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Preferred Instruments

Preferred Instruments manufactures microprocessor based controllers, instruments, and electric actuators for combustion and process applications. Preferred Instruments also manufactures oil tank gauging and leak detection equipment. A major component of the Instrument group sales is from control systems integration and personal computer based Supervisory Control and Data Acquisition systems. Preferred Utilities' industrial factory is located in Danbury, CT

BurnerMate Universal Boiler Control System

The BurnerMate Universal is a total boiler control system designed for both firetube boilers and watertube boilers. Features include advanced flame safeguard with first out annunciation, parallel positioning combustion control with oxygen trim, draft, and feedwater control


The BurnerMate Universal (BMU) is designed to control single burner firetube boilers and watertube boilers. Large servos (up to 720 ft-lb) can drive the largest FD fan dampers and stack dampers.

The BMU is available as part of a U.L. listed package with most of the leading gas and oil fired burners. It is ideal for controls retrofits for existing burners. Fuel savings is accomplished with parallel positioning combustion control with optional oxygen trim. Electricity savings is provided by the ability to control up to four Variable Speed Drives (FD fan, ID fan, FGR blower, and feedwater pump motor)

In addition to flame safeguard and combustion control, the BMU includes optional draft control and feedwater control.

The BurnerMate Universal communicates to Building Automation Systems by Modbus or optional Ethernet.

Because it is microprocessor-based and pre-engineered, the BMU is an economical alternative to more expensive PLC-based boiler controllers. The parts are in stock for immediate delivery and require no programming.


The BurnerMate Universal offers complete boiler control in an economical, off-the-shelf, pre-programmed controller. Separate processors are used for flame safeguard and combustion control for NFPA 85 compliance. Configuration is done in the field using the LCD touch pad, the optional touch screen, or our exclusive BMU_Edit software running on your PC. The BurnerMate Universal is available for immediate delivery, requires no programming, and is U.L. recognized.

BMU Functions Include:

  • Advanced flame safeguard control including first out annunciation, nuisance trip protection, and lockout snapshot
  • Combustion control (jackshaft, parallel positioning, with optional oxygen trim) utilizing up to ten servos and up to four Variable Speed Drives (VSDs)
  • Draft control
  • Feedwater control: single, two-element, or three-element
  • Large 10” color touch screen with pre-programmed graphic pages

Advanced Flame Safeguard Control: The BurnerMate Universal takes burner safety to a higher level. Instead of one digital input proving the boiler limits are made, the BMU accepts over forty boiler limits into separate digital inputs. This makes first out annunciation easy, as well as advanced diagnostics to quickly pinpoint why a boiler tripped and what needs to be done to bring it on line again. One or two flame scanners can be used for added reliability, or increased boiler safety. Flame safety control features include:

  • Support for up to three fuels (oil, natural gas, and a second gas fuel). Fuel changeover can be initiated locally or remotely either shutting down the boiler during the fuel change, or changing fuel “on-the-fly”.
  • Monitors up to nine recycling limits, up to 33 shutdown limits and one or two flame scanners.
  • All limit inputs are 120 VAC, wired in parallel, and separately annunciated.
  • During a shutdown, all major status bits are recorded and saved for up to the last 10 lockouts.
  • Adjustable time delays are available for many limits to avoid nuisance trips.
  • Optional gas valve leak test logic is included (with or without a vent valve).
  • Optional oil gun purge logic can be configured (blow-through into the furnace, or suctioned back out of the gun via scavenger pump).
  • High flue gas temperature shutdown is included for additional dry boiler protection.
  • To reduce combustibles in the furnace, the burner is driven to low fire before it is shut down.

Fuel Air Ratio Control: The BMU supports single point positioning (jackshaft) combustion control and parallel positioning combustion control. Oxygen trim can be added to either control strategy by adding a Preferred ZP probe. A link trim actuator (LTA) can be used to vary the air output in a single point positioning system. Fuel valves and air dampers are controlled by high precision (0.1° accuracy) digital electric servos. Each digital servo includes a microprocessor and all servos are networked to the BMU chassis. BMU fuel-air ratio control functions include:

