Back to the TDS blowdown control family
Return to the wider TDS blowdown controls route when you need to compare BC3250 with the broader boiler water quality control path.
Where BC3250 fits in TDS blowdown control
BC3250 sits on the model-specific end of the TDS blowdown controls route. It is used on steam boilers where dissolved solids must be monitored continuously and blowdown has to respond more consistently than a manual routine can provide.
The Spirax Sarco BC3250 boiler blowdown controller combines precision boiler water conductivity monitoring, automatic surface blowdown valve regulation, and an integrated bottom blowdown timer into a single industrial panel instrument. In industrial steam generating plants, variable steam demand, fluctuating condensate return rates, and chemical treatment dosing cause continuous shifts in dissolved mineral concentrations. Uncontrolled accumulation of total dissolved solids (TDS) triggers foaming, steam contamination, carryover of boiler water droplets into the distribution header, and premature scaling across heat transfer surfaces. Conversely, excessive or arbitrary manual blowdown wastes hot, treated boiler water, drives up makeup water consumption, and inflates fuel expenditures. The Spirax Sarco BC3250 resolves these operational challenges by continuously measuring boiler water conductivity and actuating motorized or solenoid blowdown valves precisely when dissolved solids exceed the target setpoint, while simultaneously timing intermittent bottom blowdown cycles to purge accumulated precipitated sludge.
For installations requiring a simpler compact interface without graphic trend logging, bottom blowdown timing, or EIA/TIA-485 Modbus networking, the sibling BC3150 boiler blowdown controller provides dedicated 3-digit TDS control. Where facilities require standalone boiler water sampling or manual verification before automating valve actuation, engineering teams should evaluate portable and panel-mounted conductivity meters to establish baseline water chemistry parameters.
Dual-function control architecture: continuous TDS and bottom blowdown timing
A distinctive engineering feature of the BC3250 is its integrated dual-relay architecture, which coordinates both surface TDS management and scheduled bottom blowdown routines within a single enclosure. Surface blowdown addresses dissolved mineral ions that concentrate near the evaporating water surface, whereas bottom blowdown expels dense, precipitated sludge that settles along the lower drum or boiler shell floor. Traditional boiler house installations require separate electromechanical timers or external PLC logic to automate bottom blowdown valves. The BC3250 consolidates these control functions through dedicated output relays and configurable software routines:
- Relay 1 (TDS Surface Blowdown): Configured as a single-pole changeover (SPCO) electromechanical relay rated for 250 Vac at 3 A resistive and 1 A inductive loads. It directly pilots electromechanical valves such as the Spirax Sarco BCV30, or pneumatic positioner valves (BCV1, BCV20, BCV31) to release high-TDS boiler water.
- Relay 2 (Bottom Blowdown or High Alarm): Programmable either as a dedicated bottom blowdown timer relay or as a high-TDS limit alarm. When set for bottom blowdown, it drives automated bottom blowdown ball or seat valves via timed cycles with durations programmable from 0 to 99 seconds at intervals between 1 and 99 hours.
By coordinating both blowdown mechanisms, the controller ensures that boiler water chemistry remains strictly compliant with boiler manufacturer specifications and regional water quality guidelines, such as those detailed in engineering references on water treatment, storage and blowdown for steam boilers and automated bottom blowdown systems.
Graphic display interface, bar graphs, and trend analysis
The Spirax Sarco BC3250 features an operator interface designed around a 128x64 pixel backlit liquid crystal display (LCD) and a five-button tactile keypad. Unlike legacy numerical displays, the graphic screen provides simultaneous visualization of operating values, setpoints, alarm states, and historical process trends across three coordinated operational zones:
- Primary 4-Digit Readout: Shows the active process variable (conductivity in µS/cm or TDS in ppm) with high photometric clarity, or displays programmable temperature parameters during commissioning.
- Alphanumeric Status Line: Indicates active controller states, including continuous blowdown, purge countdown, bottom blowdown status, alarm conditions, and selected engineering units.
