My contractor was supposed to install an XL824 thermostat along with a 3 ton XL16i and 3 ton TEM6. Instead of putting in the XL824, he put in an XL850 along with a relay board. When I mentioned that I did not need an XL850 because I did not have a communicating system, the installer replied that he made the switch to the XL850 and relay board because he was not sure how many wires I had leading to my old thermostat.

I am having problems with the XL850.

The temperature displayed on the XL850 thermostat does not seem to be correct. I put two other digital thermometers close to the XL850, and after allowing the two thermometers an hour to adjust to the ambient temperature near the XL850, both digital thermometers indicated a temperature 3 degrees below that displayed on the XL850. I know I can recalibrate the temperature on the XL850 to drop it 3 degrees, but that is not the only problem.

When it was a bit warm in my house yesterday morning, I opened a window and turned on the fan in the TEM6 to pull in colder air from the outside (it was about 45 degrees outside at the time) and circulate it throughout the house. The window that I opened is about 10 feet away from the XL850. Instead of the temperature being displayed on the XL850 dropping after the fan started to draw in cooler air from the outside, the displayed temperature on the XL850 quickly jumped 2 degrees higher. I am not sure why the temperature displayed on the XL850 would quickly rise 2 degrees (from 71 to 73) when much cooler air was being drawn into the house.

Another problem is that when the heat was on last night, the temperature displayed on the XL850 was very slow to change. As the house became warmer, the temperatures displayed on the two digital thermometers rose by two degrees but the temperature displayed on the XL850 stayed the same (at 71 degrees) and thus the heat pump continued to warm the house until it became uncomfortably warm. I then had to manually lower the temperature for the start of the heating cycle on the XL850 to stop the heating cycle. After I did that, less than a minute later the temperature displayed on the XL850 jumped 3 degrees higher.

I have read about accuracy problems on the XL850 and the problem of heat being trapped inside the XL850 affecting its accuracy.

I do not want to install another XL850.

The XL824 looks the same as the XL850, except that it is not a communicating model. Does the XL824 have the same accuracy problems as the XL850?

If it does, I will shift to the 803 thermostat. I know the 803 does not have dehumidify on demand, but I would rather give that up and just have more accurate temperatures throughout the year and quicker and more appropriate responses for heating and cooling from the heat pump.

Any advice would be greatly appreciated.