Earl,
After many years, the poly-iso insulation inside my trough began falling apart. The trough’s circulating warm water system failed to protect several areas – especially over bays 1 & 8. It was also not very energy efficient.
I added Winterguard Wet heat cables to the trough and wrapped all the plumbing with Reflectix radiant barrier “insulation” from Home Depot. I used two lengths of heat cable – one for bays 1-4 and one for bays 5-8.
The heat cable outlets are powered by a 10A rated relay, activated by an inexpensive, E-bay sourced temperature controller with a setpoint of 36F.
For added protection, the existed warm water loop was retained with a few modifications. Two additional remote temperature controller sensors were wired to the trough – one sensor each above bays 4 & 5. In the unlikely event that either heat cable should fail, the warm water system is automatically activated.
The temperature controller displays consistently indicate the cable heated trough temperatures of around 90F, even during “polar vortex” events.
So far, the only time the back-up system has been activated is during the pre-winter freeze protection system tests.
To answer your original question, 36F worked for me.