I have a York hvac system. AC works great. Heat is intermittent. I'll turn it to heat on thermostat and it may run for a while, but at some point (inconsistent as to how long), it turns from blowing warm air to blowing cold air. At times, I am able to turn everything off at the thermostat, wait 10minutes, and turn it back on and it works. I have someone coming out and don't want to play doctor but thought I'd ask this forum for any insight. Brrrr! Thanks! K

This is possibly a problem with the flame sensor... If you have the type of furnace that has a flame sensor. It is a stiff wire looking device that sticks up into the burner flame while the burner is burning, it senses the flame through an electric current and keeps the gas valve open. If this sensor is dirty or bad it will act the way you described. There should bo only one screw holding this to one of the burners, if you remove it be careful not to break the ceramic base, clean the wire with sandpaper and put it back in place.

You need to check for overheating too. If this is a 90% efficient furnace check the drain and trap too.