Could be sludge build up. Look around the outside of the house for a drain point, or look for a valve (usually around the pump & motorised valves) that you can open to drain it off. Failing that you may need to just cut into a pipe. Just be aware that the more often you drain and re-fill the system you will be contributing to sludge build up, so ensure you use an inhibitor when re-filling.
Ian, July 2009
