Honeywell RM7850 Relay Modules: RM7850A1027

Technical Dossier

Product Details And Specifications

Honeywell RM7850 Series: Comprehensive Module Range and Technical Overview

The Honeywell RM7850 series is a family of microprocessor-based, self-checking primary controls designed for industrial burner management systems (BMS). Deployed across heavy-process industries including petrochemical refineries, nuclear auxiliary boiler systems, offshore platforms, and large-scale district heating plants, the RM7850 has established itself as a reference-class relay module for forced-draft and induced-draft burner applications. Its integrated self-diagnostics, lockout memory, and configurable timing parameters make it a preferred replacement for electromechanical relay sequences in safety-critical combustion loops. The series operates on 120 VAC or 230 VAC supply rails and interfaces directly with Honeywell's Q7800 Series Subbase, enabling modular field replacement without rewiring the burner control panel.

The Evolution of RM7850 Architecture

The RM7850 series succeeded the earlier R7847/R7848 relay-based primary controls, introducing microprocessor supervision in place of discrete timing relays. Early production variants (circa late 1990s) established the core Q7800 subbase interface standard, which has remained backward-compatible across all subsequent RM7850 revisions. Mid-generation variants introduced extended postpurge timing options (10 s, 15 s, 30 s) and expanded the ignition trial period (ITP) configurability from a fixed 4-second window to selectable 4/10-second modes. Later variants added enhanced diagnostic LED arrays and expanded lockout history registers accessible via the S7800A1142 Keyboard Display Module (KDM). The RM7850 series is now in its mature/end-of-active-production phase; Honeywell has transitioned new installations toward the RM7890 and RM7895 series with expanded fieldbus communication options. However, the RM7850 remains the dominant installed base in facilities commissioned between 1998 and 2015, and long-term maintenance support for this series remains commercially critical.

RM7850 Full Catalog & Functionalities (SKU List)

The following SKUs represent the verified RM7850 production range, classified by timing configuration and supply voltage. Each model mounts on the Q7800A/B/C subbase and is not interchangeable with non-Q7800 wiring bases.

RM7850A1027: 120 VAC, 50/60 Hz, 30-second postpurge, 4-second ITP — standard North American utility boiler configuration.
RM7850A1035: 120 VAC, 50/60 Hz, 15-second postpurge, 4-second ITP — compact boiler and process heater variant.
RM7850A1043: 120 VAC, 50/60 Hz, 10-second postpurge, 4-second ITP — fast-cycle industrial burner application.
RM7850B1000: 120 VAC, 50/60 Hz, 30-second postpurge, 10-second ITP — extended ignition trial for heavy-oil and residual-fuel burners.
RM7850B1018: 120 VAC, 50/60 Hz, 15-second postpurge, 10-second ITP — mid-range ITP variant for distillate fuel systems.
RM7850B1026: 120 VAC, 50/60 Hz, 10-second postpurge, 10-second ITP — short-cycle variant with extended ITP.
RM7850C1000: 230 VAC, 50/60 Hz, 30-second postpurge, 4-second ITP — European/international utility boiler standard.
RM7850C1018: 230 VAC, 50/60 Hz, 15-second postpurge, 4-second ITP — European process heater variant.
RM7850C1026: 230 VAC, 50/60 Hz, 10-second postpurge, 4-second ITP — European fast-cycle configuration.
RM7850D1000: 230 VAC, 50/60 Hz, 30-second postpurge, 10-second ITP — European heavy-fuel burner variant.
RM7850D1018: 230 VAC, 50/60 Hz, 15-second postpurge, 10-second ITP — European mid-range ITP configuration.
RM7850D1026: 230 VAC, 50/60 Hz, 10-second postpurge, 10-second ITP — European short-cycle, extended ITP.
RM7850E1000: 120 VAC, 60 Hz, 30-second postpurge, 4-second ITP, with expanded diagnostic output — North American enhanced-diagnostics variant.
RM7850G1000: 120 VAC, 50/60 Hz, 30-second postpurge, 4-second ITP, CE-marked — dual-market compliance variant.
RM7850L1018: 120 VAC, 50/60 Hz, 15-second postpurge, 4-second ITP, low-fire hold output — modulating burner control variant.
RM7850L1026: 120 VAC, 50/60 Hz, 10-second postpurge, 4-second ITP, low-fire hold output — modulating burner, short postpurge.

Sourcing Hard-to-Find & Obsolete RM7850 Parts

Quality Control for the RM7850 Range

Each RM7850 unit processed by DriveKNMS undergoes a structured functional verification protocol specific to microprocessor-based burner primary controls. The test sequence includes: supply voltage tolerance verification at ±10% of rated input; flame signal amplifier gain check using a calibrated UV/IR signal source; lockout relay pull-in and drop-out timing measurement against published specification windows; postpurge timer accuracy verification across the full configured duration; and self-diagnostic cycle completion confirmation with no fault codes logged. Modules with Q7800 subbase connector pins are inspected for contact wear, oxidation, and mechanical deformation. Any module exhibiting timing drift exceeding ±5% of specification or failing the self-check sequence is quarantined and not released for sale. Test records are retained for a minimum of 24 months and are available to customers upon request.