DXM2.5 Advanced Communicating Controls

Overview

The DXM2.5 Advanced Communicating Controls is a robust, microprocessor-based heat-pump controller designed for maximum application flexibility. It features relay outputs for Compressor, Compressor Speed, Reversing Valve, Alarm Relay, and configurable relays for Fan, Fan Speed, HWG Pump, and Loop Pump, along with two accessory relay outputs. The DXM2.5 can directly communicate with and control Constant Volume (CV) EC blowers, internal variable-speed water pumps, and modulating water valves. Onboard diagnostics are provided via two LEDs.

Inputs include safety-pressure switches, low-temperature protection thermistors, condensate-overflow sensor, DIP switch selections, thermostat inputs, night-setback inputs, refrigeration-detection sensors, and an emergency-shutdown input. Additional configurable temperature-sensor inputs are available for various applications (e.g., hot water, compressor discharge, leaving air, leaving-water, entering-water). The unit includes an RS-485 communications port for interfacing with communicating thermostats or other controls.

General Operating Parameters

• Operating Environment: -40°F to 176°F (-40°C to 80°C) and up to 95% relative humidity, non-condensing.
• Storage Environment: -40°F to 185°F (-40°C to 85°C) and up to 95% relative humidity, non-condensing.

Power Requirements

• Power Draw: Normally 8VA at 24VAC; Maximum 12VA at 24VAC.
• A dedicated 24VAC, 50-60Hz, 1Ph, 40VA transformer is required for typical WSHP applications.

Relay and Connection Contact Ratings

The DXM2.5 features the following relays:

• Compressor Relay: 40VA at 24VAC
• Compressor Speed Relay: 28VA at 24VAC
• Alarm Relay: 28VA at 24VAC
• Reversing Valve: 28VA at 24VAC
• Accessory Relay 1: 28VA at 24VAC
• Accessory Relay 2: 28VA at 24VAC
• Fan Enable / Loop Pump Relay: 1 HP at 240VAC
• Fan Speed / HWG Pump Relay: 1 HP at 240VAC
• Connection Ratings: 'A' terminal: 20VA at 24VAC. Larger solenoid valve draws should be controlled via accessory relays.

Grounding

The control board must be grounded from one of the 'C' terminals.

Basic Control Features

• Single or two-stage compressor control
• Anti-short cycle protection
• High pressure cut-out
• Loss of charge cut-out
• Over and under voltage cut-outs
• Water coil low temperature cut-out
• Air coil low temperature cut-out
• Random start
• Status LED and Fault LED
• Reset lockout at unit or disconnect
• Condensate overflow sensor
• Refrigerant detection sensor
• Intelligent fault retry
• Test Mode
• Multiple blower configuration options
• Electric heat outputs
• Accessory water valve connection

Advanced Control Features

• Direct control of EC blower
• Intelligent hot water generator control
• Two accessory relays configurable for multiple applications
• Variable-speed water-pump output
• Modulating water valve output
• Night setback with override capability
• Emergency shutdown capability
• Removable thermostat connector for ease-of-installation and service
• Accepts conventional heat pump (Y,O) or heat/cool (Y,W) thermostat types
• RS-485 port for interface with communicating thermostats or other controls
• Boilerless electric heat
• Configurable inputs and outputs for advanced functions
• Stores operating conditions history for the last five faults, offering possible reasons for faults
• Client/server thermostat control of up to three units

Legend and Glossary of Abbreviations

Layout and Connections

The DXM2.5 control board has various connection points for thermostats, sensors, blowers, and other system components. Key connections include:

• Conventional Stat Connection
• Communicating Stat Connection
• Service Tool Connection
• Communications and HWG Settings
• 24V to Compressor Second-stage Solenoid
• Accessory relays for configuration.
• VFD or PWM CV EC Blower connection.
• Modulating Valve or Variable Speed Water Pump connection.
• Entering Water Temp sensor input.
• Leaving Water Temp sensor input.
• Leaving Air Temp sensor input.
• Relays for PSC Blower, CT EC Blower, Water Pump, Hot Water Pump, and HWG.
• Test Button to speed up time delays.
• Communicating CV EC Blower Connection.
• Water Coil Low Temp Limit Setting (JW3-LT1 jumper for low temp/antifreeze operation).
• Factory Low Voltage Molex Connection for Unit Harness.
• Electric Heat Connection.
• Mounting screw locations.
• Compressor Discharge Temperature input.
• Entering Hot Water Temperature input.

