Hot water coming from pipe outside house

This is bad for your heating systems health don't allow it to continue. In the attic there is a small water tank that is over flowing, When the water is coming go to the attic and check to see which tank is over flowing. There are a few possible causes.
1. The ball cock is set too heigh the water expands when heated so the water level when cold should be less than 1/2 way up the tank so that when hot the level doesn't reach the overflow outlet. To fix this bend down the arm, be careful I broke them more than once so make sure the mains water is off first.
2. The ballcock is letting past water all the time. Fit a new one.
3. This is very unlikely but a possibility. The coil in your copper cylinder has a leak allowing demostic hot water to enter the heating system. This is only possible if the heating system header tank overflow level is lower than the demostic cold water tank overflow level.