Honeywell 7800 Series troubleshooting
The Honeywell 7800 Series is a family of microprocessor burner control relay modules, such as the RM7800, usually fitted with the S7800 Keyboard Display Module. On a lockout the display shows a numbered fault code, a message and the sequence step the burner was in, which together narrow down the cause.
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Try Free Boiler Diagnostic Upload a photo of the alarm screen, the fault history or a data export. InduQT compares the evidence and suggests the next useful checks.
How it reports faults
- The Keyboard Display Module shows the fault code number, a fault message and the burner sequence status at the time of the fault.
- The relay module stores the six most recent lockouts in non-volatile memory, each with the cycle count, operating hour, fault code, message and burner status.
- A lockout is not cleared by switching power off and on; it needs a reset.
- Fault codes run from 1 to 127. Codes 105–127 are mainly relay-module self-test faults shown as *Call Service*.
Operating sequence
- INITIATE / STANDBYThe module checks itself and waits for a call for heat; limits and interlocks must be proven.
- PURGECombustion air purges the furnace; the airflow interlock must be proven.
- PILOT IGN (PFEP)Pilot Flame Establishing Period: the igniter and pilot valve are energised and flame must be proven.
- MAIN IGN (MFEP)Main Flame Establishing Period: the main fuel valve opens and main flame must be proven.
- RUNThe burner fires under its modulation control with flame supervised.
- POSTPURGEAir purges the furnace after the burner shuts down.
Fault codes
| Code | Name |
|---|---|
| 13 | *Airflow Sw. On*: airflow interlock fault during STANDBY |
| 16 | *Flame-Out Timer*: no flame detected during PFEP |
| 17 | *Main Flame Fail*: main flame lost in RUN after at least 10 s |
| 19 | *Main Flame Ign.*: flame lost during MFEP or the first 10 s of RUN |
| 23 | *Airflow Switch*: airflow interlock fault during PREPURGE |
| 28 | *Pilot Flame Fail*: pilot flame failure |
| 32 | *Airflow Switch*: combustion airflow interlock fault |
Evidence to collect
- Photo of the Keyboard Display Module showing code, message and sequence step
- The fault history (six most recent lockouts)
- Relay module model and flame amplifier type
- Flame signal reading during a normal start
Work on fuel trains, burners, pressure vessels and electrical equipment must be done by qualified personnel following the manufacturer’s documentation and local codes. Never bypass a safety device or interlock.