- Simultaneous Triple Bar Graphs: Features three vertical bar graphs showing:
- PV (Process Variable): Displays the measured conductivity as a percentage of full-scale deflection (0–100%), while dynamically recording the maximum and minimum values reached during the operating cycle.
- SP (Setpoint): Depicts the programmed target TDS setpoint arrow alongside a dotted marker representing the hysteresis recovery threshold.
- AL (High Alarm): Illustrates the high TDS limit threshold and its associated hysteresis deadband, providing immediate visual warning of water treatment upsets.
Pressing the horizontal navigation pushbuttons toggles the unit from the standard bar graph screen to a dedicated Trend Graph display. The trend screen plots TDS variations over an adjustable horizontal time base of up to 8 units in minutes, hours, or days. This on-screen historical logging capability allows boiler operators, water treatment specialists, and commissioning engineers to inspect boiler TDS response curves over previous shifts without requiring a supervisory PC or external chart recorder.
Conductivity probe compatibility and temperature compensation
Accurate boiler blowdown depends upon reliable conductivity sensing in demanding high-temperature, high-pressure boiler environments. Electrical conductivity increases with temperature at approximately 2% per °C. Without accurate temperature compensation, variations in boiler operating pressure or cooling inside external sensor chambers introduce significant measurement errors. The BC3250 accommodates Spirax Sarco industrial conductivity probes and flexible temperature compensation modes:
- Probe Support (CP10, CP30, CP32): The controller interfaces with standard high-temperature probes including the CP30 conductivity probe for direct boiler shell mounting, the CP10 for external sensor chambers, and the advanced CP32 probe.
- CP32 Scale Monitoring and Self-Cleaning: When paired with the CP32 probe, the BC3250 actively monitors sensor surface contamination. If scale deposition is detected, the controller generates warning alarms and drives an integrated 32 Vdc cleaning circuit (constant or pulsed 1 second on, 1 second off) to dissolve fouling and maintain measurement fidelity.
- Pt100 Temperature Compensation: A dedicated 3-wire input interfaces with a Pt100 RTD sensor (Class B or better) spanning 0 °C to 250 °C (32 °F to 482 °F). The controller automatically calculates temperature-compensated conductivity normalized to 25 °C.
- Manual Fixed Temperature Mode: If a physical Pt100 RTD is not installed, operators can enter a fixed operating temperature between 100 °C and 250 °C in 1 °C increments (factory default: 184 °C, corresponding to 10 bar g saturated steam).
Flexible blowdown control actions: continuous, purge, and pulsed modes
Boiler geometry, steam output, and probe installation locations dictate different blowdown actuation strategies. The BC3250 incorporates three distinct software control modes to match application requirements:
- Continuous Blowdown: When the conductivity probe is immersed directly inside the boiler shell near the normal water level, water surrounding the probe tip is continuously representative of boiler bulk water. The purge duration is set to 0 seconds. The controller holds the blowdown valve closed until conductivity rises above the setpoint (SP), opens the valve until TDS drops below the setpoint minus hysteresis, and then closes the valve.
- Purge Blowdown: When the probe is installed in external blowdown pipework or a sample tee, water around the probe tip stagnates when the blowdown valve is closed. In purge mode, the BC3250 periodically opens the blowdown valve for a programmed purge duration (0 to 180 seconds) at set intervals (10 to 60 minutes) to flush fresh boiler water past the probe prior to sampling. If the sampled TDS exceeds SP, the valve remains open for continuous blowdown. An opto-isolated burner interlock input (99–264 Vac) can synchronize purge intervals with active burner firing time, ensuring that purges only occur while steam is being generated.
- Pulsed Blowdown: On small industrial or commercial shell boilers with low water volume, opening a standard blowdown valve continuously can cause rapid level drops that trip low-water safety alarms. Pulsed blowdown cycles the valve 10 seconds open and 20 seconds closed during active blowdown demand, stabilizing water level while steadily reducing dissolved solids.
