Received the new (non) reset stat and tested for continuity. Likewise, tested again the manual reset stat, both OK.

Unfortunately, the alarm light (overheating thermostat) is still illuminated and the unit will not run; don't know what to look for next.

Any suggestions?

Thanks

As djh, you need to trace the cabling and elect flow all the way through the system. It could be something like a dry joint on the PCB, and it is the act of switching off/on which temporarily corrects the open circuit, rather than the footering with the resetable stat.

However, that doesn't really explain that it runs ok all summer, and then has an issue in winter? Maybe the poor contact shrinks enough in the cold to go totally open.

Wait for the next failure, and then do your continuity testing, BEFORE trying the old switch off/on to correct it.
Test from the mains IN across to the motor (as djh suggested). That should show NO continuity. Work your way back, connection by connection, until you get the continuity beep. That will pinpoint the offending connection.

Intermittent faults can be a nightmare, especially if you get called out to look at it - can't find a fault that has disappeared.

Thanks for the replies. Unfortunately, I'm not much of an electrical wizard. I can check continuity on simple things (stats?) but trying to work my from the mains input would be an different story.

I do have an illuminating voltage detector pen so can give that a try.

Whatever it is, cannot be too complex, just a matter of find it. Tend to believe that it is external temp related as last night, here is leafy Surrey, when it came on at 22:00, the outside temp was around -1C and with all my fiddling with the manual reset and stat connections, could ot get it into a mode where the control panel reset worked.

But this morning when the ambient temp was around 5C, it came on as normal without any fiddling at 06:30.

At the moment, the MVHR unit is off, so while it is 'cold' I have been to dig deeper. Turned it on and of course, it starts. Used my voltage checker and with my limited knowledge, all I can say is that within the circuit board cover, virtually every black wire registers live.

What I don't understand is that both motors are rated at 230V and when running, neither cable directly connected to the motor makes my tester flash. Motors running but seems like 0V, which is obviously not possible!

Not checking 'open' cables, just the insulated cable with a voltage checker. If there is current, the light flashes

I know you are all very keen for an update!!!!

The new, non-manual rest thermostat from Flexit has been install, as has a manual rest stat from one of China's finest manufacturers. The result to date is everything is working as it should with no alarm activations.

n the past however, it is my suspicion that the alarm was activating when the external temperature was near 0C and since fitting the new stats, the external temp has generally averaged 5C+, even in the early morning. I wait for the near 0C weather conditions to see what happens.