Splitting a 220v Circuit in to two 110v Circuits, is it Possible?

I'm trying to wire up a 110v attic fan but the only available breaker I have is a 220v breaker. I've heard 220v circuit is just two 110v circuits combined, so I should be able to split the 220v in to two separate 110v circuits, right?

 

Yes.

If you have a 240v breaker "available" e.g. sitting there taking up two slots in the breaker panel, all you really need to do is pull out the double breaker, then install a standard single-wide 120v breaker.

 

You'll probably only need a 15 amp breaker for your roof mushroom. What size is your 240 v breaker?

 

Very wrong. Your house has neither 110v or 220v circuits. Your house is supplied with 240 volts. It also has 120 volts derived from one leg of the 240 and the neutral.Wrong. You can use one leg of the 240 and a neutral to provide 120.

In your case since the breaker size is wrong you would reuse only the cable (except in Canada). Install a 15 amp breaker as suggested by Chandler and refill the other half of the opening in the panel cover with a blank. The white wire goes to the neutral bar and the black to the new breaker.

If the original cable is larger then #10 (30 amp breaker) let us know. You may need to use a pigtail to reduce wire size at breaker.