Industrial communications: EIA/TIA-485 Modbus RTU and infrared coupling
Modern boiler houses integrate standalone field controllers into plantwide energy management and SCADA architectures. The BC3250 includes two robust communication channels:
- EIA/TIA-485 Modbus RTU: A dedicated serial communication port configurable for either 4-wire full duplex or 2-wire half duplex industrial multidrop networks. The port supports programmable slave addresses (1 to 247) and selectable baud rates (1,200, 9,600, or 19,200 bps). Over 30 Modbus holding registers expose real-time TDS values, water temperature, alarm states, valve accumulated open hours, and fault diagnostics to external PLC, DCS, or building automation systems.
- Infrared (IR) Optical Transceiver: Built into the controller side casing, the optical transceiver allows adjacent Spirax Sarco boiler controllers mounted side-by-side on a DIN rail to exchange operating parameters and synchronization signals without physical wiring harnesses.
- Isolated 0/4-20 mA Retransmission: A galvanically isolated (100 V) current loop output updates 10 times per second with 0.1% FSD resolution, providing a continuous analog signal representing boiler water TDS or conductivity to remote chart recorders or telemetry panels.
Boiler house integration and heat recovery synergy
Automating boiler blowdown with the BC3250 delivers substantial thermal efficiency improvements when coupled with boiler house heat recovery equipment. Continuous surface blowdown discharges water at boiler saturation temperature and pressure. Passing this hot effluent through Spirax Sarco flash vessels and residual heat exchangers preheats incoming boiler feedwater while recovering low-pressure flash steam. In facilities implementing comprehensive heat recovery systems, maintaining consistent blowdown flow rates with the BC3250 optimizes heat exchanger thermal transfer efficiency compared to erratic manual discharge routines. For in-depth thermodynamic principles and calculations, engineers should consult Spirax Sarco technical tutorials on controlling TDS in the boiler water and the comprehensive portfolio of boiler controls and systems.
Engineering selection checks and ordering guidelines
Prior to finalizing equipment schedules and ordering a BC3250 controller, verify the following mechanical and electrical specification criteria:
- Sensor Selection and Mounting Location: Determine whether the conductivity probe will be installed directly into the boiler shell (e.g. CP30 probe in an internal protection tube) or in an external blowdown line (requiring purge blowdown mode). Confirm whether probe self-cleaning with a CP32 probe is required for scale-forming waters.
- Temperature Compensation: Confirm whether operating pressures fluctuate significantly, necessitating a Pt100 RTD sensor, or whether a constant fixed temperature calculation satisfies operational requirements.
- Blowdown Valve Actuator Type: Check whether the surface blowdown valve requires electromechanical 230/115 Vac on/off power (e.g. BCV30), 24 Vac/dc actuation, or pneumatic actuation with a solenoid valve.
- Enclosure and Mounting Arrangement: The BC3250 is supplied for flush panel mounting (requiring a standard 92 x 92 mm DIN cutout), chassis backplate mounting, or 35 mm DIN rail mounting within an industrial enclosure providing minimum IP54 / NEMA 4 protection.
- Communications and Telemetry: Verify whether the site requires 0/4-20 mA current loop retransmission, 2-wire or 4-wire EIA/TIA-485 Modbus RTU integration, or standalone local operation.
Ordering Example: 1 off Spirax Sarco BC3250 Boiler Blowdown Controller (mains supply 110–240 Vac, 50/60 Hz, panel/chassis/DIN mounting).
Scope of this technical information
This section preserves the published technical specifications for the Spirax Sarco BC3250 Boiler Blowdown Controller. The data, ratings, functional descriptions, certification details, input and output characteristics, default parameters, and mechanical dimensions are compiled from the authoritative English Installation and Maintenance Instructions IM-P403-89 (EMM Issue 8, Section 10) and cross-verified against Technical Information sheet TI-P403-90 (AB Issue 6). Where slight differences in standard edition citations exist between documents, the newer English EMM Issue 8 specification takes precedence. Local regulations may restrict the operation of this equipment below the conditions quoted. In the interest of development and improvement, Spirax Sarco reserves the right to update product specifications without prior notice.
