Fluffy - zero is in the correct arena. These systems have a dump valve to empty unit's water - often they jam or just fail. I've heard that turning them off at the power supply can often reset the system, others have had to remove the cover only to find the mechanism stuck which can be freed if so.
There's lot of info at whirlpool, as I had to look it up for my mum. Her's cost $320 for the guy to fit a new valve as well as a service.
