20 gallon uth in a 38 gallon viv
 

The new viv stack I'm building will have 5 vivs in it which are 38 gallon vivs measuring 40 inches across and 17 inches deep. I was wanting to recycle my 20 gallon UTH into one of the vivs for use but I didn't know if this would be a problem? I'm keeping 4 corns and 1 ball in the vivs. The ball will have a 30-40 gall UTH but I was wanting to know would a 20 gal UTH be okay for a corn in a 38 gallon viv? If not I'll save it for the tank I've currently got it on and convince the mrs. into that 6th snake that's floating around in thought : P

I'd set up the viv as you normally would, and use your probe to check temps in viv. if it does not provide the right temp zone, get another snake.
it might help your negotiations to let mrs. pick a snake.

I'm not worried about temperature wise I'm worried about size ratio, if the size is going to make a difference to the snake in this case, both UTH's would heat at the same level I'm just talking about the area covered if its a major problem.

I mean if a corn is "content" in a 20 gallon with a UTH that is for a 10-20 gallon, what makes it different in a 38 gallon as it's still the same amount of heated space that it would have in either tank.

I don't know for certain, but I suppose it would depend on the snake.

I think your logic is sound as long as the heater and rest of the tank are kept at a nominal temp. Worst case get a ceramic heater/bulb to give bring up the ambient temp.