Fasdfas My Armstrong 80 furnace will work fine for about 10-20 minutes then turns off before reaching desired temperature. I sometimes have been toggling the thermostat set temperature to get the furnace to kick-on. Furnace does blow hot air. Example: My new thermostat reads 62 but is set for 70 degrees. When furnace kicks on, it only stays on for about 10-20 minutes and increases room temp to 65. I already cleaned the flame sensor which didn't seem to make a difference.

SOURCE: Furnace doesn't reach desired temperature

On the thermostat if its a manual on then pop the cover off and look for a small dial with numbers on it. Take and move it tpoi a higher number than whats it is set at. Its called a heat anticipater and is supppose to make it so that the unit does not oveshoot or under shoot like what your experiencing. If its a digital oine then you are going to have to configure the setting in the insdtallers menu.

SOURCE: Furnace doesn't reach desired temperature

One of two things you can try. Do you own a volt ohm meter? Anytime your unit cycles short like it is doing the control circuit which on this one is 24 volts that engages relays and controls the gas valve to open and close. I am thinking the heat exchanger is getting so hot it is tripping the upper limit which for the most part is around 180 degrees. So next time it turns off take your ohm meter and with the power to the whole unit off. Either turn the breaker off or there is a light switch that is real close to unit turn it off. Undo the two wires to the gas valve and ohm them out. You should have continuity if you get nothing check each limit switch should have a couple. Undo wires to them and check. If you find one that is open more then likely the upper one. They are a safety device that keeps the unit from getting too hot.

There might be another option increase the speed of your fan motor to get the heat out of furnace quicker. Look at the circuit board notice the fan wiring going to it. The Red wire from fan is low speed look to see if it is on HEAT on the board. You should have 4 speeds to pick from you could take the black off of COOL and swap wires and test the unit by doing that. I'm going to put a solution on this so I can come back to it quicker. Even though we a just testing it for now. ken