Many of these units have flow restrictors to establish an ideal recovery rate. If the water pressure is to high - it will run through heat exchanger to fast and enough heat / energy extraction is not achieved. ( unit will not heat the water efficiently)
A consideration also has to be made to...
Step 1 is to determine the quality of water in the home so you can identify the correct method or chemical. If your on city water , a simple flush should suffice, perhaps a hydostatic flush but a check of the pressure rating would have to be taken and followed.
If your on a private system-...