Category: Thermostats and Humidistats

Troubleshooting the Oil Burner Primary Control


Table 5-4 lists possible remedies to a number of different operating problems associated with oil burner primary controls. Before checking the primary control, examine the following parts of the oil burner and ignition systems:

• Main power supply and burner motor fuse
• Ignition transformer
• Electrode gap and position
• Contacts between ignition transformer and electrode

Other system components that should be checked before examining the primary control are the oil piping to the tank, the oil filter, oil pump pressure, oil nozzle, and oil supply.

Table 5-4 Troubleshooting Oil Burner Primary Control

Symptom and Possible Cause Possible Remedy
Repeated safety shutdown.
(a) Slow combustion thermostat response. (a) Move combustion thermostat to better location. Adjust for moreefficient burner flame. Clean surface of cad cell.
(b) Low line voltage. (b) Check wiring and rewire if necessary. Contact local power company.
(c) High resistance in combustion thermostat circuit. (c) Replace combustion thermostat.
(d) High resistance in thermostat or operating control circuit. (d) Check circuit and correct cause.
(e) Short cycling of burner. (e) Clean filters. Reset or replace differential of auxiliary controls. Repair or replace faulty auxiliary control. Set thermostat heat anticipation at higher amp value. Clean holding circuit contacts.
(f) Short circuit in combustion thermostat cable. (f) Repair cable or replace combustion thermostat.
Relay will not pull in.
(a) No power; open power circuit. (a) Repair, replace, or reset fuses, line switch, limit control, auxiliary controls.
(b) Open thermostat circuit. (b)With power to relay, momentarily short thermostat terminals on relay. If burner starts, check wiring.
(c) Combustion thermostat open. (c) Repair or replace combustion thermostat.
(d) Ignition timer contacts open. (d) Clean magnet.
(e) Open circuit in relay coil. (e) Replace relay.

Combination Primary Control and Aquastat


The combination primary control and aquastat is designed for use with a constant-ignition oil burner in a hydronic heating system. The purpose of this unit is to supervise the operation of the oil burner and provide both water temperature and circulator control. A remote sensor (cadmium detection cell) is used to detect any irregularities in the oil burner flame.

Figures 5-91 and 5-92 illustrate a number of different combination primary control and aquastat units used in hydronic heating systems. In operation, the high-limit switch (SPST) will automatically turn off the burner if the boiler overheats. The low-limit circulator switch (SPDT) is used to maintain water temperature for the domestic hot-water supply. It will also prevent the circulator from operating if the water temperature is too low (i.e., below the setpoint).

On the units shown in Figures 5-91 and 5-92, a call for heat from the room thermostat pulls in relays 1K and 2K to turn on the oil burner and start heating the safety switch. Under normal operating conditions, the burner should ignite within safety-switch timing. If such is the case, the cadmium cell detects the flame, and relay 3K pulls in to deenergize the safety-switch heater. The oil burner then continues to operate until the call for heat is satisfied.

R8182H protectorelay Combination Primary Control and Aquastat

The circulator (pump) in the heating system operates when relay 1K pulls in only if the R to W contact on the aquastat control is made (see Figure 5-92). A drop in water temperature will cause the R to B (low limit) contact to be made. This acts as a call for heat, pulling in relay 2K to turn on the oil burner.

R8182D protectorelay Combination Primary Control and Aquastat

Stack Detector Primary Control


Stack-mounted oil burner primary controls employ thermal sensors to detect ignition or flame failure. A typical stack detectorthermal sensor (see Figure 5-87) consists of a bimetal element inserted into the stack (see Figure 5-88). The thermal sensor (combustionthermostat) is usually located on the stack where the elementwill be exposed to the most rapid temperature changes. Thethermal sensor should always be mounted ahead of any draft regulator.If installed on an elbow, it should be mounted on the outsidecurve of the elbow.cycling control and a thermal detector for sensing temperature changes of the flue gases (as high as 1000°F maximum temperature). The safety switch shown on the center-left of the unit is designed to lock out if the flame is not properly established. If the flame goes out during the burner on cycle, the primary control will make one attempt to restart. If the attempt is unsuccessful, the safety switch will lock out. A manual reset is then required in order to restart the burner. The primary control shown in Figure 5-89 is used with a two-wire or three-wire primary controller.

stack detector thermal sensor Stack Detector Primary Control

bimetal element Stack Detector Primary Control

A stack-mounted combination line voltage primary control and flame detector is shown in Figure 5-90. This type of primary control is used with constant-ignition oil burners and is designed for flange-mounting on a stack, flue pipe, or combustion chamber door of a furnace or boiler. It must be used with a line voltage thermostat or controller.

honeywell RA117A Stack Detector Primary Control

stack primary control Stack Detector Primary Control

Cadmium Cell Primary Controls


A cadmium cell primary control consists of a primary control assembly operating in conjunction with a cadmium detection cell.

