Question? Just ask us!
Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair
InspectAPedia ® Home
BACKDRAFTING HEATING EQUIPMENT
BACKUP HEAT for HEAT PUMPS
BANGING HEATING PIPES RADIATORS
BLOWER FAN OPERATION & TESTING
BOOKSTORE - InspectAPedia
CARBON MONOXIDE - CO
CIRCULATOR PUMPS & RELAYS
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DIAGNOSE & FIX HEATING PROBLEMS-FURNACE
DRAFT REGULATORS, DAMPERS, BOOSTERS
DUCT SYSTEM & DUCT DEFECTS
FAN, AIR HANDLER BLOWER UNIT
FLOODED HEATING EQUIPMENT REPAIR
FLUE SIZE SPECIFICATIONS
GAS BURNER Flame & Noise Defects
HEATING SYSTEM TYPES
MIXING / ANTI-SCALD VALVES
ODORS FROM HEATING SYSTEMS
RELIEF VALVE LEAKS
RESET SWITCH - ELECTRIC MOTOR
SAFETY, HEATING INSPECTION
SAFETY RECALLS CHIMNEYS VENTS HEATERS
THERMOSTATS, HEATING / COOLING
VIDEO GUIDES: Heating System Videos
WINTERIZE A BUILDING
WOOD, COAL STOVES & FIREPLACES
WOOD STOVE SAFETY
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 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.
Green links show where you are. © Copyright 2014 InspectApedia.com, All Rights Reserved.
Especially as oil prices have increased dramatically in the past few years and given the 2008 Bush Presidency economic crisis, many homeowners have a renewed interest in alternative heating energy sources.
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 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 Table. Also see How to Reduce Home Heating Costs - Heating Cost Savings Tips where we provide expert advice on how to significantly reduce your home heating[Text in process] [Illus CD 1011-1021] [DF Photos - combination unit, woodstoves, coal stoves]
- single combustion chamber wood furnace - components, characteristics, where to locate, combustion air concerns.
- how wood or combination units regulate draft
- forced draft combustion system
- dual combustion chambers, different service requirements, different operating requirements
- draft control
- barometric damper inspection
- heat exchanger design
- heat content comparison: wood, gas, oil
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:
Provide Combustion Air, Ventilation, Cooling Air for the Wood Furnace
Before looking in greater detail at wood fired furnaces and combination wood-oil furnaces, we and Carson Dunlop emphasize this safety note.
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 the Wood Furnace Combustion Air Damper Works.Sketches courtesy of 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.
Wood Furnace combustion chamber details
Wood Furnace combustion chamber cleaning details are shown in the sketches at left and below, courtesy of Carson Dunlop Associates.
Wood Furnace barometric damper inspection
DRAFT REGULATORS, DAMPERS, BOOSTERS 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 Furnace Draft Hood 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.
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.
Let me know if questions remain.
Green link shows where you are in this article series.
Frequently Asked Questions (FAQs)
No FAQs have been posted for this page. Try the search box below or CONTACT US by email if you cannot find the answer you need at InspectApedia.
Questions & answers or comments about combination fuel heating systems: wood + oil burning furnaces, boilers, other heating appliances: inspection, diagnosis, operation, maintenance, repair, safety
Check the FAQs just above, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
Search the InspectApedia website
HTML Comment Box is loading comments...
Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.