Ruud oil furnace blowing cold air, fan running constantly

I think most furnaces if the controls and heat sensors or limit switches are working right, the blower won't stay on if the heat doesn't come on. I would start checking the temperature sensors or limit switches. They will be in the heat exchanger area. To be sure what the normal reading is, you will need to find a schematic.

You might also try shutting the furnace off and then starting a heating cycle. If it starts up normally and then shuts down quickly, the problem is in the controls, may be even if it doesn't either.

Older furnaces, the blower didn't come on until a limit switch warmed up. The newer ones are on a timer on the control board, and much harder to find the problem.

One other thing is to make sure you are getting adequate air flow, no clogged filter or blocked vents.

Hi, Maggieg,
Welcome to this site, and thank you for posting a question here. I had, at one time, an oil furnace. Later, switched to a heat pump.
I do remember the furnace, during Thanksgiving of all times, decided to quit "lighting up", and was only blowing cold air. I got lucky, and on Thanksgiving day, our repair person came out, and found a bad igniter. The oil was flowing, but it would not "fire up" the oil.
If you don't know how to check all this out yourself, the best thing to do is call a repair person. They can check it out in probably 10 minutes or less, and find the problem. I do wish you the very best.

If you post back with some details, I may or may not be able to help you solve the problem. There are simple things like an ingitor that anybody with much in the way of DIY skills can find and fix, maybe before a repairman would get there.