Honeywell Prepurge timer module ST7800A1013/U

The Honeywell ST7800A1013/U is a plug-in prepurge timer module designed for use with Honeywell 7800 Series relay modules. It provides the prepurge timing of 7 seconds for the burner control system. This module is installed into the relay module compartment to ensure proper prepurge sequencing before burner ignition.

Parts of this type are selected by matching the prepurge timing requirement of the burner control system and ensuring compatibility with the specific relay module series. Installation involves disconnecting power, removing the keyboard display module, dust cover, data bus module, or remote reset module, and then pulling out the existing relay module if present. The new timer card must be set to the desired time period, aligned correctly with the slot, and inserted into the relay module compartment. After reattaching the removed components, power is restored and the burner system is run through a complete cycle to verify operation.

When items of this kind fail, the burner may not complete the prepurge cycle, leading to a lockout or failure to ignite. Symptoms can include the burner not starting, repeated reset attempts, or error indications on the control display. To diagnose, check the timer module for proper seating, visible damage, or incorrect timing settings, and verify that the relay module is functioning correctly.