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Smokey Fires and Creosote Prevention

Helpful Hints For Newmac Forced Draft Furnaces

The main reason for the formation of creosote and excessive wood smoke is due to low operating temperatures caused by any of the following: 


1. Wet Wood Supply

It is normal practice to air dry wood outside for at least a full summer season. Correct drying will take 6-12 months depending on local conditions. The wood should be cut to length, split, piled loosely or stacked and covered in a protected shelter or under plastic to keep the wood out of contact from the elements. Several weeks before Fall a supply of this seasoned wood is normally brought into a heated space to insure that the wood has optimum moisture content. Some individuals will install a dehumidifier in the same heated space to promote drying.


2. Fire Box Loading

The firebox can be loaded up to a maximum ¾ the height of the brick liner with 2” of protective ash left in the bottom of the firebox after cleaning. The initial wood load and fire should insure that a good bed of coals is established and the heating system is fired up to a temperature above 600 F . This will give the firebricks, metal heat exchanger and the venting system optimal radiant, convection and conduction heating characteristics. Normal steady state operating temperatures should fluctuate around 400 F depending on draft, combustion air supply and remaining wood fuel supply in firebox. 



1.  Stack thermometer probe should be inserted into breech at chimney base 

2.  Non approved fuel sources such as manufactured fire logs, gasoline, naptha, crankcase oil, tires, creosote or treated timber, saltwater debris, disposable diapers and plastics from household garbage should not be burned. These materials can have extreme burning temperatures as well as corrosive action on the metal heat exchanger. Poisonous or carcinogenic substances such as dioxins can be generated in flue gases from the burning of plastics.

3.  It is common practice to operate the Forced Draft Fan when reloading firebox.

4.  A heat loss calculation will accurately size required furnace output and model.

5.  Non grated models recommended when burning wood only.

The probe thermometer is the best design for accurately measuring flue gas temperature as it responds quickly to temperature fluctuations and shows the real temperature of the gases.

The thermometer should be placed at the last connection to the base of the chimney as follows:

 Thermometer Placement

Temperature Reading Result / Cause


>600 F Initial system warm–up


300-500 F  Normal operating range


<250 F Smoky fire and creosote formation or a cooling bed of coals at the end of fire


 Prevent Chimney Fires - Burn it Hot, Burn it Clean  (Burn a Little Extra Wood) 


OPERATING TIP - A properly fired solid fuel system will provide continuous heating over a 3-5 hour period. (Read sections 1-9). This can cause uncomfortable room temperatures during mild weather conditions in the Fall and Spring. For efficient short heating cycles during these times the back-up heating system should be used.


The Future Maintenance - Always use energy wisely for reduced operating costs and environmental impact. 


3. Combustion Air and the Forced Draft Fan

Both the Forced Draft Fan and its Slide Draft control regulate the amount of Combustion Air entering the firebox. Combustion Air flow is what sets the burn rate of the wood in the firebox. The final control setting should provide an optimum burning rate and should not be set below the Minimum position.


The Wood thermostat located in the main living space will automatically control the operation of the Forced Draft Fan above the fire door. By forcing extra Combustion Air into the firebox with the Forced Draft Fan the burn rate of the wood will be increased. The faster the burning rate the higher the stack temperatures of the flue gases which will minimize creosote formation.

 Forced Draft Fan is located above the Firedoor

The Forced Draft Fan (FDF) above the furnace firedoor is automatically operated by the Solid Fuel (Wood or Coal) Thermostat.


1.  Initial Set Up of Furnace Controls


a.  The Solid Fuel thermostat should be set at a normal room temperature of 21C (70F). With dual fuel operation set the back-up thermostat (oil or electric) to a lower setting of 15C (60F).


b.  The slide damper beside the FDF should be at Maximum setting. Over the heating season this can be moved towards the Minimum position to extend burn time of each load of firewood but carefully monitor for smoky fires or creosote build up. The final setting should provide satisfactory burn time and space heating without smoky fires or creosote build-up (See section 2).


 Note: The firedoor slide damper is normally closed. In GRATED models the door slide damper is located in ash door. Remember that the ash pan needs to be cleaned DAILY to provide unrestricted Combustion Airflow into firebox and prevent grates from warping.


