The furnace, in its 10th winter, does not run when the thermostat calls for heat in the morning or in the evening. It runs for three short periods and then shut itself down, when switching from night setting at 17 C (63 F) to morning setting at 20 C (68 F)or when switching from day setting at 16 C (61 F)to evening setting at 21 C (70 F) I will start when the power supply is switched off and then on. It will start and stop for three times, each time a shorter period and then shut down for good, until reset. If I raise the room temperature to the set point and then start the furnace, it will run normally by starting and stopping to maintain the room temperature at the set point. It seems that these things tend to happen when the temperature outside is relatively cold i.e. less than 10 C (14 F). There were several service calls, replaced the limit switch and the pressure switch and yet the problem did not go away. Thinking that due to the very cold temperatures the inducer motor was overloaded, I let the combustion air to be drawn from inside the house. It helped in the sense that the furnace ran for 4 hours before it quit as opposed to the 3 three short runs and quit sequence I talked about above. During those 4 hours though the temperature in the house never reached the set point though, something that I had never encountered before. Does anyone have any ideas as to what might be happening? The "technical resources" of the dealer and Carrier Technical Service Centre have not been forthcoming with cooperation or solutions!