs about combination fuel heating systems: wood + oil burning furnaces, boilers, other heating appliances: inspection, diagnosis, operation, maintenance, repair, safety

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Wood & Oil Combination Heaters, boilers, furnaces:

This article describes combination or multi-fuel heating boilers that combine burning wood with oil.

We explain how a multi-fuel heating system works and we list the special considerations that such equipment needs such as attention to combustion air supply, draft regulation, combustion chamber design and cleaning, and general safety.

These articles on heating appliances and chimneys and chimney safety provide detailed suggestions describing how to perform a thorough visual inspection of chimneys for safety and other defects. Chimney inspection methods and chimney repair methods are also discussed.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Combination Wood Burning Boilers, Furnaces, Fireplaces, Woodstoves - Special Considerations

Alternative heating sources (C) Carson Dunlop Associates

Especially when oil prices have increased dramatically in the 1970's and again during the 2008 Bush Presidency economic crisis, many homeowners found a renewed interest in alternative heating energy sources. By 2015 when oil prices were falling that interest may have taken a [probably] temporary dip.

[Click to enlarge any image]

Where firewood is available and economical, wood fired heating boilers and furnaces, and combination wood and oil heating systems that were first popularized in the 1970's oil embargo crisis still have renewed interest.

This article describes wood heat sources and special considerations in operation and safety of combination fuel wood and oil heating boilers and furnaces.

For a current comparison of the relative costs per BTU of heating oil, natural gas, firewood and electricity, readers should see HEATING COST FUEL & BTU COST TABLES.

Combination Wood & Oil Boilers & Furnaces

Wood Oil combination boiler (C) Daniel FriedmanShown at left is a combination wood-fired and oil fired heating boiler installed in a New York home and inspected by the author in 1999. Here are some basic properties of this combination wood/oil heating appliance:

- single combustion chamber wood furnace - the client has opened the upper door of the wood-side of this boiler.

- separate combuation chamber and heat exchanger for the oil fired half of the unit

- how wood or combination units regulate draft

- forced draft combustion system is used for oil while natural draft is used when the heater is burning wood

- dual combustion chambers, because they use different fuels, have different service requirements, different operating requirements and different draft and control requirements.

The air shutter on the oil burner and the oil burner motor provide foced draft when burnign oil. Boiler operation when burnign oil is controlled by an aquastat (the gray control box(es) seen on the front of the boiler. Boiler operation when burning wood is controlled by a thermostatically operated combustion air damper control that is opened or closed at the bottom of the wood side of the boiler.

We discuss the WOOD FURNACE COMBUSTION AIR DAMPER in more detail below.

Regarding concerns for chimney flues shared between oil and wood fired heaters, see exceptions to shared flue rules where Wood & Oil Fired Heaters are discussed

Alternative heating sources (C) Carson Dunlop Associates

Here are the basic components of a wood-only warm air furnace, compliments of Carson Dunlop Associates.

Many of the components on a wood fired warm air heating system such as ductwork and the blower assembly are the same as on oil or gas fired furnaces but the heating system itself is quite different:

Kerosene heater (C) Daniel Friedman

Provide Combustion Air, Ventilation, Cooling Air for the Wood Furnace

Alternative heating sources (C) Carson Dunlop Associates

Before looking in greater detail at wood fired furnaces and combination wood-oil furnaces, we and Carson Dunlop emphasize this safety note.

Watch out: Because a wood-fired furnace operates at high temperatures and needs lots of combustion and cooling air it should not be located in a confined space. Otherwise the risk of fire or improper operation are increased.

Chimney inspection and cleaning will need to be frequent to reduce the risk of a chimney fire as well. Sketch courtesy of Carson Dunlop Associates.


How Combustion Air & Temperature are Controlled on a Wood-Oil Combination Boiler or Furnace

Here we discuss first the control of wood-oil combination fuel boilers and furnaces, and next we will discuss the control of wood-fired furnaces. Below is a photo of a combination wood and oil fired heating boiler that the author (DF) inspected in New York in 1997.

Wood oil combination fuel boiler controls combustion air when burning wood (C) Daniel Friedman

Using our photo at above right as an illustration of the temperature and draft controls on the wood-side of the combination wood-oil heating boiler shown: [Click to enlarge any image]

Wood burning controls on the dual-fuel boiler:

The boiler temperature or burn rate for wood is set by turning the adjustable thermostat knob at upper right in the photo - green arrown #1.

The thermostatic control monitors the temperature in the combustion chamber / heat exchanger on the wood side of the boiler. A bimetallic spring lifts the air shutter control arm (green arrow #2) to pull the combustion air shutter control chain (green arrow #3) which lifts open (more air) or falls to close (less air) the combustion air intake door at the bottom right of the boiler (green arrow #4).

A barometric damper draft control on the heating flue is not shown.

