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downstairs
You need to position it where the temperature is directly influenced by the boiler. If there are no rads downstairs it will just call and call* but never shut down because it thinks it still needs to burn because the temperature is not being controlled. All the while overheating the upstairs.
* during the period where it’s timed to call.
Try repairing it and moving it upstairs, preferably to a position where there’s no TRV on the radiator.
Let me guess though - the ‘stat is downstairs…
Imagine pretty much any basic stat will do as a replacement but you could eBay search the model number of that one if you want a like for like replacement- if it is fucked