TOPIC: old furnace

I have a 17 year old furnace that keeps cycling on and off every 5 min. I turn on the blower to "run" mode so as not to burn up blower but is there something else I should do. Or is this a problem?

Aaron sounds like your fan limit control is bad its the dial inside the little metal box hope this helps

Aaron - sounds like short cycling to me, turning on and off too frequently.  It usually indicates a malfunction.  Is this a furnace or boiler?

If a furnace it could be a malfunction of the upper-limit switch.  This is the device that turns off the system when the desired temperature is reached.  If it is set too low, or malfunctioning, it may turn the burner off even though the temperature is still too low.  This switch can also turn it off if the filter is dirty, and AC coils are dirty, the fan is faulty or your ducts are too small.  Closing off many registers to try to adjust air flow up or down can also cause it.

You need to have it checked.  It sounds like it is electrical...

Hope this helps!

I have had one of my rent houses do this same prob. and it was just as yall say.  But I have also had this some prob with my own unit.  Turned out it was a lil pressure switch on the exhaust fan.  It was a safety switch to make sure the exhaust was exiting correctly.  Either way, you need a HVAC person.