The cadmium detection cell is considered the most effective sensor used to monitor the burner flame. It consists of a light-sensitive photocell, a holder, and a cord assembly (see Figure 5-80). The surface of the detection cell is coated with cadmium sulfide and overlaid with a conductive grid. Electrodes attached to the detection cell are used to transmit an electrical signal to the primary control. The variable resistance of this surface to the presence of light (i.e., the burner flame) is used to actuate the flame detection circuit. When light is present (in the form of the burner flame), the resistance of the cadmium sulfide surface to the passage of electrical current is very low. Consequently, as long as the burner flame lasts, an electrical current will pass between the cadmium detection cell and the primary control unit, and the burner motor on cycle will continue operating. If the burner flame should fail or if ignition should fail to occur, the resistance of the cadmium sulfide surface to the passage of electrical current will be very high. This will interrupt the passage of the electrical current to the primary control and will cause the latter to shut off the burner motor.

cadmium detector Cadmium Cell Primary Controls

The detection cell is mounted inside the burner air tube so that it faces the burner flame (see Figure 5-81). The exact location of the detection cell is determined by the oil burner manufacturer and dictated by the design of the oil burner. In any event, the detection cell must be placed so that it views the burner flame directly. The fact that the detection cell responds to any light source means that it must be located where its surface will be shielded from any form of direct or reflected external light. Moreover, the ambient temperature should be kept below 140°F because excessive temperatures can also cause the detection cell to malfunction.

cadmium detection cell Cadmium Cell Primary Controls

Sometimes a malfunctioning oil burner will cause a heavy layer of soot to accumulate on the cell surface. The cell surface should be carefully wiped to remove the soot and restore full view of the oil flame. A damaged detection cell should be replaced.

The type of primary control used with a cadmium detection cell will depend on the type of controller voltage, the type of ignition system, and the length of safety switch timing required by the installation.

The Honeywell R8184G Protectorelay primary control shown in Figure 5-82 has a transformer included in the unit to supply 24-volt power to the control circuit. This is a low-voltage primary, and it requires a 24-volt thermostat. Other models are available that require a line voltage controller (see Figure 5-83).

honeywell R8184G Cadmium Cell Primary Controls

primary control Cadmium Cell Primary Controls

The primaries illustrated in Figures 5-82 and 5-83 are designed for use with nonrecycling constant-ignition oil burners. Automatic recycling control of an intermittent-ignition oil burner can be obtained by using the primary shown in Figure 5-84. The basic differences between the constant-ignition and intermittent-ignition systems can best be illustrated by the wiring diagrams shown in Figures 5-85 and 5-86. An intermittent-ignition system contains the same components as a constant-ignition system plus the following:

honeywell R8185E Cadmium Cell Primary Controls

1. A interlock contact in the ignition circuit (T1).
2. An ignition timer heater (T).
3. An interlock contact (T2) in the circuit between the safety
switch heater (SS) and the ignition timer heater (T).

Safety switch timing can be 15, 30, 45, or 80 seconds depending on the manufacturer and model.

honeywell R8184G schematic Cadmium Cell Primary Controls

honeywell R8185E schematic Cadmium Cell Primary Controls

Oil Valves


Oil valves are used to provide on-off control of the flow of oil to the oil burner. These are normally closed solenoid valves that open when energized and close immediately when deenergized. They are variously referred to as solenoid oil valves, magnetic oil valves, or oil burner valves and are available in either immediate-discharge or delayed-discharge models.

An immediate-discharge oil valve discharges oil as soon as it is energized. A delayed-discharge valve is equipped with an integral thermistor to delay the valve opening for about 3 to 15 seconds (the length of time will vary depending on the manufacturer). This delay allows the burner fan to reach operating speed and establish sufficient draft before the oil is discharged.

