potteron pipe connection diagramme?

hi
i have a potterton floor standing boiler about 20 years old. i am upgrading to one of the new boilers what i can't work out beside the flow and return pipe i have 2 15mm pipe one is going to the otherside of the boiler and one feeds throgh a side of a 2 way valve the heating was pumped and water gravity feed. what are the 2 extra 15mm pipes? hope someone knows.
thanks
whok