Upon arrival customer stated that heat was not working upstairs and thermostat was losing power. After inspection of 5 year old system found that unit was activating the high limit switch and when it would activate it would then trip the switch and turn off thermostat. Found that supply temperature reading was 151 and return temperature was 75 degrees. Looked inside evaporator coil found no restrictions, looked in heat exchanger and found no issues. Spoke with tech support in regards to issue and found that duct system is very clogged and dirty causing a restriction on the system. There are no issues with the equipment it’s self. Gave customer estimate for duct cleaning two systems, and Uv lights. Customer declined recommendations at this time, stated he wants to get more estimates for the cleaning. Advised customer system will still have intermittent issues until duct system is cleaned.