We have had a ClimateMaster TZ-048 system since 2013 that works great when it works, but has been plagued by periodic problems. Some seem to be related to configuration, for example back in 2014 the system was cutting off in cold weather because the installer hadn't properly configured for antifreeze operation (involves cutting a jumper on the DXM2 board). They corrected that. Then later we had issues with aux heat never coming on and they corrected that. Finally earlier this summer, the system died and it was due to sloppy insulation on an internal refrigerant line that caused it to sweat and drip on the DXM2 board, eventually corroding it and messing things up. The board was replaced under warranty but we had to pay for labor. Last weekend, we had extreme cold weather here in my part of VA with overnight temps down to -5F. No surprise, we woke up to inside temps about 4 deg lower than thermostat set point and the system running non-stop, but it never caught up and eventually shut down by itself. I switched on emergency heat and stoked up our fireplace and slowly warmed up. But it made me wonder, if emergency heat worked (about +20F increase in discharge air temperature over interior temperature), why the heck didn't aux heat have a similar effect? I would have though the system went to stage 3 and triggered aux if it was running non-stop. Later in the day when temps warmed up to about 15F, I turned off emergency heat, power cycled the Geo unit to clear faults, and watched it come back online. It started heating at stage two right away (two zones calling for heat) and outputting 87F discharge air (inside of house was about 67F at that point) and then went to stage 3 (aux) 10 minutes later, and discharge air temp went up to about 115F. The house heated up quickly and I'd say the system behaved normally at that point. Some questions: I put the DXM2 into test mode and it flashed an error code 10, which is an ECM error. I believe this may have been an old fault, related to when the tech installed the new DXM2 board this summer and didn't initially set the board properly for our variable speed ECM blower. I may order one of the ACDU02 service tools so that I can check the entire fault history (with dates/times) and see for myself. Does anyone know if the ACDU02 can be hooked up permanently so that I can easily monitor the system whenever I want? Or is it meant for limited, periodic use? I'd love to mount it next to my zone controller so that I can check data whenever I want to. Was the behavior of the system simply the result of the extreme cold weather? In my mind, I can envision that our house was probably losing heat faster than the geo system could replace it, and we drove the ground loop temp down so low that the geo system wasn't able to make heat (i.e., reject cold) anymore. Eventually it may have shut down if it got below the 10F loop temp limit. Would this trigger a fault code I could read on the ACDU02? Finally, I wonder if perhaps we don't have things setup right with the thermostats and zone controller (all Honeywell). It's a two-zone system with duct dampers. Right now, the zone controller decides on whether to go to stage 2 based on the number of zones calling for heat. If one zone, it's stage 1, if both zones, stage 2. And stage 3 is based on a 10 minute timer, although Honeywell isn't explicit if this is just 10 minutes of calling for heat on any stage, or 10 minutes at stage 2. I have the option to program the Honeywell zone controller to let the thermostats control all staging, rather than base stage 2 on %zones and stage 3 on a timer. Any thoughts as to what the best approach is? And would the same approach be right for cooling in summer? The thermostats are setup to handle 3Heat/2Cool, and capable of working all three heat stages and two cool stages even though that is currently ignored by the zone controller. The problem is, the thermostats are independent of each other, and I have no idea what the zone controller does if they both call for stage 1 -- is the zone controller smart enough to tell the geo unit to go to stage 2 if two zones call in for stage 1? Again, Honeywell documentation is not explicit about this. Sorry for the long post -- been thinking about this for a few days now and have had too much time to overthink it. Appreciate any help/advice you good folks can offer.