fault finding on CH roomstat or 3 way mid positin valve

hi
just fault finding on a friends CH system and looking for some advice.
Is it possible to by pass the roomstat to confirm that its the 3 way valve that is faulty rather than the room stat.
symptons : hot water but no central heating, pump is working and obviously once the 3 way valve is pushed to manual and left there all is ok.
just wanted to confirm really that its not the roomstat.
Whats the best plan of attack in these cases please - your advice sort.
thanks dunc
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Remove the cover of the 'stat and short over the two wires that would control the system. A piece of metal conductor will do, or disconnect one wire and fix it in with the other one. REMEMBER TO TURN THE POWER OFF !!! before you do this test. Once you've fixed the two wire together and not through the switch gear in the 'stat, then you can turn the power back on and carry out the test.
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