2.  Heating Cycle and FDF Operation


a.  If room temperature is below the Solid Fuel Thermostat setting of 21C(70F) the FDF will turn ON.


b.  The FDF will push extra Combustion Air into the firebox and accelerate the burn rate. This will heat the plenum on top of the furnace. The Fan/Limit control will then engage to automatically turn on the circulating blower inside the furnace and move heated air through house duct system (See Sections 6&7). Note: Blower will run for varying lengths of time depending on heat output of fire.


c.  When room temperature reaches 21C(70F) the Solid Fuel Thermostat is satisfied and the FDF will turn OFF. The fire will still burn due to the natural draft of the hot chimney pulling Combustion Air into the firebox through the opening in the FDF (See sections 4&5).


d.  When room temperature drops below the 21C (70F) the Solid Fuel Thermostat will again turn ON the FDF.


The FDF will keep running until one of the following occurs:

1.  Existing wood supply in the firebox is sufficient to reheat living space.

2.  Firebox is reloaded and a new heating cycle initiated to reheat living space.

3.  Room temperature drops until back up heating system starts at 15C(60F).

4.  Manual Toggle Switch on FDF is turned OFF.

5.  Solid Fuel Thermostat is set below existing room temperature.


Note: The FDF can run for extended periods of time and motor will be hot to touch. Whenever opening the firedoor the FDF should be running to prevent smoke from entering furnace room.


4. Chimney Draft

Poor draft will cause a fire to smolder. The recommended draft setting for Newmac wood burning furnaces is between –0.03 to – 0.05” W.C. The recommended flue sizing for Newmac solid fuel furnaces is normally 8 “ or 7 ” diameter. The amount of dilution air entering the Barometric Draft Regulator should be minimized to prevent cooling of flue gases



a.  Seven things to increase draft are colder outside air temperatures, higher chimney, greater wind velocity, higher atmospheric pressure, higher flue gas temperatures, closed draft regulator and special chimney caps.

b.  Use approved solid fuel rated high temperature chimney only.

c.  Single wall flue pipe should not exceed 10 feet and have not more than two (2) 90 degree elbows. Combination furnaces can mount the blower section on either the left or right hand side to minimize flue pipe length.

d.  The horizontal run should slope at least ¼ rise per 1 foot run up towards the breech at the base of the chimney.

e.  Flue pipe connections must be secured with at least 3 metal screws.

f.  Code requires the flue pipe to be crimped end down toward furnace with flue pipe INSIDE smoke pipe of wood furnace.

g.  The chimney must be absolutely smoke tight throughout its entire length and must extend three feet (3) above a flat roof or two feet (2) above the ridges of peaked roofs.

h.  Flue Pipe The conduit ( piping) connecting the smoke pipe (or flue collar) of the furnace to the chimney breech must be constructed of steel with a melting point no less than 1100 C. Galvanized flue pipe can not be used.

i.  Galvanized Draft Regulators are approved for Newmac solid fuel furnaces.

j.  New flue pipe may give off odours or smoke for the first few fires.

k.  Use approved sealing methods around the breech to chimney and install fire stops between floors for inside chimneys. Liners should be properly sealed at both top and bottom. Check both chimney installation codes and manufacturer requirements for details. 


5. Combustion Air

For new home construction it is necessary by installation codes to design Combustion Air for any fossil fuel burning appliances such as oil, wood, propane, or natural gas. Any age home may need to install Combustion Air. Combustion Air is normally supplied by an insulated pipe of either 5 or 6 connected to an outside permanently open air inlet hood.  

a.  Symptoms of tight home construction can be a smoldering fire, odours and smoke back drafting into basement.

b.  Three (3) types of air systems to be designed for modern home construction are : Combustion Air , Ventilation Air for Human Health and Make-up Air for any exhaust fans connected to the outdoors.

c.  For enclosed furnace rooms there needs to be ventilation air for heating equipment cooling.

d.  Size of furnace room or open basement has no connection with providing outside Combustion Air in tight home construction.


6. Ductwork

Metal plenums and duct system are to be installed for safety and correct operating set up. Airflow is important across any heat exchanger to provide safe operating temperatures and correct operation of the Forced Draft Fan. The size of the ductwork and the return air grills should be installed to provide correct airflow to the furnaces for a CSA certified 85 F temperature rise across the heat exchanger. Ductwork should work to the following recommended sizes:


Furnace Model  Supply Return Nominal Grill Size 

WB/WG100 180 Sq.In. 250 Sq.In. 12x30 (steel truss) 

WAO/GAO 180 Sq.In. 250 Sq.In. 12”x30”

CL86/96  C/G 180 Sq. In.   250Sq. In 12”x 30”

CL115-170 C/G 220 Sq. In.   250Sq. In 12”x 30”

CL155 C/G 240 Sq. In.   280 Sq. In 12”x 30”

CL 170 C/G 260 Sq. In.   300 Sq. In 12”x30” + 6”x10”

 (See App C.)

