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I inherited an old gravity fed central heating system when I just moved. Its weird but the top up tank fills with hot water from an overflow pipe (not the cold feed controlled by a ball valve). This can't be right but I can't work out why this is happening. This makes the converted attic as hot and humid as a tropical rainforest and we have the constant sound of trickling water! Any ideas whats causing hot water from the system to constantly overflow and can anyone suggest how to fix the problem. Thanks in advance if you are able to help.

Obviously not there to see exactly what set up you have but the pipe you describe as over flow, sounds like the vent pipe which goes to feed and expansion tank via the boiler and is installed to vent the system if it over heats. You can go to www.socket2em.co.uk and click on "my plumbing" to see drawings etc.
However if you have a pump on the circuit, in some cases having the setting too high can cause water to pump out the vent especially if small no of rads.

Need to know more about your system to give definitive answer
Is your boiler solid fuel, oil or gas.?

Could be one of many things, but it sounds like the boiler stat is not working properly & making the system to hot. When this happens it will send the over heated system water up the expansion pipe into the loft top up tank.
Is the system very noisy & is the hot water taps giving out scolding water?. If so! get a boiler engineer to change the boiler stat.

J4mes ,
If you have tried turning the pump speed down and the overflow still occurs try increasing the height of the invert on the vent pipe.
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