Description
The BC3250 is an advanced digital boiler blowdown controller designed for industrial steam boilers. It continuously regulates boiler water Total Dissolved Solids (TDS / dissolved salts) by modulating or cycling a surface blowdown valve, while integrating an automated bottom blowdown timer to periodically discharge precipitated sludge. Operating via precision liquid conductivity measurement, the controller works in conjunction with Spirax Sarco conductivity sensors (CP10, CP30, or CP32), a Pt100 temperature compensation probe, and electro-mechanical or pneumatic blowdown valves. It may also be applied in contaminated condensate monitoring systems to operate condensate dump valves.
The BC3250 is housed in a robust polycarbonate enclosure suitable for flush panel cutout, standard 35 mm DIN rail, or chassis backplate mounting. It operates from a universal 110 V to 240 Vac mains supply (99–264 Vac operating limits). The front panel incorporates an illuminated 128x64 pixel graphic LCD display and a 5-button tactile keypad. No backup batteries are required; all operational parameters and calibration values are stored in non-volatile flash memory.
Main features
- Combined continuous surface TDS control and integrated bottom blowdown timer
- 128x64 backlit graphic LCD showing triple bar graphs (PV, SP, AL), 4-digit readout, and status line
- On-screen Trend Graph display logging historical TDS variations over minutes, hours, or days
- Compatible with high-temperature conductivity probes: CP10, CP30, and self-cleaning CP32
- Automatic electrochemical probe cleaning drive (32 Vdc) and scale detection with CP32
- Direct 3-wire Pt100 RTD temperature compensation input (0 to 250 °C) with manual fallback
- Three selectable blowdown control modes: Continuous, Purge (0–180 s), and Pulsed (10 s open / 20 s closed)
- Galvanically isolated 0/4-20 mA current loop retransmission output
- EIA/TIA-485 Modbus RTU serial port supporting 2-wire half duplex or 4-wire full duplex multidrop networks
- Infrared optical communication port for inter-controller coupling
- Burner interlock input (99–264 Vac) to synchronize purge intervals with active firing time
- Type-tested as a TDS controller and limiter to Vd TÜV Wasser 100 (07:2006) and UL listed
Certification and standards compliance
The BC3250 complies with the European Electromagnetic Compatibility (EMC) Directive 2014/30/EU and meets heavy industrial environment standards (Class A equipment). Full EMC compliance documentation is catalogued under reference UK Supply BH BC3250 2008.
The controller complies with the Low Voltage Directive (LVD) 2014/35/EU and is certified to the following international electrical safety standards:
- EN 61010-1:2010: Safety requirements for electrical equipment for measurement, control, and laboratory use.
- UL 61010-1 (3rd Edition, 2012-05): Standard for Safety Electrical Equipment for Measurement, Control, and Laboratory Use.
- CAN/CSA-C22.2 No. 61010-1 (3rd Edition, 2012-05): Canadian safety standards for electrical laboratory and control apparatus.
- Vd TÜV Wasser 100 (07:2006): Type tested and certified as an automatic boiler water TDS controller and limiting device.