A solenoid oil valve will make an audible click when it is opening and closing properly. If the valve fails to open after the room thermostat calls for heat, the following conditions may be responsible:

• Inadequate fuel pressure available at the valve
• An obstructed bleed line
• No voltage indicated at valve

Check the voltage at the coil lead terminals against the voltage shown in the nameplate. Also check the inlet pressure against the rating on the nameplate. If none of these conditions is causing the problem, the failure of the valve to open is probably due to a malfunctioning solenoid coil. The position of the coil is shown in the exploded view of the valve in Figure 5-76. The steps for replacing the solenoid are as follows:

1. Remove the nut on top of the valve by turning it counterclockwise.

coil solenoid oil valve Oil Valves

2. Remove the powerhead assembly from the spindle.
3. Disconnect and remove the solenoid coil.
4. Connect the replacement coil and reassemble.

Examples of delayed-discharge valves are shown in Figures 5-77 and 5-78. In both valves, the timing delay is governed by a thermistor attached to the solenoid coil. In these valves, the timing delay will vary with ambient temperature, voltage level, and other factors during normal operation. If the timing is significantly off, it may be necessary to replace the thermistor. Because the thermistor is attached to the solenoid coil, the coil must also be replaced in order to replace the thermistor.

magnetic oil valve Oil Valves

Delayed valve opening can also be obtained by using an electronic time delay wired in series with the oil valve (see Figure 5-79). Unlike the thermistor, the timing of this device is not affected by ambient temperature. On a call for heat, the valve opening is delayed for approximately 5 seconds.

time delay Oil Valves

Oil Controls


The principal functions of the oil controls are (1) to turn the oil burner on and off in response to temperature changes in the space or spaces being heated and (2) to stop the system if an unsafe condition develops. The following controls are necessary to perform these functions:

• Thermostat
• Limit controls
• Primary control
• Oil valves
• Time-delay controls
• Circulator or fan control
• Other auxiliary controls

Mercury Flame Sensors

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Many people frequently confuse mercury flame sensors (MFS) with thermocouples because each has a similar sensor that extends into the pilot flame and a tube connecting the device to the gas valve. Moreover, they have the same function in the control system of a gas-fired appliance, but there are significant differences.

The principal components of a mercury flame sensor (MFS) device are (1) a pilot flame sensor (the portion of the MFS device extending into the pilot flame), (2) a diaphragm/SPDT switch assembly located at the main gas valve, and (3) a hollow capillary tube connecting the flame sensor to the diaphragm/switch assembly.

The operation of a mercury flame sensor device depends on the evaporation of mercury. The sensor end, capillary tube, and the SPDT switch are filled with mercury. When there is enough heat produced by the pilot flame at the sensor end of the capillary tube, it vaporizes and pushes the remaining nonvaporized (liquid) mercury down the capillary tube to the bellows-type diaphragm/switch assembly located at the main gas valve. Movement of the bellows diaphragm presses against a nonadjustable, calculated spring tension with enough force to snap the SPDT switch from one set of contacts to another. This action causes the switch contacts to move from one position to another. In an MFS device switch assembly, the normally closed contact opens and the normally open contact closes. This action deactivates the igniter (after the pilot flame is proven) and opens the main gas valve to allow raw gas to flow to the burners.

Mercury flame sensors are no longer used in gas-fired furnaces and boilers, especially those equipped with solid-state control modules. However, manufacturers still produce replacement MFS units along with their compatible main gas valves.



An igniter produces the spark for direct ignition of the main gas burner in various heating applications (gas furnaces, gas boilers,gas water heaters, etc.). The Honeywell Q347 igniter shown in Figure 5-71 consists of an internal electrode with a ceramic insulator, bracket, and ground strap.

honeywell Q571 igniter Igniters

The flame-sensing rod is separate from the hot-surface igniter in most hot-surface ignition systems.

An igniter is used to provide the spark to ignite the main burner flame. Some igniters have an integral flame sensor. When this is the case, the igniter both ignites and senses (proves) the main gas burner flame.

The operating sequence of a system in which an igniter is used may be summarized as follows:

1. Room thermostat calls for heat.
2. Gas valve opens and gas flows to the burner(s).
3. Burner ignites when the gas reaches the main burner.
4. Spark igniter shuts off.