Physical Dimensions and Layout

The DXM2.5 control board has approximate dimensions of 7" high by 5" wide. It features a standard layout with clearly labeled terminals and connectors for various inputs and outputs. The board includes mounting holes for installation.

Field-Selectable Inputs and Switches

Test Mode

Test mode allows technicians to check control operation. It runs for 30 minutes with time delays sped up 15 times. Status and Fault LEDs indicate test mode or airflow codes. The Fault LED shows the most recent fault. A flash code of 1 indicates no stored faults. The alarm relay cycles in sync with the Fault LED during Test mode to indicate the last fault. Test mode can be exited by pressing the TEST button for 3 seconds or cycling the G input three times within 60 seconds.

During Test mode, the control monitors LT1 and LT2 thermistors. If the compressor is on and sensors are misreading (e.g., LT1 colder than LT2 in cooling), the control locks out with Code 9 after 60 seconds.

Retry Mode

A slow flashing Fault LED indicates the control is attempting to recover from a fault. The DXM2.5 stages off outputs and attempts to satisfy the thermostat call. If three consecutive faults occur without satisfaction, the control enters Lockout mode.

Field Configuration Options

Jumper wires should only be clipped when power is removed from the DXM2.5. Jumper 3 (JW3) must not be clipped before adding antifreeze to the water loop (10°F protection required); otherwise, freeze damage may occur and void the warranty.

Water Coil Low Temperature Limit Setting

Jumper 3 (JW3-LT1) selects the LT1 temperature limit: 30°F (not clipped) or 10°F (clipped).

Alarm Relay Setting

Jumper 1 (JW1-AL2) selects the alarm function: AL1 connected to R (not clipped) or dry contact between AL1 and AL2 (clipped).

Jumpers (Set at Factory)

• A0-2: Configures Modulating Valve or Variable-Speed Pump (for vFlow models). Set to 0-10V for motorized valves or PWM for variable-speed pumps.

DIP Switches

DIP switches should only be moved when power is removed from the DXM2.5.

DIP Package #1 (S1)

• DIP 1.1 - Unit Performance Sentinel Disable: Enables or disables the UPS feature. (On = Enabled, Off = Disabled)
• DIP 1.2 – Compressor Relay Staging Operation: Selects compressor relay engagement for Stage-1 or Stage-2 calls, used for dual-stage units or client/server applications. If set to Stage 2, there is a 3-second on-delay for the compressor and the Alarm Relay does not cycle during test mode. (On = Stage 1, Off = Stage 2)
• DIP 1.3 – Thermostat Type (Heat/Cool): Selects Heat Pump or Heat/Cool thermostat type. Defines Y1, Y2, W1, O/W2 inputs for different modes. (On = Heat Pump, Off = Heat/Cool)
• DIP 1.4 – Thermostat Type (O/B): Selects Heat pump thermostat with O output for cooling or B output for heating. (On = HP Stat with O output, Off = HP Stat with B output)
• DIP 1.5 - Dehumidification Mode: Selects normal or Dehumidification fan mode. Fan speed adjusts for cooling in Dehumidification mode. (On = Normal Fan Mode, Off = Dehumidification Mode)
• DIP 1.6 – DDC Output at EH2: Selects DDC operation. EH2 terminal outputs the last fault code if set to DDC Output, otherwise operates as standard electric-heat output. (On = EH2 Normal, Off = DDC Output at EH2)
• DIP 1.7 – Boilerless Operation: Selects Boilerless mode where only the compressor is used for heating when LT1 is above the set temperature (DIP 1.8). If LT1 is below, Emergency Heat mode is used. (On = normal, Off = Boilerless Operation)
• DIP 1.8 – Boilerless Changeover Temperature: Selects boilerless changeover temperature setpoint. (On = 50°F, Off = 40°F)

