More then likley the coils are coated with calcium deposits which are extremely difficult to remove. I cleaned mine using a full strength vinegar. You need to some how remove the chiller from your system. Then plug off the lower hole of the chiller and then fill up the coil housing with the vinegar. If you can I would let it soak overnight. The next day drain all the vinegar out of the coil housing and take the chiller outside and flush the inside of the coil housing out thoughly using a garden hose with a pressure nozel on it.
Hope this helps.
Chris