Do you know where your three port valve is? If so it should be straight forward. If the valve is not close to the time switch you may need help.
Turn your heating on for a minute or two, then off at the time clock if the valve does not move, you will see a lever which will move if the valve is, then that is where your problem lies. You can try moving the lever to see if your valve has got stuck over the summer months.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!