  • Jackshaft or parallel positioning combustion control is supported.
  • Up to ten high-precision servos are networked to the controller, and 4-20 mA outputs are available to control VSDs for FD fan, FGR fan, ID fan or FW pump.
  • Up to 14 combustion control points can be programmed for each output.
  • Separate points are available for standby, purge, and light-off positions.
  • Optional O2 trim control is supported.
  • Separate curves are available for each fuel for full-speed VSD bypass mode (in case of VSD failure) for a total of six curves per servo or analog output.
  • Atomizing (steam or air) pressure (or differential pressure) control setpoints can be input as a function of firing rate.
  • Windbox oxygen control (monitored by a seperate oxygen analyzer) allows the technician to enter a windbox oxygen vs. firing rate curve. The BMU modulates the FGR damper (or FGR fan VSD) to maintain FGR flow (as measured by windbox oxygen content) on curve at all firing rates.
  • Fuel and steam flow metering and totalizing are available. When these inputs are connected, boiler efficieny is calculated by the direct (output/input) method.
  • During boiler purge, all servos are stroked and their feedback signals checked to ensure servo integrity.

Boiler Control Functions: Many of the most common boiler control functions are incorporated in the BMU. This simplifies the control system and often eliminates the need for additional relays, timers, or other control hardware mounted in the burner management system enclosure. Additionally, the BMU accepts hard-wired inputs for burner off/on and remote firing rate that make it easy to integrate with the Preferred Chief Dispatcher lead/lag sequencer.

Other Control Functions Include:

  • Burner firing rate control (local PID control, remote firing rate input, or remote set point input)
  • Draft control with optional firing rate feed forward. Controlled outputs include a stack damper, or an ID fan VSD.
  • Feedwater control (single, two-, or three-element) Controlled outputs include a feedwater control valve or feedwater pump VSD.
  • Outdoor air temperature reset for hot water boilers.
  • Warm standby and domestic hot water override.
  • Low fire hold based on boiler shell or flue gas temperature.
  • Five auxiliary relays can be configured to control common boiler room auxiliaries.
  • Boiler cold start function allows for adjustable boiler thermal shock protection.

High Precision Digital Electric Servo Motors: The servo motors provided as part of the BurnerMate Universal are unique to Preferred. The BMU communicates digitally with a microprocessor controller board in each servo. The controller board precisely positions the servo motor and performs continuous diagnostics for safety. A high precision, sealed feedback potentiometer continuously reports the servo position to the controller board and back to the BMU chassis. The controller board includes pushbuttons for ZERO, CW, and CCW “jogging” of the servo motor. In addition, LEDs provide important diagnostic information for each servo motor. Torque ratings for the servo motors start at 3 ft-lb for small control valves and go up to 720 ft-lb for large air dampers or stack dampers.

Flame Scanners: The BurnerMate Universal system includes a line of Preferred Instruments flame scanners. Digital, microprocessor-based scanners are available in ultraviolet (UV) infrared (IR) and self-checking UV models. Flame detector and amplifier are integrated into the scanner head to eliminate the need for separate (panel-mounted) scanner amplifiers. Scanner housings are machined from anodized aluminum. Scanner mounts are 1/2” NPTF for the UV and IR versions, and 1” NPTF for the self-checking UV model. Each scanner includes a 1/8” NPTF cooling air port for hot or dirty applications. Scanners include two LEDs–one blinks intermittently depending on the intensity of the flame signal, the other LED illuminates when the scanner closes the “Flame Detected” relay output. Electrical connection is by a threaded military-style quick disconnect. The scanners output a 4-20 mA flame strength signal and a contact closure for flame proving. One or two scanners can be connected to a single BMU controller. All scanners include the same pinout and military-style quick disconnect for easy scanner change out. The BMU can be used with any flame scanner, or scanner/amplifier combination that produces a contact closure when flame is proven.

Windbox Oxygen FGR Control: Because the limits of flammability of many modern low NOx and ultra low NOx burners are extremely narrow, they require exactly the right amount of flue gas recirculation (FGR) at all firing rates, despite changing load conditions. Because open loop control can’t respond to changing conditions, and metering FGR flow is impractical, monitoring windbox oxygen content and using it as a setpoint has proven an effective means of ensuring repeatable FGR flow.

Atomizing Steam Pressure Control: Traditional means of atomizing steam pressure control are imperfect at best. Due to limited turndown capabilities of existing atomizing steam differential pressure regulating valves, the oil flame is often over-atomized at low fire, under-atomized at high fire, or both. This problem has been fixed, but using very expensive control valves with self-contained regulators/positioners. The BurnerMate Industrial controller monitors atomizing steam pressure, and modulates an inexpensive quarter turn flow control valve to deliver virtually any steam pressure required at any firing rate. Alternately, an atomizing steam/oil differential pressure transmitter can be used and the BurnerMate Industrial will hold the steam/oil differential pressure “on curve” at all firing rates.

