Hi, I've been cleaning WFP for over a year now but only been producing my own water for last 3 months. I got given one of the old Merlins (3 filters in line) by a mate when he left cleaning. From reading around the forum I get the idea they're not the best to begin with, but this one of late seems to shoot up on TDS more frequently. I use DI to finish off but even with refilling the vessel the last week in September I checked and today its up at 008 again! So I'm looking for advice on whether any feel it's time to replace the membranes - my mate said they were put in last year - or switch to a 4040? And if I go 4040 what sort of pre-DI TDS can i expect?A few stats are: I use about 900-1200 liters a week, tap TDS is around 300-315ppm, pre-DI is about 34-45ppm. Not sure what water from outside tap is but it is about 60l per minute. Any suggestions on how to proceed are more than welcome. Apologies if a bit wordy, I'm new to all this! Thanks.