Devimat 550 Underfloor Heating Not Working

26 Oct 2007
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United Kingdom
My underfloor heating seems to have stopped working. Symbol to show it's below setpoint temperature comes on, however the mat doesn't heat up whether on manual or timer. It worked erratically last year and sometimes required the consumer unit switch turned on and off for it start heating up. However this time that doesn't seem to work. I have checked to see whether there is current going to the mat from the controller but there isn't. Is a faulty controller the likely problem?
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Having to cycle the consumer unit to get it going sounds odd. Is the underfloor on its own circuit? If so it might be the circuit breaker that's iffy, or the MCB connection could be loose - erratic operation as described often points to loose connections.

Could also be a faulty controller - are you able to test whether power is reaching the controller (screen could be operating on battery backup)?

Lets go back to basics.

Check voltage into the DEVI controller. 230v AC is what you need!

Set to manual and wind up the temperature. Wait for click. Do you now get 230v out to the element.

Get ohm meter.
Check resistance of mat (result will depend on mat size, somewhere around 100 Ω.)
Chweck resistance of floor probe. That will be in the kilohm range.

Compare the resistances against the values provided by the installer when the mat was laid (you have got those, havent you? :cry: )
or post the results back on here.
230v into Controller. Switched to manual, click of thermostate but no current out to element. Left for 24hrs and never any current. I don't have the installer values as it was installed before I moved in. However the controller doesn't show any error codes when operating. I can supply resistance value if they are relevant without the installer values?
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Have followed the troubleshooting in the guides. Resistance values are 8.83 for sensor, 36 for mat. Seems to me that it is the controller which has failed, as thought.

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