Because this system uses a flow control valve instead of a pressure regulating valve, a seperate atomizing steam shutoff valve is not required. Simply program the controller so that atomizing steam pressure is zero during boiler standby, and the flow control valve will be driven fully closed. Many boiler technicians like to open the atomizing steam valve during purge to blow any water out of the line and get the line hot prior to oil burner lightoff. The BurnerMate Industrial provides seperate points for standby, purge, and ignition to accomodate this practice.

Fuel Flow Meter Display and Totalizing: To monitor and track fuel usage (and savings) the BMU displays and totalized fuel flows for up to three fuels. Fuel flows are also available on Modbus to communicate with a plant Energy Management System (EMS)

Digital Communication: The BurnerMate Universal communicates via a secure, safety rated digital protocol with the required LCD keypad and the servo network. The BMU chassis communicates via Modbus to the optional touchscreen OIT and optional CommStation OIT-Bridge. Both the touchscreen OIT or OIT-Bridge act as protocol converters and can speak Modbus, Modbus over Ethernet, or a number of other open digital protocols to a Building Automation System (BAS) or Energy Management System (EMS). Both the touchscreen OIT and the OIT-Bridge are supplied pre-loaded with over 75 standard graphic pages. Boiler overview screens can be field-selected to resemble the boiler being fired.

Analog Communication: The BurnerMate Universal accepts industry standard 4-20 mA, 0-5 VDC, 1-5 VDC, thermocouple, and thermistor inputs. Analog inputs are self-powered by the BMU. Engineering units, or scaled values are entered by parameter selection during the BMU setup. Analog outputs are 4-20 mA and are available only for the devices shown above. All other outputs are on the BMU servo network.

Digital Inputs & Outputs: All digital inputs to the BMU are 120 VAC from industry standard switches and contacts. Each of the most common boiler/burner limit switches is pre-assigned to a particular digital input. Spare limit inputs are provided.

Auxiliary Relays: Five relay outputs are available, each configurable for the following functions: common alarm, auxiliary fan starter, fuel 1,2,3 auxiliaries, common auxiliaries, limits made, outside air louvers, hot water pump/valve, LWCO auto-blowdown valve, flame on, fuel 1,2,3 open. Relay outputs are rated 120 VAC 5 amps and are configured by parameter selection.

Tandem Oil/Gas Valve Servo Option: Another slick feature designed for retrofit applications, the BurnerMate Industrial allows you to replace the jackshaft actuator on gas & oil fired with a Preferred servo and maintain the linkage to the oil and gas valves. The air damper is taken off the linkage and driven by a dedicated servo for parallel positioning control of both fuels. A ZP probe can be added to the system for parallel positioning with oxygen trim control. This features reduces the number of servos required for a gas/oil burner retrofit, and speeds installation. Separate curves are provided for standby, purge, and ignition positions for oil and gas. The revised software positions the gas/oil valve (jackshaft) actuator based on the BMS fuel selection and the corresponding fuel’s curve data.

“Link Trim Actuator” (LTA) Oxygen Trim Servo Option: The BurnerMate Industrial provides an easy way to retrofit jackshaft systems to parallel positioning with oxygen trim. Simply replace the existing jackshaft actuator with a new Preferred servo and install a Link Trim Actuator (LTA) in the drive rod to the FD fan damper. The controller will modulate the jackshaft servo in response to load demand, and the LTA to keep the stack oxygen “on curve.” The LTA has been redesigned to use the BMU-SM style servo, and the integral linkage redesigned to virtually eliminate hysteresis. This system works in conjunction with a Preferred ZP oxygen probe wired directly to the controller chassis.


  • Optional 10″ color touch screen contains over 75 pre-programmed graphics pages.
  • Flame scanners available in UV, IR, and UV self-check.
  • Servos available up to 720 ft-lb torque for large FD fan damprs and stack dampers.
  • Interfaces with optional zirconium oxide oxygen probe for oxygen trim control and increased fuel savings.
  • Drives up to four Variable Speed Drives (VSDs) for electricity savings.
  • Interfaces with the Preferred Chief Dispatcher plant master, lead/lag controller.


