Tags: Primary Control

Combination Primary Control and Aquastat

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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

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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

Servicing a Gas Burner Primary Control

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Access to the wiring terminals of the primary control illustrated in Figure 5-4 is obtained by loosening the screws that secure the chassis to the base. When remounting the chassis, be sure to tighten all mounting screws because they also serve as electrical connections.

No attempt should be made to repair a primary control except for tube replacement. Vacuum tubes are used in Honeywell primary controls. Never replace them with radio tubes. If a primary control is defective, the entire chassis should be replaced with a good one.

RA890F wiring Servicing a Gas Burner Primary Control

Operating controls located in the T/T circuit (see Figure 5-6) should be of the low-voltage, two-wire type. A low-voltage transformer for this purpose is built into a Honeywell Protectorelay. Safety controls located in 1–6 terminals must be two-wire, line voltage type. With the exception of the line switch, no controls should ever be placed in the line ahead of the 1–2 terminals of the primary control.

RA890F wiring connection 2 Servicing a Gas Burner Primary Control

Before assuming that the primary control is defective, be sure to check the pilot, pilot adjustment, flame detector circuit, and all operating and safety controls; proper operation is also dependent on these external factors. The flame circuit can be more accurately checked by the use of a microammeter to read flame current. Normal operation requires a current of 2 microamperes or more.

Never push relays in manually because it can result in accidental opening of the main diaphragm valve. Be sure to turn off the electrical power before removing the primary control chassis from the base.

Gas Burner Primary Control

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The primary control shown in Figure 5-4 is a solid-state electronic relay used on gas, oil, or combination gas-oil burners. It is designed to provide operational control of the burner in response to the room thermostat and limit controls, and to instantly shut off the burner in the event of flame failure. Figure 5-5 illustrates a typical wiring diagram for a Honeywell RA890F Protectorelay primary control used in a control circuit for a gas-fired boiler.

honeywell RA890F Gas Burner Primary Control

RA890F wiring Gas Burner Primary Control

This primary control is used with rectification-type flame detectors to sense the presence or absence of flame. The heart of the flame detector circuit in a gas-fired system is an electrode inserted in the pilot flame. In the wiring diagram, shown in Figure 5-6, the flame rod is connected to terminal F of the primary control.

RA890F wiring connection Gas Burner Primary Control

In the event of pilot flame failure, the flame detector circuit responds to control the gas valve in the manifold. When there is an absence of flame, the primary control shuts off the supply of gas by closing the gas valve or by keeping it closed if it is not already open.

These units are designed to be fail-safe. Abnormal conditions in the flame detector circuit, such as an open circuit, short circuit, or current leakage to ground, simulate absence of flame and cause the system to shut down. Safety controls such as temperature or pressure limits or low-water cutoffs are connected ahead of the switching terminals of the relay so that shutdown of the burner occurs even in the event of a relay malfunction such as fused contacts.

The main valve circuit is deenergized 8?10 of a second after flame failure occurs. On starting up or after flame failure, a trial-for-ignition period of approximately 45 seconds maximum occurs. During this ignition period, only pilot gas is allowed to flow to the burner. If the flame circuit is not completed within this time period, safety lockout of the relay occurs, causing a total shutdown of the system. Manual reset is then required to restart.

Two sets of relays are contained in a Honeywell RA890F Protectorelay primary control. The load relay (left hand) supplies current to the No. 3 terminal to control the blower motor. The flame relay (right hand) responds to the load relay but only if allowed by the flame-detecting electronic network of the relay. The flame relay can supply current to the No. 5 terminal controlling the gas valve only if the load relay has also pulled in.

The load relay is responsive to the thermostat or other operating control connected to the T/T terminal provided that safety controls, located in the 1–6 circuit, indicate that safe conditions exist for main burner operation.

On an interruption of power to the No. 1 and No. 2 terminals of the primary control, the relay returns to the standby position. When power is restored, normal operation is resumed except that the starting cycle is maintained longer than usual while the vacuum tube is warming up. Relay positions and their effect on the burner are listed in Table 5-1.

table 5 1 Gas Burner Primary Control