1.  For solid fuel furnaces there are specified safety requirements for ductwork that require 6” clearance for 6 ft. from the supply bonnet. Labels on furnace casing clearly indicate MINIMUM installation requirements. Also 6 ft and 6” clearance required for Return ductwork on WG/WB.

2.  The installation of ductwork for any wood burning furnace should be properly designed to provide gravity feed during power failure. It should be correctly sized with proper transitions and sloped to assist ‘gravity’ operation during power failure. See operation manual for correct safety procedures.

3.  Recommended minimum plenum height is 24”.

4.  Both Supply and Return air ductwork has to be fully enclosed . ie NO JOIST PANNING

5.  Blockage in ductwork or over return air grills can cause unsafe operating conditions.

6.  Insure installation of Return Air grill(s) maintains free area requirements.

7.  It is normal practice to locate Return Air Grills on main floors of living space and not in basement area where furnace is located.

8.  Do not install Return Air Grills directly on plenum.

9.  Do not install Return Air Grills in an enclosed furnace room.

10.  Size insulated air conditioning ductwork to maintain free area in table above.



a.  Excessive Heat Loss on parts or all of duct system. According to accepted design practice any ductwork located in unheated areas such as a garage, basement or crawl space should have sealed joints and insulated.

b.  Open Ended Ductwork where by the return air is taken from a different area than the supply air to living space. Both supply and return need to be installed to the SAME heating space to give a Close Looped System.


For example: An incorrect installation may get its return air completely from an unheated garage space with no supply leads into this space. This would cause the operator to fire the heat exchanger above an 85 degree temperature rise. For a properly designed forced air system the return air temperature should be no lower than 55-60 F to maintain certified temperature rise of 85F across the heat exchanger. 

Proper sized and installed Return Air grills are important to the safe operation of any forced air system but especially with solid fuel furnaces.


When installing Return Air grills they are normally situated centrally on each floor of the main living space. When branches are installed the SUM of each of the return air branches should equal the MINIMUM recommended Return Air requirements as stated on the certification labels. It is better to oversize Return Air ductwork and grills.


The Free Area of each grill should be matched to the cross section area of the ductwork connecting to it. Typical trussed steel Grill Sizes are:


Nominal Grill Size Free Area Sq. In. Connecting Ductwork
6 x 10 45 * 8” x 10”
6 x 30 135 8” x 16”
8 x 30 180 8” x 22”
10 x 14 105 8”x 16”
10 x 24 180 8” x 22”
12 x 30 270 8” x 34”
(* 8x16 usual minimum)

All Return Air ductwork should be fully enclosed and of metal construction. Any ductwork connecting to Return Air grill between joist space should be designed to maintain minimum requirements.


 Proper Joist Space Installation Maintaining Free Area Requirements


·  Blockage in ductwork or over return air grills can cause unsafe operating conditions

·  Do not install Return Air Grills directly on plenum.

·  Do not install Return Air Grills in an enclosed furnace room.

·  Slope both supply and return air ductwork to promote gravity feed during power failures.

·  Refer to installation manual for complete installation recommendations

7. Blower Setting

The blower speed must be set to give certified 85F temperature rise across the heat exchanger. Inadequate airflow from a slow blower speed could cause limiting problems and a shutdown of the Forced Draft Fan.


1.  Correct blower speed will also optimize flue gas temperatures.

2.  Two speed blower operation not recommended in solid fuel system.

3.  For belt drive blowers insure that the belt tension allows 1-1/2” to 2” of free play.

4.  The circulation blower located inside furnace casing will turn ON and OFF automatically during heating cycle due to Fan Limit Control (See section 9).

  Blower will run for varying lengths of time depending on heat output of fire.


8. Filters

Plugged air filters could restrict airflow and cause furnaces to operate above CSA approved 85F temperature rise across heat exchanger. This could cause limiting problems and a shutdown of the Forced Draft Fan.

·  Electronic Air cleaners not recommended with solid fuel systems.


9. Fan/Limit

The eight inch (8”) Fan/Limit provides two (2) functions:


·  Fan - Correct On /Off operation of the circulation blower in the furnace.

·  Limit - Safety device to shutdown the Forced Draft Fan (or combination burner) if the plenum temperatures become too high.


