Most filter installations that leak do so at a fitting rather than at the filter, and usually because the wrong type was used or the right type was made up wrongly. There are three families in domestic filtration and each seals by a different principle, which is why the technique that works for one ruins another.
Tapered threads, which seal on the thread
National pipe thread is tapered, so the seal is made by the threads wedging together, assisted by sealant. Use thread tape wound in the direction of tightening, or a suitable pipe dope, and remember these are not meant to be tightened to a shoulder. Overtightening a tapered thread into a plastic housing port splits it, sometimes days later rather than immediately. Never use thread tape on a fitting that seals with an O ring or a washer, where it does nothing useful and can hold the face apart.
Compression fittings, which seal on the olive
A nut compresses a ring against the tube and the fitting body, and the seal is that ring rather than any thread. The tube must be cut square and clean and inserted fully to the shoulder before tightening. Tighten firmly and then stop, since overtightening crushes the olive out of shape and creates the leak it was meant to prevent. An olive is not reusable once compressed, so a joint remade after being taken apart needs a new one.
Push fit fittings, which seal on the tube surface
A stainless collet grips the tube and an O ring seals against its outer surface, which means everything depends on that surface being round, smooth and undamaged. Cut with a proper tube cutter, never scissors or side cutters, which crush the tube oval. Push past the collet until the tube seats against the internal shoulder, then pull back to confirm the grip. To release, depressurize and press the collet squarely while pulling, since pulling alone scores the tube.
Common to all three
Support the housing on a bracket rather than letting its weight hang on the fittings, because a full sump works joints loose over months. Check every joint by hand after pressurizing, and again an hour later, since fittings frequently weep only after pressure settles. Use fittings rated for the pressure and temperature, particularly on a well where the pump cycles pressure constantly. Our whole house filter guide covers the pipework layout.
Questions people ask about water filter fittings
Should I use thread tape on every fitting?
No. Only on tapered threads that seal on the thread itself. On O ring or washer sealed fittings it can hold the faces apart.
Why did my compression joint leak after remaking it?
The olive is not reusable once compressed. A joint taken apart and remade needs a new olive to seal properly.
Why do push fit connections leak?
Almost always the tube: cut out of square, crushed or scratched where the O ring seals against its outer surface.
When should I check the joints?
By hand right after pressurizing and again an hour later, since fittings often weep only once pressure has settled.