Honeywell Control box ST7800A1047/U

The Honeywell Controller ST7800A1047/U is a plug-in purge timer module designed for use with the Honeywell 7800 Series Relay Module. It provides the prepurge timing function, with a prepurge timing of 40 seconds. This module is installed into the relay module slot to control the prepurge cycle in burner systems.

Parts of this type are selected by ensuring compatibility with the specific relay module model and verifying the required prepurge timing. Installation involves disconnecting power, removing the keyboard display module, dust cover, data bus module, or remote reset module, and pulling out the existing relay module. The new timer card must be set to the desired time and inserted into the relay module slot, then all removed modules are reattached and power restored. Always run the burner through a complete cycle to verify proper operation.

When items of this kind fail, the burner may not perform the correct prepurge cycle, leading to improper operation or safety shutdowns. Symptoms can include the burner failing to start, intermittent operation, or error codes related to purge timing. To diagnose, check the timer module for correct settings and ensure it is properly seated in the relay module slot.