Ok, a couple days back I made a post about the outside unit and the Furnace running at the same time when heat was chosen on a Ruud split system. I go that figured out finally but now sometimes I check the thermostat and notice that it has Heat On but the Furnace never kicks on. Example: Set Thermostat to 65 degrees, furnace runs and gets to 65 or 66 degrees and cuts off as it should. Periodically check hear the Furnace run check thermostat and is 65 or 66. Go to bed and wake up the next morning (or it could be when periodically checked) check thermostat and it shows 63 degress and Heat On but nothing is running or trying to run that I can tell. I can turn the heat switch off or ocool on the thermostat and then back to heat and it will cut on like it should. I have also found that if I adjust the temp up to a higher temp for the Set Temp it will cut on as well. Does anyone know what the issue might be or has any one experienced the same issue?

I'd start by changing the batteries on the thermostat.

If that doesn't help, you need to take thew cover off the burner compartment of the furnace. Post the make and model of the furnace from the rating plate in the burner compartment.

Observe the sequence of events that occurs when the furnace starts up normally.

Next, use a multimeter to measure the voltage between C and R and C and W when the furnace fails to turn on properly. You should have 24 VAC at both points if the thermostat is calling for heat.

If there is 24 VAC at both points and the furnace hasn't started, report on how far the furnace has gotten in that ignition sequence.

I am at work at the moment but I can tell you that yesterday afternoon I noticed that the issue had occurred so I went down and pulled the cover off and it appeared as if it were not required to be on at all(no heat requested) as there was not sound of anything running nor any ignition.

When you pulled the cover (if you pulled the blower door cover) you would need to hold in the door safety switch to allow power to the furnace. With this switch held in, do the test that SP suggested. We need to know if the stat is calling for heat, and the call is making it to the furnace. One step at a time here. Let us know what you find and we will take the next step. Good luck, take care.

Sorry it took a while, here goes it seems everything is OK as far as voltage that I can tell... hoping I am checking it correctly. With panels off it seems as though the "exhaust" fan comes on some times the ignitor lights and other times more often it does not... this is followed by the fan coming on even if there is no fire. In these times it does not attempt to come back on and heat again that I have found and if it does ignite it finishes and does not come back on till the temp drops when the no ignition problem most likely will occur.

At this point, I suggest calling in a local hvac tech. Only they will be able to diagnose the system correctly. This truly sounds like a problem that will require hands on testing. Good luck, and please let us know what they find.