Technical specifications
| Nominal mains voltage | 110 Vac to 240 Vac at 50/60 Hz |
|---|---|
| Permissible operating voltage range | 99 Vac to 264 Vac, 47 to 63 Hz |
| Power consumption | 7.5 W maximum |
| Operating location | Indoor use only |
| Maximum operating altitude | 2 000 m (6 562 ft) above sea level |
| Ambient operating temperature | 0 °C to 55 °C (32 °F to 131 °F) |
| Maximum relative humidity | 80% up to 31 °C (88 °F), decreasing linearly to 50% at 40 °C (104 °F) |
| Installation overvoltage category | Category III |
| Pollution degree | Degree 2 (as supplied); Degree 3 when installed in an enclosure (minimum IP54 or NEMA Type 3, 3S, 4, 4X, 6, 6P, or 13) |
| Enclosure ingress rating | Front panel only: IP65 (verified by TRAC Global) and NEMA Type 4 hose down (UL approved) |
| Enclosure construction | Polycarbonate casing; silicone rubber front keypad (60 shore) |
| Panel mounting screw torque | 1.0 to 1.2 Nm |
| Conductivity probe types | CP10, CP30, and CP32 high-temperature probes |
|---|---|
| Conductivity measurement ranges | 0–9.99, 0–99.9, 0–999, and 0–9 990 µS/cm or ppm |
| Minimum conductivity threshold | >= 1 µS/cm at 25 °C |
| Conductivity accuracy | +/-2.5% FSD (+/-5% FSD on the 0–9.99 range) |
| Measurement resolution | 0.1% FSD |
| Probe excitation drive | ac, 4-wire circuit (eliminates polarization) |
| TDS conversion factor (pH term) | Selectable 0.50 to 1.00 (factory default 0.70) |
| Neutralising factor | 0.70 |
| Temperature compensation sensor | Pt100 RTD, Class B or better, 3-wire dc drive |
| Temperature compensation range | 0 °C to 250 °C (32 °F to 482 °F) |
| Manual temperature setting | 100 °C to 250 °C in 1 °C steps when Pt100 is not fitted (default: 184 °C / 10 bar g) |
| Temperature compensation accuracy | +/-2.5% FSD (overall system accuracy +/-5.0%) |
| Burner interlock input | 99 Vac to 264 Vac, maximum 2 mA (opto-isolated) |
| Probe cleaning voltage | 32 Vdc maximum; constant or pulsed (1 s on, 1 s off) |
| Relay contacts | 2 x single-pole changeover (SPCO) electromechanical relays |
|---|---|
| Maximum relay voltage | 250 Vac |
| Relay contact ratings | 3 A resistive at 250 Vac; 1 A inductive at 250 Vac; 1/4 HP (2.9 A) at 250 Vac ac motor; 1/10 HP (3 A) at 120 Vac ac motor; Pilot duty C300 (2.5 A continuous) |
| Relay 1 function | Surface TDS blowdown valve actuator (open / close) |
| Relay 2 function | Bottom blowdown timer actuator OR high-TDS / fault alarm contact |
| Bottom blowdown duration | 0 to 99 seconds (or 0 to 9 seconds if purge blowdown active) |
| Bottom blowdown interval | 1 to 99 hours between automatic cycles |
| Analog retransmission | 0–20 mA or 4–20 mA current loop, configurable span |
| Retransmission open-circuit voltage | 19 Vdc maximum |
| Maximum retransmission load | 500 ohms |
| Retransmission isolation | 100 V galvanic isolation; 10 updates per second; 0.1% FSD resolution |
| Serial communications | EIA/TIA-485 Modbus RTU protocol, slave address 1 to 247 |
| Modbus baud rates | 1 200, 9 600, or 19 200 bps (8 data bits, no parity, 1 stop bit) |
| Modbus physical topology | 4-wire full duplex or 2-wire half duplex |
| Infrared optical port | Lateral optical transceiver, Master or Slave mode (default Slave) |
| Connector type | Rising clamp plug-in terminal blocks with screw retention |
|---|---|
| Conductor wire gauge | 0.02 mm² (24 AWG) to 2.5 mm² (12 AWG) solid or stranded |
| Insulation stripping length | 5 mm to 6 mm |
| Terminal screw tightening torque | 0.5 to 0.6 Nm |
| TDS probe cable | High-temperature screened cable: 4-core (CP32), 4-core (CP10/CP20 via junction box), or 2-core (CP10/CP30). Wire gauge 1.0 to 1.5 mm² (18 to 16 AWG). Maximum length: 100 m (328 ft) |
| Pt100 temperature cable | 3-core screened copper instrument cable, 0.5 to 1.5 mm² (20 to 16 AWG). Maximum length: 100 m |
| Modbus EIA/TIA-485 cable | Screened twisted pair (STP), characteristic impedance 120 ohms. Maximum run: 1 200 m (4 000 ft) |
Mechanical dimensions and cutout requirements
The controller enclosure is designed for versatile panel or cabinet installation. For flush instrument panel mounting, a 92 mm x 92 mm square cutout is prepared. The supplied rubber sealing gasket and decorative clip-on bezel provide an IP65 weather seal against panel faces between 1.0 mm and 3.0 mm thick. Alternatively, the chassis backplate mounting arrangement utilizes four 4.2 mm diameter fixing holes spaced 112 mm vertically and 67 mm horizontally. For cabinet rail installations, an integrated rear spring clip secures the unit onto standard 35 mm DIN rails (EN 50022 / BS 5584).
