Rads working irratically, cold downstairs while hot upstairs

[tiggerjr82]

OK I have a vaillant boiler, not sure of the model, its not written on there! It is a condensing combi boiler, about 4-5yrs old i think

The other day the heating was on upstairs and down but I had no hot water even tho the hot tap was on on the boiler settings. I turned the rads off on the boiler and i got hot water. Now, I get rads upstairs red hot but cold downstairs, then after a day they came on downstairs too. They are controlled by thermostatic valves and all are turned on - why would downstairs not come on when its soooo cold and then apparently work fine later?

Both taps and rads on the boiler are set to be on continuously

Thanks

 

[Agile]

It sounds as if the diverter valve may be sticking!

Tony

 

[tiggerjr82]

I see, how would I fix that?