Quality Assurance: The boiler control system shall be manufactured and supported in the United States by Preferred Instruments. The Boiler Control and Burner Management systems shall be manufactured in accordance with UL 508A (CSA C22.2 #14 for use in Canada). Simply supplying UL recognized individual components is not sufficient. The assembled control cabinet as a whole must be inspected for proper wiring methods, fusing, etc., and must be labeled as conforming to UL508A. Inspection and labeling shall be supervised an OSHA approved Nationally Recognized Test Lab (NRTL). The system shall comply with NFPA 85 “ Requirement for Independence,” the flame safeguard system shall be provided with independent logic and power supplies and shall be physically separated from the combustion control logic.

Parallel Positioning Combustion Control: A parallel positioning combustion control system with oxygen trim and (optional) VSD control shall be provided for each boiler. Each system shall be designed to provide continuous boiler operation within boiler design limits with a high level of safety and energy efficiency. As required the system shall provide continuous monitoring and control of steam pressure (or water temperature), water level, combustion air and fuel ratio & flue gas recirculation. The system shall be fully integrated to the burner management system to provide fully automatic, safe and reliable startup and shutdown.

Oxygen Trim System: Provide a boiler breeching mounted in-situ, zirconium oxide oxygen analyzer for each boiler. Extractive type oxygen analyzers are not acceptable for combustion control. The probe shall be of a suitable length to sense the oxygen level in the middle third of the breeching. All wetted parts shall be stainless steel. The oxygen analyzer shall:

  • Include continuous self-diagnostics with diagnostic codes for at least 10 common faults.
  • Automatically send the trim control to the ‘null’ position and trigger the alarm dry contacts in the event of an oxygen analyzer fault.
  • The detector shall be field replaceable without removing the probe from the stack and shall not require special tools.
  • The analyzer shall automatically perform periodic detector cell impedance tests to be used by the operator as an indication of calibration shift.
  • Analyzer calibration shall be pushbutton semi-automatic (no trim pots) with English language prompts and diagnostic messages. Analyzer output shall be field selectable as 0-10% or 0-21% without field re-calibration.

Flame Safeguard System (FSG): Integral to the control system furnished shall be a Burner Management System (BMS) /Flame Safeguard System (FSG) controller. The system shall be designed to ensure the safe start-up, on-line operation, and shutdown of fuel firing equipment. Burner management system components shall be located in the combustion control cabinet and shall be fully integrated for automatic sequencing of light off and shutdown. Microprocessor-based FSG shall provide: safety interlocks, flame monitoring protection, and timed sequences. Sequences shall include forced draft fan start and stop, furnace purge, burner light off and shutdown and post-purge. The FSG shall be capable of firing up to three fuels (two gas fuels, one oil fuel), one fuel at a time. Fuel changeover shall from oil to gas, or gas to oil firing shall be accomplished “on the fly” without boiler shutdown per NFPA. A panel front-mounted English language, four line, twenty character LCD message display shall be provided to display flame signal strength, startup and shutdown sequence status, alarm, system diagnostic, first-out messages and burner historical information.  Historical information shall include the status of all limits and servos for the last ten lockouts.
To ensure boiler low water cutouts are working correctly, the controller shall be capable of performing a daily automatic water column blow-down test. The user shall select the time of day and duration of the water column blow-down test. The controller shall ensure the low water cutoff switches are functioning correctly, and alarm the operator or lockout the boiler if it detects a switch malfunction. To prevent nuisance trips, the flame safeguard system shall accommodate two flame scanners with one required to prove flame. The controller shall provide 120 VAC or 24 VDC scanner power, and accept two analog inputs for flame strength indication. Flame safeguard system shall include oil gun post purge for oil firing. Assured low fire cutoff shall be provided. For additional nuisance trip protection, field adjustable time delays shall be provided for F.D. fan start, fresh air damper, minimum air flow, low draft cut out, and fuel pressure limits. Five field selectable auxiliary relays shall be included for common alarm, auxiliary fan start, blow-down, flame on, fuel valve open, hot water pump or valve. To protect against dry firing, an option shall be available for high flue gas temperature lockout. To ensure air switches are functioning, minimum air flow pressure switch and purge air flow pressure switch safe start check shall be included.

Feedwater Control: Provide a boiler water level controller capable of single-, two-, or three-element feedwater control with the ability to automatically switch between control strategies dependent on system demands.

