This is same another question but not understanding. I am trying to replace a heat pump thermostat on a fuel master plus 1-1. The heatpump has LP furnace for back up heat. It was a Lennox Thermostat and I am replacing with a Honeywell. On the Lennox I have R, V, VR, F, M, L, A, Y, X. Honeywell is E, AUX, Y, G, O/B, L, R, and C. I need help. I have a terminal cross reference but do not see where to put A. The fuel master plus has a heat relay, blower relay, k-relay, another one not sure. I don't understand.

The A is a new one on me and I have been around a long time. You or a On Site professional are going to have to track that wire down and see where it goes and what it controls then figure out where to put it.

Better make sure you call a Trane dealer. Trane likes to be different from the rest of the industry. Don't be surprised to find out that your new stat isn't compatible with your system.