Honeywell MS8120A1205 Retrofit-Ready Actuator for S2024 Series Control Systems
The Honeywell MS8120A1205 (S2024-2POS-SW2) is a spring return damper actuator engineered for seamless integration into legacy HVAC and building automation control systems. Whether you are replacing a failed unit on an aging air handling unit, upgrading a pneumatic-to-electric conversion project, or restoring a decommissioned control cabinet to full operation, the MS8120A1205 delivers the mechanical torque, electrical compatibility, and field-proven reliability that retrofit engineers demand.
This actuator is purpose-built for two-position (open/close) damper control within the S2024 series platform. It accepts a standard 24 VAC control signal and returns to the fail-safe closed position via an internal spring mechanism upon loss of power — a critical safety feature for smoke control, economizer, and outside air damper applications. The unit’s direct-mount bracket system is compatible with standard NEMA and ISO damper shaft configurations, eliminating the need for custom adapters in most retrofit scenarios.
Upgrade Compatibility Table
| Parameter | MS8120A1205 (S2024-2POS-SW2) | Retrofit Notes |
|---|---|---|
| Control Signal | 24 VAC, 2-position | Compatible with standard BAS relay outputs; verify transformer VA rating |
| Torque Output | 35 in-lb (4 Nm) | Confirm damper blade area and static pressure before substitution |
| Spring Return | Yes — fail-safe closed | Matches original S2024 series fail-safe logic; no relay inversion required |
| Mounting Interface | Direct-mount, 3/8″–1/2″ shaft | Use Honeywell Q618A or equivalent universal bracket for non-standard shafts |
| Auxiliary Switch | SW2 — dual SPDT end-switch | Verify BAS feedback wiring; SW2 provides open and closed position confirmation |
| Communication | Hardwired (no BACnet/Modbus) | For protocol-based control, pair with Honeywell W7750 or equivalent DDC controller |
| Power Consumption | ~4 VA running / 7 VA stall | Verify 24 VAC transformer capacity when replacing multiple actuators on one circuit |
| Replacement Compatibility | Replaces MS8103, MS8120, S2024 series variants | Confirm terminal block pinout against original wiring diagram before energizing |
| Commissioning | Manual override lever included | Use override to verify damper travel before connecting control wiring |
| Warranty | 12-Month Warranty | Covers manufacturing defects; includes pre-shipment functional test |
Retrofit Planning for Existing Automation Systems
A successful retrofit begins well before the MS8120A1205 arrives on site. Start by pulling the original control panel drawings and identifying the 24 VAC transformer feeding the actuator circuit. In older S2024 installations, a single Honeywell AT72D transformer may be powering four to six actuators simultaneously — adding the MS8120A1205’s 7 VA stall load without recalculating total VA demand is a common cause of nuisance tripping and premature transformer failure.
Next, inspect the existing terminal block wiring. The S2024-2POS-SW2 variant uses a three-terminal connection: common (C), normally open (NO), and the SW2 auxiliary switch feedback pair. If the original installation used a Honeywell MS8103A1030 or an earlier MS4109F without the SW2 option, the BAS controller’s feedback input may need to be re-mapped in the DDC program. Controllers such as the Honeywell Excel 10 W7751 or Spyder W7753 support configurable universal inputs that can accept the dry-contact feedback from the SW2 switch without hardware changes.
For projects involving a full control cabinet upgrade — replacing a pneumatic panel with a direct digital control enclosure — the MS8120A1205 is typically installed alongside a Honeywell W7750 VAV controller, a TR21 or TR24 transformer, and a Q7055 or Q7056 subbase for zone valve and damper coordination. In multi-zone AHU applications, the Honeywell Excel 500 controller or its modern equivalent provides the supervisory logic needed to sequence damper positions across multiple air handling units without conflicting control signals.
I/O expansion is another common retrofit requirement. When the existing DDC panel lacks sufficient digital output points to drive additional actuators, a Honeywell XL-15C I/O expansion module or a third-party DIN-rail relay module can be added to the control cabinet. Ensure that the expansion module’s output relay is rated for 24 VAC inductive loads — resistive-rated relays will fail prematurely when switching actuator coils.
