Is this a tankless system? If so, then this won't help.
If it is running all of the time and not overflowing, it is one of two things:
- the valve is not shutting off, and water is getting too high and running down the overflow tube. Check this by taking the tank cover off and looking. If this is the problem, replace the fill valve (cheap and easy - less than $10).
- or, the flapper valve is not sealing. This could be that it is worn out, the chain is caught and holding it up, or the seat is dirty or corroded. Again, replacing the flapper valve with the appropriate new one usually stops it.