Trane 3 ton XL19i, 2TWZ9036 two stage, dual compressor heat pump, TWE040 variable speed air handler, Trane residential zoning assembly ZSASSMAL0100, Master Scheduler Thermostat ZUSTATALMS10.
Had fault/defrost light on zone controller board go into a diagnostic indication mode: 10 separate 1-second flashes followed by a single 10 second period of continuous on, repeated ad infinitum. We were told to power system down, wait, then power up again to see if diagnostic cleared.
It did clear, but now we’re not getting any AC. Master controller display briefly shows compressor first stage on, but display goes off after 2 seconds and compressor never actually comes on. Our guy has us on his back burner since we “don’t really need AC at our altitude,” but told us to see if our compressors will run in test mode. They do.
Just out of curiosity, we tried heat pump heat and aux heat as well. Again no compressor action on heat pump heat. Aux heat shows as if on at master stat display but never actually gets powered up.
Our guys have only seen one zoned heat pump in their life (ours). They have no idea what the 10 1 second diagnostic flashes mean, but will “find out from Trane” one of these days. They said something about a possible stuck limit switch, but Trane says it can’t happen.
So what can make the test mode work but not the master stat? Bad stat, bad controller, bad something else? The system has been working flawlessly summer and winter for almost two years including summer highs of 96 and winters down to 15 below. (The heat pump supplies sufficient heat down to about 20 in stage 1 and 10 in stage 2). Any ideas on the 10 1 second flashes?