The Boiler Control Circuit
Burner controls are generally wired in series to the solenoid valve on the fuel pump or to the gas valve on the gas-fired equipment. One electrical supply leg runs from the burner switch through each of the safety devices to the solenoid valve. Any safety or control device on this leg can break the circuit. The other electrical supply leg can break the circuit.
The other electrical supply leg runs directly to the burner, fuel solenoid or gas valve. If any of the safety switches on the control leg of the circuit opens, the circuit to the solenoid valve is disrupted, fuel is bypassed at the fuel pump and burner firing stops. Any number of control devices can be wired into this series.
Low Voltage Control Circuits
It becomes important that the operator and the controls themselves be protected from high voltages, especially as voltages increase to 230 V and higher. This is accomplished by stepping or reducing the supplied voltage to the control circuit down to 24 V using a transformer. The main switch is wired in the 24 V leg. When the main switch is turned on, a relay or magnetic contractor is activated which completes the main, high voltage circuit. All control and safety devices are wired into the low voltage circuit. When activation of a safety device breaks the high voltage circuit.
This allows switches in the system to be designed for use with low voltage current. Safety is enhanced with low voltage controls. The operator is isolated from the high voltage circuit. The switch the operator touches only receives 24 V.
All controls and switches on the low voltage side, wired in series, control a single high voltage magnetic contact. Low voltage controls are used on most three-phase equipment. Otherwise, duplicate or special switches and safety devices would have to be installed.
Wiring And Safety
High pressure cleaning equipment generally operates in a wet environment. Consequently, it only makes sense that all electrical connections and enclosures should be protected against moisture.
Most full-featured equipment uses weatherproof electrical connections and enclosures and totally enclosed, fan-cooled electric motors. Codes and regulations appear to require that any cleaning equipment with electrical systems should be weatherproof. However, much of the equipment currently in use and a substantial amount of the equipment manufactured today uses non-weatherproof electrical connections and enclosures and open drip-proof motor designs.
The requirement for ground fault circuit interrupters on new 115 V high pressure cleaning equipment in fact parallels similar requirements for GFCIs on swimming pool equipment, which, like high pressure cleaning equipment, is operated in a wet environment.
The fuel solenoid valve is a magnetically-operated valve that controls fuel flow to the burner nozzle. When current flows to the solenoid valve’s magnetic coil, a magnetic field is generated. The magnetic field lifts and holds the valve ball against the fuel bypass port, blocking the port. Fuel flow is directed through the outlet port to the burner nozzle and the burner fires. When current flow to the coil ceases, the magnetic field ceases to exist.
The valve ball is no longer held up against the bypass port. The ball falls, blocking the outlet port. Fuel flow to the burner stops. Without fuel, the burner cannot fire. Since a completed circuit is required to hold up the valve ball so fuel can reach the nozzle, any break in the circuit automatically causes the valve ball to drop, blocking the outlet port, and burner firing stops. The fuel solenoid is wired into the control system in most cleaners with trigger gun control. In open gun systems the entire burner motor circuit may be wired directly to any boiler controls.
Control and safety devices help reduce the possibility of burner safety problems. The more control and safety devices incorporated in a cleaning system, the smaller the likelihood of safety problems occurring.
The more boiler controls or safeties in a cleaning system, the greater the chance that either proper operation or failure of a safety device will be misunderstood by the customer as a boiler malfunction. To the operator, the boiler simply does not work. Actually, a potential threat to operator safety has been prevented. Many United States manufacturers have avoided problems with perceived boiler unreliability by simplifying or minimizing safety systems.
The Burner Switch Alone
On a hot water cleaner with an open gun, the only burner control may be the burner switch. The burner is simply turned on while water is flowing through the system and turned off when heat is no longer needed. The burner switch may be wired to the main switch so that the burner cannot be activated until the main switch is turned on. Or the burner switch may simply be the second position on a rotary switch. There are no safety devices in this configuration. Inherent boiler inefficiency and dead-reckoning fuel nozzle sizing are the only factors which keep water output temperatures below 212 degrees Fahrenheit.
The Burner Switch And High Temperature Limit
A high temperature limit switch added to the previous configuration will break the circuit to the burner assembly when water output temperature reaches a pre-set limit. This high-limit switch is normally on and the circuit is broken when the extreme temperature is reached. When the switch cools, the circuit is restored and burner operation resumes. The limit switch is wired in series with the burner switch. Lack of any flow-actuated control should limit this configuration to open gun equipment.
The Burner Switch, Thermostat And High Temperature Limit
Adding the thermostat in the series of control and safety devices allows for variable selection of desired output temperature achieved by intermittent burner operation controlled by the thermostat with the added safety of the high temperature limit switch as a fail-safe. Lack of any flow-actuated control should limit this configuration to open gun equipment. In this configuration the burner circuit is temporarily interrupted by the thermostat turning the burner on and off automatically to keep water output temperature within the temperature limits set by the thermostat. If the thermostat fails, water output temperature will continue to rise until the high-limit switch interrupts current flow and the burner ceases to fire.
Burner Switch, Flow Switch, Thermostat And High Temperature Limit
With the addition of the flow switch the burner control system automatically cuts off fuel to the burner nozzle whenever water flow is stopped or the equipment goes into bypass mode without the boiler ever having to reach a temperature limit. Equipment may be configured so that the flow switch is used with only a thermostat, only a high temperature limit switch or as the only control device.
Circuit Board Control
Any combination of control and safety devices may be used in conjunction with a circuit board containing one or more controlling microchips. Instead of actually breaking the circuit, the devices act as sensors for the microchip, which then acts as a controlling device when preset parameters are met.
To isolate a circuit board problem, simply apply proper voltage to solenoid valve’s magnetic coil. (This may be 24 Volts, low voltage, in systems with circuit boards.) if the solenoid valve works properly and there are no other problems in the system, you may have a circuit board problem.
Other controls may be used in boiler control circuits as well. These can include flow and pressure switches, timers and low fuel shutoffs. In some cleaner designs more than one flow switch used.