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Is this Heat pump? if so what are the outside temperatures?
If heat pump there are controls connected to the thermostat that may at about 40f stop the heat pump, and switch to aux or emergency heat. If the aux is Electrical heating coil, you may not have power to the relay for the Electrical coil. If the Aux heat is gas, there is a problem with the controls. If none of the above are true, I would suggest your programable thermostat is calling for a lower temperatue at 8pm. I know termostats are not fun to program. It will take you some time to learn how, if you have the owner's manual. otherwise find the manual, learn, find a friend or family menber that knows how. Last resort change thermostat to non programable or have heating tec come by and help. Make sure everything else is ok in the heater.
I assume that you have a heat pump system. If this is the case, the outside temperature is probably lower than the heat pump is sized for and it is using the auxiliary heating system to keep up. The auxiliary heating system usually is an electric coil in the heat pump unit which supplements the heat being produced by the heat pump. This is normal operations and does not indicate a problem with the heat pump, it is just really cold outside. If it is not colder than usual outside, there may be a problem with the heat pump and you should call a heating contractor to check it out.
If the compressor is running in heat mode but not producing any heat then the change over valve may be stuck. That is a job for a pro. If the thermostat isn't applying power to the changeover valve then a thermostat change is easy to do.
The control that monitors the cutoff temperature may need to be adjusted. Your hetpump may also work effectively at 16 degrees. you have not left any information on the make or model of the heatpump or the thermostat.
See if it happens only when your temperature of the room is more than 3 degrees of your setpoint temperature. Since you have a heat pump, the auxiliary (electric heat strips) is use for backup of the heat pump when it can't keep up. The heat pump will run by itself if it can maintain temperature within a couple degrees. If it is too cold outside to handle the load or you set the temperature more than 2 degrees past room temperature, the auxiliary will turn on. The thermostat has a memory of how long it takes to reach desired temperature. If it goes into auxiliary mode trying to satisfy setpoint during the last call for heat, it will start the next call for heat in both aux & heat pump mode until the thermostats algorithm (timed program) allows only the heat pump to run. Not sure if you have an air to air, or geothermal heat pump. The outside air temperature will affect only the air to air system ie. when the temps drop outside, the heat coming out of your vents will be cooler...the air to air systems drops efficiency when the outdoor air drops.
Friend! All thermostat is calibrated to cut in and cut off at the end of - or + 1 degree end. Example: If you set 23 degree, it will cut off in 21 degree and cut in @25 degree. So accordingly you set the temp. Secondly, don't set temp near to your room temp.Minimum difference should be 3 degree. If your room temp 12 degree, set at least 15 degree for heat and if room temp 28, set at least 25 degree for cool. Hope it's helpful?
I believe the unit you are describing is a heat pump. Heat pump are great when the temperature isn't below 30 With low ambient temps. outside it is much more efficient and comfortable to run aux. electric heat from the heat pump.
I think you probably have a heat pump. Aux heat is electrical resistance heat coils that produce heat when the heat pump can not produce enough to keep the temperature up (caused by low outside temperature or a problem with the refrigerant system). This resistance heat can be switched on by the emerg. heat button. It also can come on automaticlly (sp) and will be indicated by the aux being shown on the thermostat. Sometime if you set the heat up several degrees by thermostat the aux will come on to help the system catch up to the desired temperature setting. The emerg. heat is a way to turn on the AUX manually. If you do not have a heat pump system then it is possible that you have a heat pump thermostat installed on a non heat pump system. Emergency heat and Aux are both the secondary.
OK. Here is a Trane Weathertron BAYSTAT240 thermostat wiring picture. This heat pump w/ electric (emergency) heaters is running right now. Notice the G terminal has green wire, the R terminal has red wire, the white wire is jumpered from W to X2, etc. The black wire loop you see in the center hole is unconnected.
This unit is adjusted by leveling the base - loosen the screw to the left of "R" and the one to the right of "F". Twist the base clockwise to RAISE the indicator. This makes the mercury in the switch turn ON at a different temperature. Remember to tighten those 2 screws again.
You will have to experiment several times to make the unit read exactly right. If the tech installed the base exactly level, the unit would read right (it says here).