DIP Package #2 (S2)

• DIP 2.7 – Auto Dehumidification Fan Mode or High Fan Mode: Selects fan mode. In Auto Dehumidification, fan speed adjusts during cooling if H input is active. In High fan mode, fan operates at high speed when H input is active. (On = Auto Dehumidification Mode, Off = High Fan Mode)
• DIP 2.8 – Factory Setting: Normal position is On; do not change unless instructed.

DIP Package #3 (S3)

• DIP 3.1 – Communications Configuration: Selects DXM2.5 operation as a communicating server (On, default) or client device (Off).
• DIP 3.2 – HWG Test Mode: Forces HWG pump output operation for up to 5 minutes. (On = HWG test mode, Off = Normal HWG mode)
• DIP 3.3 – HWG Temperature: Selects HWG operating setpoint. (On = 150°F, Off = 125°F)
• DIP 3.4 – HWG Status: Controls HWG operation. (On = HWG mode enabled, Off = HWG mode disabled)

Safety Features

The DXM2.5 includes several safety features to protect components:

• Anti-Short Cycle Protection: 7-minute delay for compressor.
• Random Start: 5-80 second random start delay upon power-up or after returning from Night Setback/Emergency Shutdown.
• Refrigerant Leak Detection: If a leak is detected, all outputs disengage, and the fan engages.
• Extended Compressor Operation Monitoring: Automatically turns off the compressor relay after four continuous hours of operation, waiting for the short-cycle protection time.
• Fault Retry: The Fault LED slow flashes during retry attempts. If three consecutive faults occur without satisfaction, the control enters Lockout mode. LT1 and LT2 faults are factory set for one try and enter lockout mode after the first fault.

CAUTION: Do not restart units without inspection and remedy of faulting condition. Equipment damage may occur.

Fault Codes

The DXM2.5 provides detailed fault codes indicated by the Fault LED and Status LED, often in conjunction with Test Mode. Faults are categorized as Primary (HP, LP, LT1, LT2, CO) or Secondary (Over/Under Voltage, UPS, Swapped LT1/LT2).

Lockout Mode

In Lockout mode, the Fault LED fast flashes. The compressor and fan outputs are turned off. Soft reset involves removing the thermostat call; hard reset requires disconnecting power. Fault codes are stored in non-volatile memory.

Lockout with Emergency Heat

If configured for Heat Pump mode and in Lockout, activating the W input triggers Emergency Heat mode, activating the fan and auxiliary heat outputs.

Table 3: LED and Alarm Relay Output

This table details the behavior of the Fault LED, Status LED, and Alarm Relay for various operating conditions and fault states, both with and without Test Mode active.

