This simple bypass is made with three ball valves. Flow is from the right in the picture.
The picture shows the unit in "Service" position. Valves on the left and the right are open; center valve is closed. Water flows through the filter.
To bypass, close the right and left valves and open the center. Water flows around the filter.
The bypass will work without the right or the left valve, but it's best to have both because it allows isolation of the filter completely from the house plumbing if desired.
The filter is protected from freezing in winter with insulation and a moveable box.
This filter is not pretty these days, but it has been in continuous trouple free service for over 10 years at this location.
(whole house & well units)