Sooo... A little more has come to light. My neighbor apparently left his door open all night (or potentially multiple nights). I'm not positive if it was just the second night I was having issues or both nights. Apparently, he came by to fix his thermostat (that was wired wrong) and forgot to shut his main 12'x18' garage door. Apparently, his unit got down in the 30s inside his unit. I'm not even positive that he was running any heat in his unit at all prior to the day he left the door open. The unit on the other side of my unit has not been sold and also was not heated at all. Here are some plots of the two days in question.
In addition to the heat issues, you'll also see a spike in temperature in the late afternoon each day. The thermostat is positioned just right that the sun shines through in the afternoon to hit directly on the thermostat. I don't think this happened in the summer with the higher sun and more northern sunset. The picture below is from a couple hours ahead of the sun hitting the thermostat but you get the idea.
I did a couple things to help fix things:
1. I set the thermostat down to 50 right away to at least stop the bleeding. The heat has not run again (not even once) even when it got down to 30 degrees the Friday after Thanksgiving. I'm not ready to call success yet and need to put the temperature back up to test things. Absolute temp outside is important but the relative delta inside to outside is also important.
2. Ecobee has remote sensors, so I put one on the back wall. I also set up a custom "comfort setting" call AwayAvoidSun that
only uses the sensor on the back wall. I run the thermostat on the Away setting all day/night averaging the two sensors and then run the AwayAvoidSun setting with just the sensor on the back wall from 1pm to 5pm. So far, that looks like it is fixing the temperature spike but I haven't had a really sunny day yet to make sure.
3. I contacted the builders/HVAC contractors and asked them to check my settings. The HVAC installers put in the heat pump and backup electrical resistence heater with a generic thermostat and a later installer set up the Ecobee. Right now, the Ecobee turns off the heat pump when the external temp gets below 35 degees and it is the resistive heating alone after that. The heat pump is notionally a 3.5 ton unit but heat pumps very in output based on the external temperature. It puts out over 50 MBh at 65F, 40 MBh at 40F, and only 31 MBh at 35. The resistance heater is a 15kW heater that is equivalent to about 51 MBh. I want the HVAC folks to let me know if the system should be working in series with the 35 cutoff, if the cutoff should be different, or if the two should be working in parallel at lower temperatures. It appears that the heat pump can actually produce heat with a COP over 1 (more heat the energy expended) all the way down to -5F. Mind you it only produce 12 MBh at -5F at a COP of 1.45. Still, addting 12MBH to the 51MBH is still additional heat capability and as I understand it - with resistive heating anything with a COP over 1 is more efficient. (Again - no expert here.) I just don't know if the system can handle being used in parallel.
4. It appears this has all gotten the attention of the builders who have realized that the unsold units need to be heated or else the sprinkler systems could freeze. They are currently running electric and installing furnaces in the unsold units. If the unsold unit next to me is at least kept at 40 degrees all winter that should at least help.
I'm not sure all is fixed but it is a start. I want the builders to get the units next to mine heated and then I'm going to turn the heat up some to see if I can get it to maintain 68 degrees in the winter if I want. I do not expect to keep it that warm all the time but I would like to be able to go down there in the winter and use the balcony space for a party or social event.