Fault Code Descriptions

• Fault Code 2 (High Pressure): Triggered by the High-Pressure switch opening.
• Fault Code 3 (Loss of Charge): Triggered by the Loss of Charge switch being open for 30 continuous seconds during compressor operation.
• Fault Code 4 (Water Coil Low Temperature Limit - LT1): Triggered if LT1 thermistor is below the limit for 50 seconds and rising slowly.
• Fault Code 5 (Air Coil Low Temperature Limit - LT2): Triggered if LT2 thermistor is below the limit for 50 seconds and rising slowly.
• Fault Code 6 (Condensate Overflow - CO): Triggered when the Condensate Overflow sensor detects overflow for 30 continuous seconds.
• Fault Code 7 (Over/Under Voltage Shutdown): Occurs when control voltage is outside 18VAC-31.5VAC. Self-resets if voltage returns to range. Alarm Relay closes after 15 minutes of shutdown.
• Fault Code 8 (Unit Performance Sentinel - UPS): Warns of inefficient heat pump operation (e.g., high LT2 in heating, high LT1 or low LT2 in cooling).
• Fault Code 9 (Unit Performance Test-UPT/Swapped LT1 and LT2 Thermistors): Occurs during Test mode if LT1 and LT2 thermistors are swapped or misread.
• Fault Code 10 (EC Blower Fault): Detected if EC blower feedback is low or configuration is incorrect.
• Fault Code 11 (Low Air Coil Pressure Switch): For Dedicated Dehumidification units; triggered by low-refrigerant pressure.
• Fault Code 12 (Low Air Temperature): For Dedicated Dehumidification units; triggered if LAT thermistor is below 35°F for 30 seconds during cooling/reheat/constant fan.
• Fault Code 13 (Internal Flow-Center Faults): Detects pump faults (locked-rotor, low-voltage, no flow, bad sensor) in internal flow centers.
• Fault Code 15 (RDS Sensor Fault): Indicates loss of communication with or a fault detected by the Refrigerant Detection System sensor.
• Emergency Shutdown (ESD): Activated by an external common signal to the ESD terminal, shutting down the unit.

Refrigerant Detection System Fault Codes

The Refrigerant Detection System (RDS) monitors refrigerant sensors. If refrigerant is detected above a threshold, the blower is enabled, compressor disabled, and pilot relay enabled. Faults are detailed in Table 4.

Table 4: RDS Fault Codes

Operating Descriptions

Unit Operation Descriptions

• Power Up: Unit waits for inputs and safety controls to be normal. Default anti-short cycle delay is 7 minutes.
• Standby/Fan Only: Compressor is off. Fan outputs may be active for constant fan or high-speed fan operation.
• Smart RV Control: Reversing valve (RV) relay cycles only when the thermostat calls for a heating/cooling mode change.
• Heating Stages (1, 2, 3): Control compressor, fan, and electric heat (EH1, EH2) based on demand and stage configuration. EH2 activates after 10 minutes in Stage 3.
• Emergency Heat: Fan output activated, EH1 on immediately, EH2 on after 5 minutes.
• Cooling Stages (1, 2): Control compressor, fan, and RV based on demand and stage configuration.
• Night Low Limit (NLL) Staged Heating: OVR input initiates heating, advancing stages based on NLL demand duration.

Blower Configurations

The DXM2.5 supports various blower configurations:

• No Blower: K1 becomes loop-pump relay, K2 becomes HWG-pump relay.
• 2-Speed PSC Blower: K1 is blower-enable, K2 is blower-speed.
• Constant Volume (CV) EC Communicating Blower: Direct control via communications. K1 is loop-pump, K2 is HWG-pump.
• Single-Speed PSC Blower: K1 is blower relay, K2 is HWG-pump relay.
• 2-Speed Constant Torque (CT) EC Blower: K1 is blower-enable, K2 is blower-speed.
• Constant Volume (CV) EC (PWM) Blower: Direct control via PWM signal. K1 is loop-pump, K2 is HWG-pump.
• VFD Blower: Direct control via VFD for target LAT or discrete speeds. K1 is VFD-enable relay.

Internal Flow Center [IFC] Operation

Monitors EWT, LWT, and pump feedback to maintain temperature difference across the water coil and keep LWT within limits.

Proportional Valve Operation

Monitors EWT and LWT to control a proportional valve, energizing it 60 seconds before compressor start to establish flow. Maintains temperature difference and LWT limits.