| Panel cutout | 92 mm x 92 mm (+0.8 mm / -0.0 mm) DIN standard |
|---|---|
| Minimum panel thickness | 1.0 mm (with bezel) or 2.0 mm (without bezel) |
| Front bezel dimensions | 120 mm wide x 140 mm high (including terminal shrouds) |
| Depth behind panel | 52 mm behind front mounting flange |
| Lateral unit spacing | 15 mm minimum clearance between adjacent units for thermal ventilation |
| Fixing screws | Two M4 x 25 mm screws with fibre sealing washers (supplied) |
| Unit weight | Approximately 500 g (1.1 lb) |
Default factory parameter settings
| Parameter Menu | Description | Permissible Range | Factory Default |
|---|---|---|---|
| MODE | Manual valve position | OPEN or CLOSE | CLOSE |
| DATA: UNITS | Engineering display unit | µS/cm or ppm | µS/cm |
| DATA: TEMP | Temperature engineering unit | °C or °F | °C |
| DATA: PH TERM | TDS conversion factor | 0.50 to 1.00 (0.01 step) | 0.70 |
| INPUT: T COMP | Temperature compensation | 100 °C to 250 °C (212–482 °F) | 184 °C (10 bar g) |
| INPUT: TDS - SENSOR | Probe model selection | CP10, CP30, CP32 | CP30 |
| INPUT: PURGE DURATION | Pre-sample purge flush | 0 to 180 seconds (1 s step) | 000 s (continuous) |
| INPUT: PURGE INTERVAL | Interval between purges | 10 to 60 minutes | 30 minutes |
| INPUT: PURGE BURNER | Burner firing dependency | Normal or Cumulative | Cumulative |
| INPUT: TDS - SP | Conductivity setpoint | 0 to 100% FSD | 50% FSD |
| INPUT: TDS - HYST | Setpoint deadband hysteresis | 0 to SP | 5% FSD |
| INPUT: TDS - CAL | Calibration target value | 0 to FSD | 4 000 µS/cm |
| OUTPUT: CLEAN DRIVE | Probe cleaning mode | STANDARD or PULSED | STANDARD |
| OUTPUT: CLEAN DURATION | Electrochemical clean time | 0 to 99 seconds | 20 seconds |
| OUTPUT: CLEAN INTERVAL | Clean cycle frequency | 1 to 99 hours | 12 hours |
| OUTPUT: RETRANS - 4 mA | Analog output low span | 0 to FSD | 0 µS/cm |
| OUTPUT: RETRANS - 20 mA | Analog output high span | 0 to FSD | 100% FSD |
| COMMS: ADDRESS | Modbus RTU slave address | 1 to 247 | 1 |
| COMMS: BAUD | Serial transmission rate | 1 200, 9 600, 19 200 bps | 9 600 bps |
| COMMS: IR | Infrared bus mode | MASTER or SLAVE | SLAVE |
Scope of these installation and maintenance instructions
This section preserves the official installation, wiring, commissioning, and maintenance procedures for the BC3250 Boiler Blowdown Controller, derived from Spirax Sarco document IM-P403-89 (EMM Issue 8). It covers mandatory mechanical mounting rules, electrical isolation, overcurrent protection, sensor and signal wiring diagrams, EIA/TIA-485 Modbus network wiring, quick-start commissioning, and maintenance routines. Safe operation of this equipment can only be guaranteed if it is properly installed, commissioned, operated, and maintained by qualified personnel in full compliance with these instructions and relevant national regulations (including BS 7671, EN 12953, EN 12952, and EN 50156).
