Yes, it shuts off before reaching the thermostat temp. If the thermostat is set to a higher temperature. It is very cold outside, approx. 10-30F. After it initially started shutting off without reaching the thermostat temp, I wired the thermostat input wire to constant
12v, and the furnace will run initially around 3 hours.
I did not replace the limit switch with a new one, however, I did swap it out with the one on my the old NT30 which I saved. No change. And I have heard it is rare for them to go bad. I have not wired a bypass for this yet to confirm, however.
And I already have a Dino board, that was part of the rebuild. I just purchased supplies to add a fourth vent, because I figured it can't really hurt. Afterwards, if the problem still exists I will likely use a jumper wire to confirm it is the limit switch.
I had used that manual many times, but never thought to look for airflow requirements. I am suspicious of that being the issue.
Mayco, do you have an 30k NT30SP with only 3 vent tubes?