Special DXM2.5 Application Notes

These notes detail applications based on accessory relay configuration:

• Cycle with Fan: Accessory relay 1 cycles with fan.
• Cycle with Compressor: Accessory relay 2 cycles with compressor.
• Digital Night Setback: Accessory relay on when NSB input is grounded. Requires compatible thermostat.
• Mechanical Night Setback: When NSB is grounded, other inputs are ignored. OVR input can provide a 2-hour override for heating calls.
• Water Valve/Slow Opening: Accessory relay energizes 60 seconds before compressor start.
• Outside Air Damper: Accessory relay turns on with Fan Enable relay, with a 30-minute delay after NSB return.
• Humidifier: Accessory relay 2 is on when H input is active.
• Hydronic Economizer: Modifies normal cooling; accessory relay 1 activates instead of compressor output for first-stage cooling demand if H input is active.
• Dedicated Dehumidification Mode Operation: Supports cooling, cooling with CWR, and heating. Uses OVR for Low Air Coil Pressure switch and T4 for Leaving Air Temperature sensor.

Thermostat Inputs

Table 5 details how various thermostat inputs (Y1, Y2, W, O, G, NSB, OVR, ESD, H) and DIP switch settings result in specific system demands and outputs for different operating modes.

EC Blower Operation

DXM2.5 controls EC blowers directly, allowing selection of target airflow and blower-off delays. Soft Start Ramping adjusts airflow over the first 2 minutes of operation. Default operation occurs if configuration is incorrect.

Hot Water Generator Operation

Controls HWG pump via K2 relay based on S3 DIP settings and temperature inputs (T5, T6). Features include HWG setpoint selection and test mode.

Other Outputs

• Electric Heat: EH1 and EH2 activate during Heating Stage 3, Emergency Heat, and Boilerless Operation.
• Status LED: Indicates operating status (Green).
• Fault LED: Indicates current status or flashes fault codes (Red). Primary faults are retained; Secondary faults are retained only if no Primary faults exist.
• Communications: Single RS-485 port for thermostats and other controls.
• Pressure Switches: Normally closed, open upon fault.
• Condensate Sensor: Faults if impedance is less than 100,000 Ohms for 30 seconds.
• Thermistor Temperature Sensors: NTC type (LT1, LT2). Table 7 shows part numbers; Table 8 provides calibration points.

Table 8: 1% Sensor Calibration Points

Provides nominal resistance values for NTC thermistors at various temperatures.

Nominal Resistance per Temperature

Table 9: Nominal Resistant per Temperature

A comprehensive table listing nominal resistance (kOhm) for thermistors at corresponding Celsius and Fahrenheit temperatures.

Basic Troubleshooting Information/Service and Application Notes

General troubleshooting involves verifying inputs and outputs. If the board is confirmed operational, the issue lies elsewhere.

DXM2.5 Field Inputs

Conventional inputs are 24VAC from the thermostat. Verify using a voltmeter between C and Y1, Y2, W, O, G.

Sensor Inputs

Paired wires connect sensors to the board. Continuity can be checked at the board connector. Thermistor resistance should be measured with the connector removed and compared to charts.

DXM2.5 Outputs

Compressor relay is 24VAC. Alarm Relay can be 24VAC or dry contacts. Electric heat outputs are 24VDC.

Test Mode

Accessed via the TEST button for 30 minutes. Sped-up time delays aid diagnostics. Alarm Relay cycles with Fault LED to indicate the last fault. Can be exited by pressing TEST for 3 seconds or cycling G input three times within 60 seconds.

DXM2.5 Thermostat Details

• Anticipation Leakage Current: Max for Y1 is 50 mA, for W is 20 mA.
• Thermostat Signals: Y1, Y2, W, O, G have 1-second recognition. R and C are from the transformer. AL1 and AL2 from Alarm Relay. A is paralleled with compressor output for well water solenoid valves.
• Safety Listing: UL 873, CE IEC 60730.

Configuration and Advance Troubleshooting Information

A communicating thermostat or diagnostic tool is recommended for configuring advanced features and troubleshooting.

System Configuration

Factory settings can be adjusted via the communicating thermostat's Installer menu for ECM target airflows, control options, loop configuration, and field-replacement controls.

