hot radiators but no hot water

Hi,

I have a gulf stream 2 combi-boiler and have been experiencing problems!! i have all of a sudden stop getting hot water but my readiators are boiling hot!!! the setting on the programmer are set to \\\'all day\\\' and the switch on the boiler is set to \\\'summer\\\' there seems to be 3 pumps for my system, one of them is making a strange noise!! this one has a label written on it saying \\\'DHW\\\' and it there are 3 speed settings on the pump, 1,2 and 3. The label on this particular pump says \\\'must be set to 3\\\'

the pipes right at the bottom of the tank lead out of the hot water tank to thie long little box, which an electrical wire is going into... the out on one of the pipes is labeled \\\'to taps\\\' and this is freezing.. there is another pipe from the main stop cock going into it.. and then there are another 2 pipes coming in from the tank which is boiling hot...

any idea??? thanks for any help!!

does this "little box" look like

may be different colour

no! this little box.. is about 30 cms long and 15cm wide like a rectangle and it is made out of metal with 4 pipes going in/out of it... 2 on the far left and 2 on the far right...