HMI screen updates are often overlooked during actuator retrofits. If the facility uses a Honeywell EBI (Enterprise Buildings Integrator) or a third-party SCADA system, the damper status graphic linked to the original actuator’s feedback point must be re-verified after wiring the SW2 contacts. A mismatch between the graphic’s normal state and the actuator’s physical position can cause false alarms and operator confusion during the first weeks of operation.
Finally, document the as-built wiring before energizing the new actuator. Photograph the terminal block, record the transformer VA rating, and note the DDC point address assigned to the damper output. This documentation becomes essential when the next technician performs preventive maintenance or when the BAS is upgraded to a BACnet IP backbone using a Honeywell WEBs-AX or JACE 8000 controller.
Downtime Control During System Migration
Minimizing production or occupancy disruption during an actuator replacement requires a structured sequence. Begin by scheduling the changeover during a low-occupancy window — early morning or weekend — and notify the facility manager at least 48 hours in advance. Before de-energizing the circuit, use the MS8120A1205’s manual override lever to position the damper blade in the desired hold position (typically fully closed for outside air dampers, or fully open for exhaust dampers), preserving ventilation balance during the swap.
Retain the original DDC program logic unchanged during the initial commissioning phase. Do not modify setpoints, PID parameters, or sequence-of-operations until the new actuator has completed at least one full open/close cycle under BAS control and the SW2 feedback has been confirmed at the controller. This approach protects the original control logic and allows a rapid rollback if the replacement actuator reveals a wiring discrepancy.
For critical applications — hospital operating rooms, pharmaceutical cleanrooms, or data center cooling corridors — consider staging a Honeywell MS8120A1205 as a pre-tested spare on the shelf before beginning the retrofit. Pre-shipment functional testing, included with every unit we supply, confirms actuator travel, spring return force, and SW2 switch actuation before the unit leaves our warehouse. This eliminates the risk of discovering a defective replacement during a live system outage.
Once the new actuator is mounted and wired, perform a manual override test, then a BAS-commanded open/close cycle, and finally verify the SW2 feedback status at the DDC controller before returning the system to automatic mode. Total field time for a single actuator swap, including documentation, typically runs 45–90 minutes for an experienced technician.
Retrofit Support FAQ
Q: Is the MS8120A1205 a direct drop-in replacement for the MS8103A1030?
A: Mechanically and electrically, yes — both units share the same 24 VAC two-position control interface and direct-mount bracket. The key difference is the SW2 dual end-switch option on the MS8120A1205, which provides open and closed position feedback. If the original installation did not use feedback wiring, the SW2 terminals can be left unconnected without affecting actuator operation.
Q: What wiring changes are required when replacing an S2024 series actuator without an auxiliary switch?
A: No changes to the control wiring are required. Connect the 24 VAC hot and common to the C and NO terminals as per the original diagram. If your BAS controller supports position feedback, connect the SW2 normally open contact to a digital input point and configure it as a damper-open status indicator. Update the DDC graphics accordingly to avoid false fault alarms.
Q: How do I verify compatibility before ordering if I cannot locate the original model number?
A: Measure the existing actuator’s shaft clamp diameter (typically 3/8″ or 1/2″), confirm the control voltage (24 VAC), and check whether the damper is two-position or modulating. If the damper operates open/closed only and the control signal is a simple relay contact, the MS8120A1205 S2024-2POS-SW2 is the correct selection. Contact our technical team with photos of the nameplate and terminal block wiring for confirmation before ordering.
Q: What does the 12-month warranty cover, and is pre-shipment testing included?
A: Every MS8120A1205 unit undergoes a pre-shipment functional test covering actuator travel, spring return force, and SW2 switch actuation. The 12-month warranty covers manufacturing defects in materials and workmanship from the date of shipment. It does not cover damage resulting from incorrect wiring, voltage mismatch, or mechanical overload. Warranty claims are processed within 5 business days upon receipt of the defective unit.
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