MK3s Upgrade and Thermal Runaway - with my personal solution
So, I had a problem with this for over a week. After a lot of reading and reflection I finally figured it out. Posting here for anyone else that might find themselves in the same predicament that I had.
I upgraded both my MK2.5 and MK3 to the new "s" upgrade. On both printers I was also switching to the new tilted print fan for the first time. Both upgrades went well, but when printing the benchy and another model, my MK2.5S would always error out with a thermal runaway somewhere along the way. Particularly when a model reached a large flat point well above the heat bed. The MK3S acted somewhat similarly, but not quite as bad. I could not get a full model out without a thermal runaway error. WHAT??? Never happened before. What gives?
Tried this, tried, that no luck. Then I began to wonder... Both printers had problems in the past (one tumor, one broken heat break) that eventually led me to replace the entire hot end in both printers many months ago. Living in the US and not wanting to wait on Prusa, I bought genuine E3E v6 hotends from Amazon. Both came with a heater that had blue wires rather than the red ones that were original the the printer. Figured so what, blue wires, schmoo wires. Both printers did just fine with the new hot ends for months, so nothing to worry about.
After trying everything else, I did more research online and discovered that the "genuine E3D v6" hot ends are configured with 30 watt heaters now. The blue wires meant that I had 30 watt heaters in my replacement hot ends. The Prusa machines are sent with 40 watt heaters. I just swapped out the 30 watt heaters for 40 watt heaters and all of my problems are solved.
So, if you are getting thermal runaway errors because the heater cannot keep up with the print fan cooling, make sure you have 40 watt heaters. Even though the 30 watts heaters did just fine in both printers before the upgrade, they do not do so well with the improved cooling of the new design.