Discuss Mitsubishi Ecodan 8.5KW one zone gone offline and missing from main controller in the Plumbing Zone area at PlumbersForums.net
It is unlikely that a service engineer has disabled a heating zone. You can only disable zone 4 and above with a 8.5KW Ecodan. The base configuration is 3 zones - 2 heat zones and 1 dhw into the The Main Board (FTC2). If you have one heat zone functioning, test the second zone by changing over J3 J4 (thermostats) and or J9 J10 (zone valves). If it is the board that has failed, I would involve MHI, it is an expensive item to replace and is electronically matched to the serial number of the ASHP. If it is the external units that have failed it is a simple like for like replacement.
I am sure that you are aware that zone valves for an ASHP need to be fast acting and are not configured in the way you would for a gas or oil fired system. The thermostat and associated zone valve are wired independently into FTC2 or any associated extension module. This is in part to ensure that the zone valve is confirmed to be fully open before the ASHP is activated.
During any troubleshooting on the problematic zone, ensure that you do not trigger the ASHP will all zone valves closed. You can inadvertently do a lot of damage to the heat pump in a few minutes.
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