• Airflow Selection: Adjusts ECM target airflow and blower-off delays for each mode.
• ECM Airflows: Independent selections for heating/cooling stages, dehumidification, and constant fan. Minimum/maximum limits are fixed.
• Non-EC Configuration: Airflow selections are unavailable if not controlling an ECM blower.
• Heating/Cooling Off Delays: Adjustable from 0 to 255 seconds.
• Option Selection: Sets selected control options.
• LT2 Setpoint: Set to ANTIFREEZE ONLY for water-to-water units with antifreeze; otherwise, set to WATER.
• Motorized Valve: Set to ON if a motorized valve with end switch is used with a communicating thermostat; OFF otherwise.
• Unit Configuration: Selects Heat Pump Family, Unit Size, Blower Type, and Loop Type for proper operation, especially with ECM blowers.

Service Mode

Provides functions for troubleshooting: Manual Operation, Control Diagnostics, Control Configuration, and Fault History.

• Manual Operation: Bypasses normal timings to activate inputs, Test mode, ECM blower, internal flow center, or proportional valve.
• Control Diagnostics: Displays status of inputs, sensor values, voltages, and operating status/parameters.
• DIP switch Configuration: Shows current DXM2.5 configuration.
• Fault History: Stores status of inputs/outputs during faults (last five lockout conditions). Displays operating mode and time of fault.
• Fault Conditions: Options to view temperature, voltage, flow, I/O, configuration, and possible causes for stored faults.
• Clear Fault History: Clears stored fault history.

DXM2.5 Client/Server Addressing

Multiple DXM2.5 units (up to three) can be controlled by a single thermostat. Server unit is configured first. Clients are addressed by holding the TEST button. DIP switch S3-1 must be ON for the server and OFF for clients.

Functional Troubleshooting

A flowchart guides troubleshooting steps, starting from checking main power, unit startup attempts, lockout conditions, and specific operational issues like only fan or compressor running.

Troubleshooting Tables

Detailed tables list faults, possible causes, and solutions for various issues, including:

• Main power problems
• HP Fault Code 2 (High Pressure)
• LP/LOC Fault Code 3 (Low Pressure/Loss of Charge)
• LT1 Fault Code 4 (Water coil low-temperature limit)
• LT2 Fault Code 5 (Air coil low-temperature limit)
• Condensate Fault Code 6
• Over/Under Voltage Code 7
• Unit Performance Sentinel Code 8
• Swapped Thermistor Code 9
• Low Water Flow Code 13
• Leaving Water Temperature Low Code 14
• Refrigerant and RDS Code 15
• No Fault Code Shown (Unit Short Cycles, Only Fan Runs, Only Compressor Runs)
• Unit Doesn't Operate in Cooling
• Modulating Valve Troubleshooting

Performance Troubleshooting

This section addresses performance issues such as insufficient capacity, high head pressure, low suction pressure, and high humidity, providing possible causes and solutions.

• Insufficient Capacity/Not cooling or heating: Checks include dirty filters, airflow restrictions, duct leaks, refrigerant charge, metering device, reversing valve, thermostat location, unit sizing, scaling, and water temperature.
• High Head Pressure: Potential causes include airflow issues, overcharging, non-condensables, restricted metering device, water flow, and water temperature.
• Low Suction Pressure: Related to reduced airflow, water temperature, insufficient charge, or high airflow.
• Low Discharge Air Temperature in Heating: May be due to insufficient charge or poor performance.
• High Humidity: Can be caused by high airflow or an oversized unit.
• Only Compressor Runs: Checks thermostat wiring, fan motor relay/motor, and reversing valve.
• Unit Doesn't Operate in Cooling: Checks thermostat setup, wiring, and reversing valve operation.
• Modulating Valve Troubleshooting: Verifies AO-2 jumper setting, output signal voltage, and valve operation.

Revision History

Provides a log of updates made to the manual, including section changes, added content, and corrections.