Honeywell ST699 Common Problems

When it comes to managing a hot water tank and gas central heating system, having a reliable thermostat and control unit is crucial. The Honeywell ST699 is one such device that many homeowners rely on. However, like all electronic devices, it is not without its quirks and problems. Let’s delve into a common issue and its possible solutions.

The Problem:

The Honeywell ST699 system generally works fine when calling for either hot water or central heating. However, users have reported a glitch where if the system calls for both hot water and central heating simultaneously, the ST699 unit appears to malfunction. Specifically:

1. The digital display goes blank, and the orange indicator lights turn off.
2. The system doesn’t trip a circuit breaker or blow a fuse.
3. Power cycling the unit (turning it off and then back on) seems to reset it, and it works fine until the same scenario happens again.

Potential Causes and Solutions:

1. Power Supply Issues: The most common theory is that the ST699 might be losing its mains power supply, possibly due to wiring discrepancies. Users who haven’t made any recent modifications to their system should double-check where the unit gets its Live and Neutral feeds. Non-standard or faulty wiring could be the root of the problem.
2. Device Age: The Honeywell ST699, being an older model, might be reaching the end of its lifespan, especially if it’s shutting down under full load (both channels). A potential solution would be to consider replacing the device. Newer models, like the ST799, are designed with improvements and might provide more reliability.
3. Device Replacement Difficulties: For those thinking of replacing the ST699 with the newer ST799, it’s essential to know how to safely remove the old unit. Typically, there are two slits on top of the unit. By inserting a flat head screwdriver into these slits and gently pulling the panel from the top, users can release and remove the device. Additionally, there might be a tab on the bottom corner that, when pushed, allows the unit to lift off its top clips.

Conclusion:

While the Honeywell ST699 has served many users well over the years, as with all electronics, it can face issues over time. For those experiencing the described problem, checking the power supply and wiring is a good first step. If the issue persists or if the unit is old, considering an upgrade might be the best solution. Always remember, when dealing with electrical systems, safety should be the top priority. If in doubt, seeking professional assistance is always recommended.