John Keiser wrote:

As far as lasting "forever", I should have added that I had to replace the
Grundflos circulating pump after 30 years [bearing failure] and the delta
temperature control unit after 25 years [triac failure]. No complaints
since these components gave good daily service for decades - but not
"forever."

Our system was apparently installed in the early 1980s and is still
all original except for the water tank. The March pump needed a new
impeller and the Delta controller still works, although the screws
holding its circuit board came out completely because of vibration.
The controller was built with 100% generic parts and is so simple I
was able to make a copy it. I don't remember there being a triac in
it (freeze protection is through recirculation), but triacs are cheap.