I read the reviews on Amazon and some of the posts on HVAC talk. I was curious to see if there it is a design problem, or people complaining about the interface being to difficult to use.

It looks like to me there is a design problem. The heat generated by the device (you can't call these things thermostats in my opinion) is causing the temperature reading errors. I also think the communication system is not very robust and prone to errors. The newest software may have fixed this. It doesn't look good when Trane can deliver a device that has to do simple communication.

The most important function a thermostat must do correctly is read the ambient temperature. If it can't do that then it is worthless in my opinion. If it were me, I would have serious reservations about buying the XL950. I recommend either getting a Trane system that does not require the XL950, or looking at similar equipment from Carrier or other manufacturer.

We had a 950 installed with a new system two years ago, and it took several visits, almost a year, three thermostat units, and (finally) an external sensor to get it working properly. I had no idea it was a widespread problem.

We moved and are having the same system put in, but going with the 850 this time. Our Honeywells won't work because the Trane is a communicating system. Though now I wish we had read more reviews. There was nothing about it when we first got it, because it had just come out. We thought our experience at the previous house was an anomoly.

ETA: Before getting the external sensor (a little bigger than a quarter, attached to wall maybe 12 inches from the tstat), we ran a fan pointed at the 950 all the time. The external sensor was the better fix, but still not a choice I'd knowingly make. We're using the same HVAC company as last time, and if the 850 has the same problem, I will have them switch out the system for a non-communicating one. We will not dance around the issue a second time!

Thank you all. @mike_home, yes I agree it certainly seems like XL950 has a design problem, and that it fails (often) to do the basic and most critical thing ... accurately read ambient temp. @pipiep Keep us posted please on exp. with XL850, which I have read less about, but it seems like it is basically same design with a smaller (less heat producing) screen; nonetheless, not sure it will be much better. Do you recall approximate cost of the remote temp sensor? Have you inquired whether the Trane equipment you are getting will work with a non-Trane Tstat such as Ecobee, or Honeywell - that is something I am going to explore.

I don't want to rule out Trane, as many (but not all) of the pros on this forum seem to favor Trane (Carrier) over Lennox. I have good feeling from long face-to-face meeting with Trane and Lennox dealer/designer, but less so for Carrier.

@Air and energy Appreciate your comment which really is not completely on topic of the thermostat, yet still of some help. Surprised at your concern about the replacement part costs as for 10 years these would be covered by Trane parts warranty, right? Of course one is on the hook for labor unless you buy the extended labor warranty which is by a 3rd party insurer and most on this forum have serious concern about whether that company would really be there when you need them to pay.

from OP and what I have learned speaking with Trane installer:

1) Only Trane XL950 or 850 will be fully communicating with heat pump and air handler. One can use a Trane relay and use any 3rd party 24v Tstat, but will loose the fully communicating functionality.

2) Remote temp sensor for 950 or 850 equipment cost is about $45 and very little to run the small wire a foot or two from the unit. If I end up doing Trane, and given some of the reported horror stories on forums about the 950/850 doing very poor job of reading ambient temperature, personally I will run the wire in the wall during construction, so if remote temp sensor should prove to be necessary, then I am prepared. Seems like relatively cheap insurance policy.

Can you provide any more information about the Trane relay? For instance, a part number or how the installation works? I have the XL850 and am having the same temperature control problems. Thank you!

When the contractor put in our Trane XL16i and TEM6 air handler in late 2016, he was supposed to install an 824 thermostat but instead put in an 850. The 850 was terrible. When it was in heat mode, it would display the same temperature for a long time while the room became uncomfortably hot and then the temperature in the display would quickly jump up about 3 degrees and then turn off the air handler. Another problem was when we lost power. We had to manually reset the unit to get it working again each time we lost power. That could be a real problem when you are away for an extended period during the winter and the power goes out. You could be left with no heat while away on vacation. I do think there is a design problem in the 850 and likely 950 if it is the same form.

We eventually put in an 803, as I feared the 824 would have the same problems. Even though the 803 does not have dehumidify on demand, it does keep the temperature very comfortable and the temperature in the display is very accurate. Also, with its battery backup, we do not have a problem when power comes back on after power loss.

As a side note, as Air and Energy Soutions mentioned, a Trane unit can be very loud. I was able to reduce the noise in our XL161 to acceptable levels by first loosening the bolts holding the compressor to the cabinet floor by one full turn (suggested by someone at Trane phone help) and then installing a Brinmar sound blanket.