Draft Control: The controller shall provide two-element draft control utilizing a Preferred Instruments JC-XMTR draft transmitter. Burner firing rate shall be used as a feed forward for improved response to load changes. The control shall provide both automatic and manual damper control. All adjustments shall be made from the front panel display in engineering units.

Flue Gas Recirculation Valve Control: The controller shall have a characterizable setpoint curve for damper or variable speed fan output signal. All the logic required to insure that pre-purge, postpurge, light-off, and burner modulate cycles are automated shall be provided within the controller. Alternatively, the control will regulate FGR according to a preset windbox oxygen setpoint curve.

Windbox Oxygen FGR Control: The controller shall accept an analog input for burner windbox oxygen to be used as a measure of flue gas recirculation rate. During commissioning, a windbox oxygen vs. firing rate curve shall be established. The controller shall modulate the flue gas recirculation valve (or flue gas blower motor VSD) to maintain windbox oxygen on the pre-established curve despite changs in ambient conditions.

Atomizing Media Pressure Control: The controller shall accept an input for atomizing steam pressure or atomizing steam/oil differential pressure. During commissioning, an atomizing steam pressure (or atomizing steam/oil differential pressure) curve shall be established. The controller will modulate an atomizing steam flow control valve to keep the atomizing steam pressure on curve at all firing rates.

Additional Control Requirements:

  • Minimum number of f(x) Curves to be provided per servo: 6
  • Minimum number of points per f(x) Curve to be provided: 11
  • Cold FGR low fire cutback shall be provided when FGR is utilized for NOx reduction.
  • Separate curves shall be provided for FD fan full speed bypass of VSD in case of VSD failure.
  • Controller shall include the capability of receiving a remote firing rate input and remote set point input.
  • Controller shall include dual outdoor reset setpoint curves (normal & setback)
  • Controller shall include warm standby start / stop cycle.
  • Controller shall include low fire hold, and cold start warm up ramping.
  • All external or auxiliary power supplies necessary for electronic transmitters (or final control element) shall be included.
  • Boiler control software shall be U.L. recognized and inaccessible to prevent tampering. Unit commissioning shall be by parameter selection, not requiring ladder logic or blockware programming.
  • The controller shall accept standard 4-20 mA, 1-5 VDC, or RTD inputs for analog inputs. No special sensors shall be required.
  • Controller shall receive and display inputs for fuel flow, air flow, and steam flow. Controller shall be capable of future upgrade to fully metered combustion control by replacement of the flame safeguard microprocessor chassis only.

OIT Touch Screen: Provide as an option a ten (10) inch Operator Interface Terminal (OIT) designed to provide local operation, graphic display of information, alarm message display, historical and real time trending, remote controller tuning, x/y plots of fuel-air curve data for intuitive commissioning, Ethernet connectivity and standard internet browser remote communication.  The OIT shall contain a minimum of 75 graphic pages and be networked to the boiler control and burner management systems. The OIT shall provide graphic pages allowing step-by-step commissioning of the controller parameters using English language prompts and selections. The system shall be an industrial hardened operator interface terminal. The terminal shall be web enabled and allow remote monitoring via a standard internet browser and support Modbus TCP/IP Master, TCP/IP Slave, RS-485 Modbus Master, and Ethernet communications.

High Torque Servo Features:

  • Easy pushbutton set-up, not requiring the adjustment of internal or external potentiometers.
  • Servo zero, span, and direction of travel shall be accomplished by push button configuration.
  • Totally enclosed, dust tight, and splash-proof covers.
  • Provide a separate direct acting digital servo actuators for the fuel gas and fuel oil.
  • Electrically isolated shaft position feedback potentiometer, integral brake, 90° rotation in 25 seconds.
  • The actuator shall be capable of being stopped, started, or instantly reversed without loss of power or overloading.
  • Servo actuator positioning accuracy: 0.1 degrees. Servo full stroke safe start check shall be provided.
  • For high torque applications such as watertube boiler air dampers, servo torque shall be rated minimum 70 ft-lbs with 0.4 degree accuracy.
  • No servo feedback adjustments shall be required with pushbutton zero setup. Adjustable travel limit switches shall be integral, with re-adjustment not requiring new fuel air ratio curve re-entry.
  • Servos shall be cycled during each light-off cycle, and the feedback from each servo monitored to ensure safe actuator operation.
  • Servos shall be Preferred Instruments, model BMU-SM or BMU-UM (high torque).