Sounds like there is as Big Pete suggested something wrong with the return system.
It certainly sounds like there should be:
some sort of thermostatically controlled circulating pump in the system which could have been removed or never installed
the system may not be vented properly,
the return line may be blocked or the check valve used to prevent the circulating water from flowing back into the feed system if fed from the domestic hot water system is either leaking, missing, installed backwards or again never installed.
It should also be noted that if water to the purifier is gravity supplied from an separate header tank a check valve would not required. If the water is supplied from a header tank I would check to see if the return line is properly piped into the header tank and not into the feed pipe to the purifier
I've come across similar issues on a domestic hot water return system in the past and many cases it was either an issue with the pump or the check valve.
Troubleshooting 101 "Don't over think it - K.I.S.S. it"