If the Fan/Limit is defective or is not sensing in the proper plenum location either of its functions can be affected. Approved Fan/Limit settings are:


CL 86/96 C/G Fan Off = 95F Fan On = 130F High Limit = 180F

CL 115-170 C/G Fan Off = 95F Fan On = 130F High Limit = 210F

WAO/GAO Fan Off = 95F Fan On = 130F  High Limit = 200F

WB/WG 100 Fan Off = 100F Fan On = 140F High Limit = 180F


·  Button should be pulled out to Automatic setting

·  WG100 and WB100 have Fan/Limit already installed directly on furnace.


Combination Furnaces

Wood Add Ons


The WAO/GAO products can only be added to an OIL furnace with a firing range of 0.75-1.10 usgph

9. Maintenance

The build up of creosote and soot on the inside of the heat exchanger will promote further deposits. More importantly these layers will lower the heat transfer to the supply plenum or bonnet. A daily inspection of the inside of the heat exchanger should be carried out to insure that any material build-up is removed from interior surfaces.



·  There should be a major cleaning of the heat exchanger in the Spring with the bricks removed to clean out ash deposits in this section. All inside surfaces should be cleaned prior to summer shut down.

9. Maintenance (cont.)


·  The flue pipe should be regularly removed and back section of heat exchanger cleaned.

·  In WB/WG100 and WAO/GAO the Scandinavian Baffle inside firebox will have to be regularly flipped to maintain a flat profile 


10 . Mild Weather Operation.


During moderate temperature fluctuations normally experienced in the Fall and Spring the back up heating system should be used. The use of small fires for low heat output will cause a smoldering fire and promote creosote build up. The wood furnace should be used when prolonged cold outdoor temperatures are established.


11. Thermostats


There are a wide variety of thermostats available with varying functions. A low cost or standard mercury bulb thermostat will provide satisfactory control of home temperature in a solid fuel system.


In a single zone forced air system the thermostat(s) should be installed on the main floor in a central location following manufacturer’s instructions.


 Do not place thermostat in areas which may affect its operation such as:

·  Near cold air return grills

·  Above lamps, tube radios or TVs

·  Near a radiator or warm air register

·  Behind closed door or other obstructions affecting air across the thermostat

·  Near a window or door frequently opened to the outside

·  Excessive vibrations


Avoid sources causing abnormal sources of radiant, convection or conduction heat.


Air temperatures can vary within a home by 5 to 10 F from room to room or from floor to ceiling. The thermostat will provide a relative temperature reading that will provide a comfortable range of temperatures throughout the home. A heat loss is important to provide the optimum system design for a comfortable home environment.


It is normal to experience warmer home temperatures when heating with wood. The furnace blower will operate automatically to cool the heat exchanger. The air temperature will usually be 5 to 10F higher than when heating with oil. If lower home temperatures are desired when burning wood:


·  Smaller fires can be built (See Section 2)

·  Slide draft settings adjusted to minimize combustion air entering the firebox

(See section 3)

·  Lower draft with Barometric Draft Regulator( See section 4)


Recommended thermostat anticipator settings:

 Combination Add On

Wood Thermostat 0.4 amps  0.4 amps

Oil Thermostat 0.4 amps  0.2 amps

Note: An amperage of 25 volts or greater will short out anticipator destroying thermostat and warranty. Disconnect thermostat wire until ready for operation.



Duties and Responsibilities


Any solid fuel installation is guided by CSA B365 Installation Code for Solid Fuel Appliances. Combination furnaces also follow the requirements of CSA B-139 Installation Code for Oil Fired Equipment. However local authorities will have the final say on any regional code requirements.


1.  It is the installers responsibility to make certain that the equipment operates according to the manufacturers instructions and that all safety devices function properly. In Canada the W.E.T.T. course is recommended for all installers. Test equipment for wood burning furnaces should include draft meter, stack thermometer and duct thermometer.


2.  All operation procedures and warranty instructions should be reviewed with the homeowner emphasizing regular maintenance and correct operating procedures.


3.  More details on wood burning tips and facts are located in the Newmac Installation, Operating and Service Manual.


4.  Newmac WAO or GAO Add On units can only be used with an oil fired forced air furnace rated between 0.75-1.1 gph


5.  Newmac assumes no liability for either the use or misuse of recommendations in this general information guide.


The Future Maintenance


To reduce environmental impact burn wood at optimum temperatures, clean furnace regularly, and be vigilant in energy conservation practices in the home.


To reduce environmental impact dispose of used mercury thermostats at a certified environmental depot.

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DMC Firewall is developed by Dean Marshall Consultancy Ltd