Watch out: if the wood-oil boiler uses a single chimney and flue and a common barometric draft regulator on that flue for both fuels (unsafe, and illegal in some jurisdictions) it will not be possible to adjust the draft regulator for optimum performance of both fuels. There may also be creosote fire hazards when burning different fuels in the same flue.

Oil burning controls on the dual wood-fuel boiler:

In our photo of a dual fuel wood-oil boiler at above right, the yellow arrow 6 points to the primary aquastat and yellow arrow 5 points to a second high limit aquastat control. Combustion air for the oil burner is provided through the oil burner air shutter and by a squirrel cage blower fan in the oil burner. Draft will be regulated by a barometric draft regulator on the flue vent connector (not shown).

How the Wood Furnace Combustion Air Damper Works

Wood furnace combustion air control (C) Carson Dunlop Associates Wood furnace combustion air control details (C) Carson Dunlop Associates

Sketches courtesy of Carson Dunlop Associates. Photo at below right - Daniel Friedman. [Click to enlarge any image]

Alternative heating sources (C) Carson Dunlop Associates

Forced draft wood furnace operation: As the sketch (left) shows, instead of relying on natural draft, a forced-draft combustion wood air furnace uses an electric blower fan to feed air to the wood fire.

Heater controls can turn off the blower fan and on some models adjust the airflow rate as needed.

Alternative heating sources (C) Carson Dunlop Associates Alternative heating sources (C) Carson Dunlop Associates

Wood Furnace combustion chamber details

Alternative heating sources (C) Carson Dunlop Associates

Wood Furnace combustion chamber cleaning details are shown in the sketches at left and below, courtesy of Carson Dunlop Associates.

Alternative heating sources (C) Carson Dunlop Associates Alternative heating sources (C) Carson Dunlop Associates

Wood Furnace barometric damper inspection

DRAFT REGULATOR, DAMPER, BOOSTER are devices used to regulate the draft on oil-fired heating equipment such as furnaces, boilers, or water heaters.

The barometric damper or draft regulating device we are discussing here is normally used only on oil-fired heating equipment, not on gas-fired equipment.

The inspection requirements such as assuring that the damper is level, properly located, and operating freely are provided in our heating section in the Draft Regulator article linked-to just above.

The equivalent draft control on gas fired heating systems is discussed
at DRAFT HOOD, GAS HEATER On gas fired equipment.

Wood Fired Heating Furnace & Boiler Control Operation

Reader Question: how to control an outdoor wood fired furnace

I have a outdoor wood furnace which has a thermostat on it out there it controls the blower! I want to run a thermostat in the house so when it get warm enough in house it will over ride the other thermostat? Where and how many wires will the 2nd thermostat need? I thought being just to turn blower on and off? - D.M. 11/19/2013


I would like to help but don't know enough - I'm afraid that posing a solution without fully understanding the system is dangerous.

Reader follow-up:

I have a outdoor wood furnace and it has blowers on it that blows the heated air into our duct work! It has a thermostat on it that kicks the blowers on when temperature reaches 100 degree and  suits off when it hits 200 and I need to wire a thermostat in the house that will shut off the blowers when the house reaches the desired temperature!  I think I can do it but was wanting a second opinion!


Any furnace depends on air flow through its heat exchanger and supply plenum to avoid cracking, holes, leaks and damage to the heat exchanger that otherwise are likely to occur if the exchanger is subjected to high temperatures. That's why the limit switch on a normal furnace will keep the blower fan running for some interval after the thermostat says the call for heat has been satisfied. So you would not want to add a control that by some other means turned off the warm air blower.

To be clear, the in-house thermostat talks to the fan limit switch. When you connect your thermostat to the furnace's own limit control, the thermostat will end the call for heat but the switch won't immediately shut off the air flow at the furnace, as I've explained.

Take a look at FAN LIMIT SWITCH if you want to read more about this topic.

In the special case of wood fired heating equipment there is a bit more to understand:

Because wood fired heating equipment might run at higher temperatures than say a gas fired furnace, and because we usually cannot absolutely and completely and suddenly "shut down" the wood fired furnace (that is you can't completely start and stop a wood-burning fire unless you douse it with water), wood fired boilers and furnaces operate a bit differently. Instead, during a heating cycle, the wood fired heater's controls will close down the combustion air supply to the fire, slowing the fire but not turning it off completely.

What this means to you is that it is possible that in order to protect the wood fired furnace from overheating, it may want to blow more air through its heat exchanger (and into the building) at times than the thermostat actually needs. It depends ... on the sophistication of the controls on the furnace.

Finally, we do not usually want to run wood-fired equipment always at its slowest, lowest heating settings because of the risk of dangerous creosote formation in the appliance and in the chimney.


Continue reading at FIRE CLEARANCES INDOORS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see HEATING COST SAVINGS METHODS - Heating Cost Savings Tips where we provide expert advice on how to significantly reduce your home heating


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WOOD-OIL COMBINATION HEATERS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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