I am able to get hotwater from my ecotec vaillant boiler but not any heatin?

I am able to get hot water but no heating from my vaillant ecotec boiler, we can press the plus and minus buttons and it will overide but then switches off again almost straight away, but then occaisionally will remain on for a few hours.. we have a wireless digistat and this is constantly running out batteries as it is trying to get the boiler to kick in to get to the set temp, but obviously nothing is happening.