The duration of the spark operation must be within the igniter manufacturer’ s specified lockout timing period. The igniter manufacturer will provide a chart of the ignition control lockout times in the service literature for the igniter. The example shown in Table 5-3 is for Honeywell’ s Q347 igniter.

table 5 3 Igniters

The electrode spark gap in the igniter must be within the specified maximum (see Figure 5-72). If the gap is not within specifications, it will have to be adjusted for optimum performance.

igniter gap adjustment Igniters

The flame rod of a combined igniter–flame sensor unit must be immersed 1 inch in the burner flame to produce the best flame signal (see Figure 5-73). Examples of poor flame conditions and their probable causes are illustrated in Figure 5-74.

flame rod immersed Igniters

The flame signal can also be adversely affected by a bent bracket, bent rod, or cracked ceramic insulator. Sometimes the bracket can be bent back into shape. If the rod is bent or the ceramic insulator is cracked, the igniter should be replaced.

poor flame condition Igniters

Always check the specifications of the replacement hot-surface igniter before installing it. Not all igniters have the same voltage or warm-up time as the original design.

The igniter used in a hot-surface ignition operating system differs in design from the type used in intermittent pilot or direct-spark ignition systems described in the preceding paragraphs (see Figure 5-75).

hot surface ingniter Igniters

Be careful when replacing a hot-surface igniter because they are fragile and easily damaged. Sometimes a crack in the igniter surface is so small that it is not visible. A cracked hot-surface igniter may still work, but it will have a much shorter service life. After it is installed, check the hot-surface igniter for any inconsistencies in its glow pattern.

Hot-Surface Ignition Module


The principal components of a hot-surface ignition system are the hot-surface ignition module, a line voltage silicon carbide igniter (also sometimes called a glow stick or glow plug), a remote flame sensor, a 24-volt AC ignition-detection control, and a 24-volt (AC) redundant gas valve (see Figure 5-69). The flame sensor is designed to detect the presence of a flame. It can be mounted remotely on multiple burners or next to the gas burner.

hot surface igniton Hot Surface Ignition Module

The hot-surface ignition module, similar to the one shown in Figure 5-70, is a microprocessor-based gas ignition control designed for direct ignition gas-fired appliances. It provides direct main gas burner ignition, remote sensing, and prepurging. It will retry for ignition and has a fixed time for flame lockout.

honeywell hot surface igniton Hot Surface Ignition Module

Some hot-surface ignition modules have self-diagnostic capabilities. A diagnostic light on the HSI module provides the following information:

• If the diagnostic light on the module flashes on and off one time at initial startup, the unit is functioning properly.
• If the diagnostic light is lit continuously, there is most likely an internal problem with the module. Check for an internal problem by interrupting the line power or 25-volt thermostatic power for a few seconds and then restore it. If the burner still fails to ignite, replace the module.
• If the diagnostic light continues to flash, the problem is in the external components or wiring.

For HIS modules without self-diagnostic capabilities, a qualified HVAC technician or electrician should troubleshoot the system with the appropriate test equipment. The test equipment should include the following:

• A volt-ohm meter for checking both the voltage and the resistance.
• A precision microammeter for checking the flame sensor output and location.
• A pressure gauge (low scale) for checking gas pressure.

Extreme caution must be taken when working on a hot-surface ignition system. Because of the high voltage present, there is always the potential for serious electrical shock.

If the unit is not equipped with a self-diagnostic light, closely follow the troubleshooting suggestions provided by the manufacturer. These will be specific to the make and model. Some simple things to look for include the following:

• Checking to make certain the manual knob on the gas valve is in the on position and gas is available at the inlet piping
• Checking the outlet gas pressure to make sure it matches the nameplate rating
• Checking the wire leads to the gas valve for proper connection or damage

Direct-Spark Ignition Module


The direct-spark ignition (DSI) module illustrated in Figure 5-67 is a low-voltage, solid-state unit that controls the gas valve, monitors the burner flame, and generates a high-voltage spark for ignition. DSI modules are available with or without a purge timer and with separate or combined igniters and flame sensors. Typical wiring connections for a direct-spark ignition system are illustrated in Figure 5-68.

direct spark ignition module Direct Spark Ignition Module

direct spark ignition module wiring Direct Spark Ignition Module