I have a ultra 80 sx. Everything works fine with the exception of when reaching set temperature on the thermostat the fan stays on without the burners being on.

It is normal for the blower to continue blowing even though the thermostat has reached set temperature . There is a temperature sensor in the heat exchanger on your furnace and as long as there is still heat there to be blown out the blower will continue until the heat exchanger cools. This is typically only for a min or so and if the blower runs for a long period then you may need your furnace serviced. Hope this helps.