1. General safety information
Warning: The BC3250 operates on dangerous voltages up to 264 Vac. Isolate mains supply before touching terminals or opening panel enclosures. Installation and maintenance personnel must be suitably qualified to work with equipment operating at hazardous voltages. All wiring materials and installation methods must conform to IEC 60364 (Low-voltage electrical installations), EN 50156 (Electrical equipment for furnaces and ancillary equipment), and local electrical codes (NEC / CEC in North America).
The product must be installed in an environment that minimizes heat, vibration, mechanical shock, and electrical interference. It is designed for indoor use only; do not install outdoors without a weather-protected secondary enclosure providing minimum IP54 / NEMA 4 rating. Do not attempt to open the controller casing; it is factory-sealed with no user-serviceable internal components.
2. Typical boiler control system arrangements
The BC3250 is deployed across standard Spirax Sarco boiler control system (BCS) architectures. In a standard BCS1 arrangement for industrial steam boilers, the controller receives conductivity signals from an immersed probe (CP30 or CP32) and drives an automated surface blowdown valve (e.g. BCV30) to purge concentrated solids into a blowdown vessel or cooler, while coordinating bottom blowdown discharge.
3. Mechanical installation
The BC3250 must be installed strictly in the vertical orientation to ensure proper convection cooling and readability of the graphic LCD display. Provide a minimum lateral gap of 15 mm between adjacent controllers. Three mounting configurations are supported:
3.1 Flush instrument panel mounting
- Cut a square aperture 92 mm x 92 mm (+0.8 mm / -0.0 mm) in the panel face. Panel thickness must be at least 1.0 mm when using the decorative bezel (or 2.0 mm without bezel).
- Drill two 4.2 mm diameter screw clearance holes on the vertical centerline spaced 112 mm apart (56 mm above and below aperture center).
- Peel the protective backing from the supplied cellular rubber gasket and apply it firmly to the front mounting flange of the controller.
- Insert the controller from the rear of the panel cutout, position the front bezel over the flange, and secure using the two supplied M4 x 25 mm screws and fibre washers. Tighten screws evenly to 1.0–1.2 Nm. Warning: Do not use screws longer than 25 mm, as this creates an electrical shock hazard.
3.2 Chassis backplate mounting
Drill four 4.2 mm diameter holes in the mounting backplate corresponding to the 112 mm vertical by 67 mm horizontal chassis hole pattern shown in Fig. 15. Secure the unit using M4 screws, nuts, and lock washers. Do not drill into the plastic controller case or use self-tapping screws.
3.3 Standard 35 mm DIN rail mounting
The rear casing is equipped with a spring-loaded latch mechanism compatible with standard 35 mm symmetrical DIN rail (EN 50022 / BS 5584). Hook the upper mounting bracket over the top edge of the DIN rail and press the bottom edge firmly inward until the spring clip latches securely. To disengage, insert a flat-blade screwdriver into the release slot at the bottom to ease the spring latch downward.
4. Electrical installation and wiring safety
Ensure that all electrical wiring complies with the following mandatory safety requirements:
- Overcurrent Protection: The building electrical distribution must provide overcurrent protection for all phase conductors. Install a 3 A quick-blow fuse or miniature circuit breaker (MCB) on the mains supply line. Protect the burner interlock circuit with a 1 A quick-blow fuse. Protect relay output circuits with separate 3 A quick-blow fuses.
- Phase Alignment: All relay contact circuits must be powered from the identical electrical phase as the controller mains supply. Supplying relays from different phases is strictly prohibited.
- Local Disconnect Device: A dedicated, clearly labelled disconnecting switch or circuit breaker complying with IEC 60947-1 / IEC 60947-3 must be installed in close proximity to the controller, easily accessible to operating personnel. It must disconnect all phase conductors simultaneously without interrupting the protective earth (PE) ground path.
