INTRODUCTION TO OIL FIRED FORCED AIR FURNACES# 1
Oil furnaces have two main systems, heat producing and heat distributing systems
The heat producing system consists of the oil burner, fuel supply, heat exchanger and the
combustion chamber.
The heat distribution system consists of the furnace blower assembly, duct work and
vents.
The sequence of operation for the furnace is; The thermostat calls for heat, the oil burner
motor, fuel oil pump, combustion blower wheel and ignition are all powered at once. The
fuel/air mixture is forced under pressure through the nozzle orifice, into the combustion
chamber where it is ignited by the electrodes. When the fire has heated the heat
exchanger to a set temperature, the indoor blower will come on, blowing the heated air,
through the duct work and into the living space. When this space is heated to a
predetermined ( thermostat setting) temperature. The burner motor will shut down, but
the indoor fan will continue to run to cool the heat exchanger to a set temperature.
The furnace models are the Lowboy, Up-flow, Down- flow and Horizontal.
Lowboy- used in spaces with little head room
Up-flow- Vertical, the air is pulled in at the bottom, forced across the heat exchanger, and
out the top
Down-flow-Vertical, the air is pulled in at the top , forced across the heat exchanger, and
out the bottom into the duct system.
Horizontal-used in an attic and/or a crawl space
Fuel oil is a fossil fuel and is delivered in liquid form and stored in a tank. Number 2 fuel
oil is used in residential and commercial. Number 2 fuel oil is dyed RED for tax
purposes. Tank types are Above Ground, Below Ground, or Indoors.
Above ground- Made of steel 12 ga., fiberglass or a combination of both. 230 to 300
gallons. Maintenance; paint tank every 3 years
Underground- steel (STI-P3) and fiberglass.
STI-P3- (Steel Tank Institute-Protected 3 ways) 1. External corrosion resistant. 2.
Sacrificial Anodes. 3. Dielectric bushings Maintenance; Check anode yearly, replace if
needed.
Indoor oil storage-Warmer oil aids in oil flow ,combustion and efficiency.
Fuel oil supply systems, there are two systems, one pipe and two pipe,
In the one pipe system only one pipe runs between the tank and burner where it is
atomized, mixed with air and ignited. Air must be removed from this system manually. If
the tank is above the burner, a single pipe system can be used.
In the two pipe system, there are two pipes, one pipe acts as the supply and the other as
the oil return to the tank. If the tank is below the oil burner, a two pipe system is
recommended. Oil lines should be 3/8 OD copper, never use cast iron or PVC pipe for oil
lines. Never use compression fitting in an oil system. If needed ,use a non- hardening, gas
resistant pipe joint compound. Make all runs as short as possible and use as few fittings
as possible. Outside oil lines should always be insulated, all underground lines should be
run in some type of containment (PVC). A shutoff valve should be installed just outside
the furnace in the line from the tank. A vent alarm should be installed on the fill vent to
prevent over filling. The oil filter should be installed between the tank and the oil pump
to prevent solid objects from damaging the pump or plugging the nozzle.
Oil safety valve (OSV)
The oil safety valve acts to stop oil flow in the event of a leak in the suction line.
Combustion is the process of burning fuel to generate heat.
In order for combustion to take place, there must be ample amounts of fuel, oxygen, and
heat. During the combustion process, theoxygen reacts with the fuel oil to produce heat,
carbon dioxide and water vapor.
Preparing oil for combustion.
Fuel oil will not burn in its liquid form and must be changed to a gaseous state for
combustion to take place. Fuel oil is forced under pressure through the nozzle, which
reduces the liquid to small droplets. This process is called atomization. The droplets are
then mixed with air(which contains oxygen). Heat is introduced as a spark from the
electrodes, ignites the droplets and as the temperature rises the droplets become vapor
and burn. Oil is fed under pressure to a nozzle swirling the oil in one direction while Air
is forced through the tube that surrounds the nozzle is being swirled in the opposite
direction.
Gun type oil burner.
Modern oil burners are referred to as high pressure burners, that operate at 100 PSI.
Many newer furnaces use a flame retention burner, these burners have become extremely
popular because they operate at higher speeds, have higher static pressure, have an
improved air-oil mixing, lower emissions, higher efficiency, cleaner combustion, less
maintenance costs, a hotter flame, and a tightly formed flame, which has a more uniform
flame that leaves less unburned fuel behind.
Burner motor
The oil burner motor provides power to the combustion fan and the oil fuel pump, a
flexible coupling connects the shaft on the burner motor to the shaft on the fuel oil pump.
Motor speeds can be 3450 or1725 rpms, so the speed of the pump must match the motor
speed. These motors are usually permanent split capacitor (PSC) motors, they have
excellent running efficiency, and no external parts to wear out.
Burner combustion fan is the squirrel cage type with adjustable air inlet vent built into the
housing. The fan forces air through the air tube to the combustion chamber to aid in the
combustion process.
Fuel oil pumps There are two types , single stage and two stage. Pumps perform three
major tasks, The pump moves the oil from the tank to the burner for ignition. The pump
acts as a filter for particulate matter that has found it’s way past the oil filter. The pump
also has the ability to be adjusted to the pressure desired. Pumps are available in both
single stage and two stage pumps.
Single stage can be used with both one pipe or two pipe systems as long as vacuum does
not exceed 12 inches. Both pumps have bypass plug which MUST be removed on the
single line system so oil can re-circulated back to the pump inlet.
Two pipe systems have two sets of gears, one to pull oil from the tank and one set to push
the oil to the pump. two pipes systems can operate at a higher vacuum level. Two stage
pumps are recommended for high lift applications. The bypass plug is NOT removed.
Both pumps have an internal screen at the inlet. Pump pressures are measured in “water
column inches”