Honeywell RM7800 Series
Burner Control Guide:
Select, Wire, Troubleshoot

The RM7890 is the most widely specified relay module in the 7800 series — a microprocessor-based on/off primary burner control for automatically fired gas, oil, or combination fuel single-burner applications. It replaced the RA890 Protectorelay® as the standard for commercial and industrial burner control, and it's the model most technicians mean when they say "RM7800."

The RM7890 includes five LED indicators showing burner sequence status, dynamic self-check logic, and nonvolatile memory that retains the last six lockout histories after power loss. It requires a plug-in flame amplifier, Q7800 subbase, and an S7800A Keyboard Display Module (KDM included with most suffix configurations).

The RM7890B variant adds a Valve Proving System (VPS) feature — requires the S7800A1142 KDM to program. Common part numbers include RM7890A1015, RM7890B1014, RM7890C1005.

Choose the RM7890 when: You need a standard on/off primary control for a gas, oil, or combination burner, you're replacing an RA890F/G/H Protectorelay, or you need the most commonly stocked and serviced relay module in the 7800 family.

The RM7895 extends the RM7890 with two critical additions: a dedicated blower motor output terminal and a plug-in ST7800A purge timer card for selectable pre-purge time. This makes it the correct choice when the application or local code requires a specific purge duration before ignition — typical for larger commercial boilers, process burners, and applications governed by NFPA 85.

Purge time is set by selecting the appropriate ST7800A purge timer card, which plugs into the relay module. Available purge times range from 0 seconds to 10 minutes depending on the card selected. The RM7895 provides a higher level of safety certification than the RM7890 due to the proven pre-purge cycle.

Choose the RM7895 when: Your application requires configurable pre-purge time, you have a power burner with a dedicated blower motor output, your insurance or code requirement specifies a purge-proven control, or you're upgrading from an RM7890 and need the additional safety margin.

The RM7840 is designed for modulating burner applications — where firing rate varies based on load demand rather than simple on/off control. It provides full modulation control, selectable pilot configurations (interrupted or intermittent), and all the safety supervision features of the RM7890/7895 series, with the additional outputs and logic required to command a modulating valve or actuator.

Common in large commercial boilers, industrial process burners, and any application where precise heat output control and fuel efficiency at varying loads are required. The RM7840L variant includes intermittent pilot control; the RM7840G uses interrupted pilot sequencing.

Choose the RM7840 when: Your burner operates at variable firing rates, you're controlling a large commercial boiler or industrial process heater with modulating capability, or your system uses a firing rate controller that requires modulation outputs from the burner management system.

The RM7850 combines modulating burner control with an energy-saving purge feature — the burner purges at low fire (not high fire) — reducing the volume of heated air expelled from the firebox during purge while maintaining safety. This reduces the amount of heated air pushed out of the firebox during purge, improving overall system efficiency in high-cycle applications.

The RM7850 is typically found in larger commercial and light industrial applications where energy savings from reduced purge losses are measurable. It requires a flame amplifier, Q7800 subbase, and the S7800A KDM.

Choose the RM7850 when: You need modulating control with the efficiency benefit of a flame-supervised (energy-saving) purge strategy, particularly in high-cycling systems where repeated purge events represent meaningful heat loss.

The RM7838 is a semi-automatic primary burner control — designed for burners that use a manually lit pilot rather than an automatic igniter. The operator manually lights the pilot, the RM7838 proves the pilot flame via the amplifier, then automatically opens the main fuel valve and supervises the main flame for safe operation. If main flame is lost, the control shuts down and locks out.

The RM7838B adds Valve Proving System (VPS) capability. Common in older industrial installations, industrial ovens, kilns, and some process applications where a manual pilot procedure is part of the established operating sequence.

Choose the RM7838 when: Your burner uses a manually lit pilot, you're replacing an older semi-automatic control, or your process application uses a manual startup sequence with automatic flame supervision during run mode.

The base RM7800 designation covers the original modulating relay module variants: RM7800E (energy-saving purge), RM7800G (modulating, interrupted pilot), RM7800L (modulating, intermittent pilot — most common), and RM7800M (on/off, intermittent pilot). These are the foundation models from which the broader 7800 series expanded.

The RM7800L1012 is one of the most commonly encountered base models in field service — it covers modulating, intermittent pilot applications and ships with the S7800A1001 KDM. The RM7800L is the most field-common base model for modulating intermittent-pilot burner applications.

Choose an RM7800 base model when: Your application specifies a specific RM7800E/G/L/M suffix, you're doing a like-for-like replacement on an existing installation, or your burner OEM specifies one of these variants by model number.