No HW but HW+CH works fine?

Hi

Just bought a new house but got problems with the hot water. When I switch both HW+CH everything's ok. If I just have CH then they fireup ok. However when I just switch the hot water on then the boiler comes on for a few minutes and then shuts off.

The system has a Worcester 15ri boiler and drayton 3 way valve. I've changed the valve but still no joy.

I'm no expert but these are my observations. When I switch the HW on from the programmer, the 3 way valve moves to the HW side. You can feel water flowing in/out of the valve but its cold.The boiler then comes on after a few clicks. None of the pipes to/from boiler warm up. The boiler then switches off after a 2/3 minutes.

Cheers