MCZ Hydro stoves - External Thermostat problems or unusual operational characteristics

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This article relates to any of the MCZ Active System Hydro stoves that have the external interface panel as seen below (found at rear of boiler). These models include Ego, Musa, Suite, Club and Star:

Rear interface panel of MCZ Active Hydro Stoves

If the stove is installed and set up to operate in response an external thermostat (see User Manual PP 51-52; also, article Connecting to an S-Plan or Y-Plan system), the stove will respond to a call for heating and fire to deliver the heat energy required. If it is noted once installed, that the boiler does not appear to fire when heat is called for, the following checks should be made:

1. Ensure that the 'External Thermostat' function has been enabled (within Settings Menu, see User Manual P 52).

2. The 'Auto Eco' function Must be enabled for the stove to correctly respond to external thermostat calls. From the stove first switching on and completing an initial heat up cycle to attain the set target boiler temperature (Set H2O), when a call for heat is received the stove will modulate back to high power if it is at minimum fire, or will commence ignition if Auto Eco ran for the set time and it then shutdown into standby. Note that there will be a 5 minute delay after the stove completes shutdown before commencing reignition in response to a new call for heat.

3. Ensure that the external wiring to the 'Therm' terminals (see picture above) is producing a call for heat. This can be done by checking for continuity across the two wires from the external thermostat wiring, however NOTE: The input to the terminals for the external wiring must Not be mains voltage: A clean, volt-free contact is required. A further check can be made by removing the external wiring and inserting a simple wire jumper across the 'Therm' terminals. This should cause the boiler to respond (or at least cause 'TOn' to be displayed on the control panel - see below).
Further observations can be made via the control panel. When an external thermostat calls for heat, the control panel will display 'T on'. Once this call is removed, the display will show 'T off': Again, this will only be the case provided that 'External Thermostat' is enabled.

If after the above checks the stove still fails to respond to any external call, the problem may lie internally. The interface panel (which has a mini PCB attached to its rear) connects to the Motherboard via a serial ribbon cable: The fault may lie with one of these two components. To check if the cable is at fault:

  • Switch off and electrically isolate the stove.
  • Remove the right side panel for access.
  • Locate the rear of the interface panel and the ribbon cable attached to it that goes to the motherboard.
  • Detach the cable from the motherboard, then refer to the pictures below - the orientation from the rear of the boiler will be inverted:
Ribbon cable connecting rear interface to Motherboard

Pin locations on ribbon Cable for External Thermostat

These pictures identify the two pins on the cable that connect to the two 'Therm' terminals. Place a wire jumper across the 'Therm' terminals, then check for continuity across the corresponding cable terminals. If there is no continuity, the cable is at fault and requires replacement. If however there is continuity, it is likely the interface panel has failed and that will need replacing.

Unusual operational characteristics

With regards to MCZ Hydro stoves installed on S-Plan systems - there have been several reports of installations using an older version of the S-Plan wiring solution than is currently described in article Connecting to an S-Plan or Y-Plan system, where the stove has exhibited unusual operating characteristics, incorrect information displayed on the control panel, possibly not responding to calls for heat, etc.

In this older version of the wiring, a relay was not used in the circuit between the microswitches on the motorised valves, and the 'Therm' terminals on the rear of the stove: Instead, the microswitches (grey and Orange wires) linked directly to the stove, thus in themselves providing the volt-free closed contact to generate a call for heat.

It is believed (although not proved conclusively) that certain types of motorised valves may be inducing voltage into the volt-free micro switch circuits, which in turn produce the unusual characteristics described above. If this does seem to occur, the solution is to instead use a relay (greys becomes perm. live and oranges provides power to relay coil) as seen on the current wiring diagrams.