Diagnose and Rectify Intermittent Ducted Heating / Controller Issue

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• Heating Professionals Quote Request #1694567

Adam requested 3 quotes from Heating Professionals in St Kilda East

  The thermostat controller (T8602D Chronotherm IV) for our ducted gas heating appears to have an intermittent issue where despite the flame symbol indicating that the heating should be on (with the temperature setpoint above that indicated by the actual temperature sensor), the actual heating unit and fan does not come on (despite the 5 minutes minimum time off for the compressor having been passed).

The problem can usually be rectified by:
1. Decreasing the temperature setpoint lower the current temperature (which causes the controller flame symbol to turn off)
2. Using the system key to set the controller to the ‘off’ selection
3. Removing the thermostat controller head unit from the backplane for 5-10 seconds then remounting
4. Using the system key to return the controller to the ‘heat’ selection, and selecting a temperature

setpoint higher than the current temperature
This will then usually result in the heater and fan coming on as desired, however after a period of time it will stop working again and does not switch back on (unless the reset rectification process above is repeated again). Strangely the issue does not usually occur during the day and mostly occurs in the evenings.

The attached photo shows the wiring hookup to the controller backplane, with only the ‘W’ (heat relay) and ‘R’ terminals connected, and no wire on the ‘G’ (fan relay) terminal. From this wiring, and the controller fan operation (setup #2 setting indicated in table 2 below), it appears that the heating unit itself should control the fan operation. As such, I wonder whether it’s possible that the issue could be that the heating unit in the roof cavity is not correctly commanding the fan to operate as expected. However this doesn’t make sense given that the rectification process of resetting the controller, (as described above), usually fixes the issue, at least temporarily.

Appreciate any proposal to diagnose and resolve this issue. I have interrogated the controller settings with results attached together with the T8602D thermostat instructions.