Spot Render to file?

This might sound odd, but I have a scene where I am updating one part of the image and didn't want to have to render the whole scene again. I wanted to complete a spot render and save time. I can easily merge the original renders with the spot renders in Photoshop etc IF I can output the spot render to file. ((I think that makes sense))

Is there a setting somewhere where I can change the render target to be a new window on the spot render tool?

Thanks in advance