Heater Repair – How Heaters Work – By PHP

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How Heaters Work

If you understand how this heater operates, you will be able to service and maintain it. The principle is simple. No oil pump is needed. The oil is supplied by a siphon system. Air pressure is used for the siphon., therefore it is important to maintain proper air pressure adjustment. Correct air pressure is the most important factor for proper heater operation.

Air enters into the rear of the heater through the air intake filter. It is then drawn through the compressor, mounted on the motor. The compressor increases the air pressure, which is regulated by the pressure relief valve. Carbon, which may be picked up from the carbon vanes of the compressor, is removed from the air by the air output filter. The filtered air at the correct pressure then enters the nozzle adapter through the air line. As the air passes through and exits the nozzle, it creates a partial vacuum in the center of the orifice which siphons the oil from the tank. The filtered oil is atomized with the air as it is injected into the combustion chamber.
The transformer supplies power to the spark plug, which provides a constant spark inside the combustion chamber. Oil sprayed from the nozzle directly hits the spark and is immediately ignited. A HSI heater works differently.

The fan draws air through the back of the heater in the space around the fan motor compressor assembly. This air performs three functions. It cools the motor, transformer and other heater components, insuring their long operating life. The air also conducts heat through convection from the combustion chamber providing heat to the area to be heated. The remainder of the air enters the burner at the rear of the combustion chamber through the vents in the burner in a swirling pattern. This air mixes in the combustion chamber with the aerated oil spray from the nozzle to give complete fuel combustion, burning with minimal odors or waste.
Power is supplied to the heater by a three-wire electrical cord. When plugged in, the heater will begin to function. No switches are needed for operation; to stop, merely pull the plug.
Your heater is equipped with a flame-out safety control. The purpose of this system is to stop all electrical power to the motor and ignition transformer in approximately 20-35 seconds if ignition should fail to occur on start up or lack of fuel. An HSI heater has a different safety control.

Air Pump

The heater’s air pump consists of a rotor (c) with four carbon blades (d) rotating inside a pump body(b). The rotor is driven directly by the motor (a) and is attached to the motor shaft by means of a plastic insert (f). As the motor rotates, the carbon blades travel outward rubbing against the inside surface of the steel pump body (d). The rotor’s position inside the pump body is such that it is off centered with the pump body and a .003/.004 of an inch gap is set at the uppermost quadrant (g). As the motor rotates, the air between the blades (h) is compressed and routed to the nozzle through the air line.
Air Filters The air filtering system consists of an air input filter and air output filter. The air input filter is located at the right rear of the motor and its purpose is to filter all incoming air prior to entering the air pump.
The filter design is such that it can be cleaned in a mild, soapy solution, thoroughly dried and used over again.
The air output filter is located under the plastic end cover. This filters purpose is to prevent any carbon dust (from rotor or blade wear) from entering the air passages in the nozzle. This filter is non-cleanable and should be replaced when considerable build-up of carbon dust is observed.

Nozzle

As mentioned previously, the purpose of the air pump is to compress air and deliver it to the nozzle. The compressed air, as it travels through the nozzle, creates a negative pressure and lifts the fuel from the fuel tank. The fuel from the fuel tank and the compressed air are mixed at the nozzle which results in a very fine mist of fuel being sprayed into the combustion chamber. The air pump/nozzle combination eliminates the need for a conventional type fuel pump. It is important for the service technician to understand the nozzle operation. In many cases concerning improper operation of a heater, the problem is the result of a seal leak or a restriction (dust/dirt) being present within the nozzle. It should also be pointed out that each model heater requires different nozzles due to different fuel flow rates.

Combustion System (Burner Head)

The burner head is mounted to the rear of the combustion chamber. The purpose of the burner head is to meter and mix the amount of air entering the chamber to obtain proper combustion characteristics. The nozzle is mounted in the chamber from the burner head and is surrounded by fins, so that the amount of air entering the chamber from the burner head is equal and balanced. The size of the slots in the burner head differ between models.

Motors

The motors used on the low pressure heaters are fractional horsepower motors ranging from 1/35 HP on the smallest heaters to 1/4 HP on the largest heaters. The motors used can be grouped into two categories. The first category is the shaded pole motor(FIG. 5.1). The shaded pole motor contains a single winding and does not require an external start relay for operation. These motors are used on the smaller heaters (30,000, 35,000 & 50,000 BTU) and are easily identified by observing that only two wires (usually red and white) are necessary for operation.
The second category is the split-phase motor(FIG. 5.2). This motor contains two separate windings. The first winding being the auxiliary or start winding and the second being the main or run winding. This motor requires an external means of switching the power off to the auxiliary winding after the motor reaches a predetermined speed. This power off switching is accomplished by a motor start relay or solid state relay. These motors are identified by observing that three wires (black, red and white) are routed from the motor.

Motor Relay

The motor start relay is used to de-energize the start winding on split-phase motors. As mentioned in the Motor Section, remember that split-phase motors have both an auxiliary (start) and a main (run) winding. For proper motor operation, both the auxiliary and main winding are energized during startup. After the motor starts and reaches approximately 80% of rated speed, the auxiliary winding must be de-energized to prevent overheating. This is accomplished by the use of a motor start relay. The operation of the relay functions because the main winding amperage decreases as the motor speed increases. The coil of the relay is wired in series with the main winding. The contacts of the relay are in series with the auxiliary winding. When voltage is applied to the motor, the high inrush current through the main winding creates a magnetic force in the relay coil and closes the contacts of the relay, which energized the auxiliary winding. As the motor speed increases, the current through the relay decreases and the contacts open, with gravity, de-energizing the auxiliary winding.

Solid State Relay

The solid state relay is used to de-energize the start winding on split-phase motors. As mentioned in the Motor Section, remember that split-phase motors have both an auxiliary (start) and a main (run) winding. For proper motor operation, both the auxiliary and main winding are energized during start up. After the motor starts and reaches approximately 80% of rated speed, the auxiliary winding must be de-energized to prevent overheating. This is accomplished by the use of a solid state relay. The operation of the relay functions as a positive temperature coefficient resistor. When voltage is applied to the motor, the main and auxiliary windings are energized as current flows through the solid state relay which is connected in series with the auxiliary winding only. The resistance is increased over time in the solid state relay where it eventually rises to the point theoretically no current flows, de-energizing the auxiliary winding.

Safety Control

The safety control circuit consists of a photocell (light sensitive cell) and a safety control. The photocell is used to sense the presence of light inside the combustion chamber. The photocell varies its electrical resistance in relation to the light rays which contact its surface. When the heater is operating properly, the photocell sees sufficient and the proper color light, which keeps its resistance low. When the photocell does not sense the proper color or amount of light, the resistance of the photocell increases and ceases current to flow to the solid state trigger circuit, which in turn sends current flows into the circuit breaker and heats a (bimetal) bar causing the bar to warp. The warpage releases the spring loaded reset button, which brakes all power to the heater.