My Ducane furnace flashes 4 times, runs for 5 minutes, and then the burner goes off. When the limit switch is jumped it runs good. What needs to be changed here?
NEVER JUMP A LIMIT SWITCH TO ALLOW A GAS APPLIANCE TO RUN. THIS IS ONE OF THE MOST IMPORTANT SAFETY IN THE SYSTEM AND IS REQUIRED FOR PROPER AND SAFE OPERATION OF THE UNIT.
Limit switches may only be jumped by technicians to test and rule out certain components and are not to be left bypassed.
Your furnace seems to be going off on High limit. When the fan runs for long enough after the furnace shuts down on high limit, it will cool the limit switch to the point where it can close again and re run the heating cycle. This is why the unit is cycling.
There are many things that can cause this. Listed in order from most common to least common:
Always check your furnace filter first. This may save you the cost of a service call. If your filter is clean then have a licensed HVAC service technician look at and repair the unit. This problem will either be a very very minor issue, or a very very major issue.
Mnc123 is absolutely correct. The limit switch is there to prevent catastrophic failure due to overheating. You can end up either destroying your unit or causing a fire.
I recently ran into a similar problem with my home central heating unit. The fan would run initially to check that there's air flow, then I hear the natural gas burners fire, and then it just turns off after 30 seconds or so. After 10 seconds of silence, it starts back up again. And this repeated 3 or 4 times in a row before going into a halt mode for 30 minutes to an hour.
Turned out in my case to be a stuffed up air filter. Replacing the filter made the problem go away. It simply wasn't getting enough air to cool the system down. So it shut itself off, allowing it to cool.
The HVAC tech guy came out to examine the system, and he told me all of that. But he said that if it continued to be a problem after changing the air filter, it's probably a faulty limiter switch. In which case, he said he could come out again and replace the limiter. Of course it could still malfunction after that, but replacing the air filter and the limiter switch are the first things you have to do in order to eliminate them as possibilities. After that are more tricky repairs, like replacing the controller board.
Air filters have to be changed at least once a year. I didn't change mine in over 2 years, so it had built up too much dust. I ran out and bought 3 at a time and will change them twice a year now.