sounds as though the fault is with this new thermostat.
If you turn the stat up to it's max setting does the heating come on within 5 minutes? Some of the newer stats have a delay period before they send a signal to turn on. (to reduce cycling)
You could also try using a new set of batteries (not rechargable). I have had the odd set come nearly flat that work for an hour or so then cause the faults you describe.
If you turn the stat up to it's max setting does the heating come on within 5 minutes? Some of the newer stats have a delay period before they send a signal to turn on. (to reduce cycling)
You could also try using a new set of batteries (not rechargable). I have had the odd set come nearly flat that work for an hour or so then cause the faults you describe.