- Screening and Segregation: All sensor, analog retransmission, and Modbus serial cables must be screened. Connect cable screens to the designated screen terminals or earth studs at the controller end only. Never run low-voltage sensor cables in the same conduit or trunking as high-voltage mains or heavy motor wiring.
5. Sensor, signal, and communication wiring
5.1 TDS conductivity probe connection
Use high-temperature screened multi-core cable (maximum length 100 m / 328 ft) between the probe and controller. For the CP30 probe, connect the two active probe cores to the TDS probe input terminals, and ground the overall cable shield to the controller screen earth. When installing the CP32 self-cleaning probe, terminate the 4-core screened cable to provide both ac measurement drive and 32 Vdc pulsed cleaning power. Ensure junction boxes installed near the boiler shell maintain IP65 weather sealing.
5.2 Pt100 temperature compensation probe
Connect a 3-wire Class B Pt100 RTD using screened copper instrument wire (0.5 to 1.5 mm²). The 3-wire bridge configuration automatically compensates for lead resistance up to 100 m. If temperature compensation is operated in manual fixed mode, leave Pt100 terminals unpopulated and program the operating temperature in the commissioning menu.
5.3 EIA/TIA-485 Modbus RTU network
For industrial SCADA or PLC networking, connect screened twisted-pair cable (nominal characteristic impedance 120 ohms) to the Modbus communication terminals. The port supports:
- 2-Wire Half Duplex: Link terminals A to A and B to B across all multidrop slave devices, connecting signal ground (COM). Ensure a 120-ohm terminating resistor is fitted across lines A and B at the extreme ends of the bus.
- 4-Wire Full Duplex: Separate transmit (TX+, TX-) and receive (RX+, RX-) pairs for high-throughput polling. Maximum cable run is 1 200 m (4 000 ft).
6. Commissioning and calibration procedure
Commissioning is performed using the five front panel pushbuttons and illuminated LCD screen:
- Security Access: Press and hold the OK button for 5 seconds to access configuration mode. Enter the 4-digit security passcode to prevent unauthorized modification.
- Configure System Parameters: Set engineering units (µS/cm or ppm), temperature units (°C or °F), and TDS multiplier (default 0.70).
- Configure Inputs: Select probe model (CP10, CP30, or CP32). If purge blowdown is utilized, set purge duration (10–60 s) and purge interval. Set temperature compensation to Pt100 or enter fixed boiler saturation temperature.
- Calibrate Conductivity (TDS - CAL): With the boiler at normal operating temperature and load, obtain a fresh water sample from the boiler water sample cooler. Measure sample TDS using a calibrated laboratory conductivity meter corrected to 25 °C. Access the
TDS - CALmenu item and adjust the displayed value to match the laboratory reading. Press OK to lock calibration into non-volatile Flash memory. - Configure Outputs and Alarms: Set the target TDS setpoint (
TDS - SP) to the boiler manufacturer's recommended dissolved solids limit. Set setpoint hysteresis (TDS - HYST, typically 5% FSD). Configure Relay 2 for either bottom blowdown timing or high alarm duty.
7. Maintenance and fault finding
The BC3250 controller requires no periodic preventive maintenance, lubrication, or routine component replacement. Regularly verify that the panel bezel gasket remains clean and free of chemical buildup. Periodic operational checks include:
- Probe Inspection: Inspect the conductivity probe tip annually for scale buildup or magnetite deposition. When using the CP32 probe, check the graphic status line for probe fouling indicators.
- Water Chemistry Audit: Check boiler water TDS weekly against the controller display using a portable conductivity meter and sample cooler. If readings diverge by more than 5%, recalibrate following Section 6.
- Diagnostic Error Codes: In the event of sensor fault, open circuit, or excessive temperature, the controller displays alarm codes on the information line (e.g.
TDS SENSOR FLT,TEMP SENSOR FLT,HIGH ALARM). Press and hold OK for 5 seconds to acknowledge and clear transient alarms after rectifying field wiring.