If I wanted to stop humidification during a call for cooling using a step down trans to 24 and not the 24 hum terminals new furnaces have how is it done,using a relay ? With a board and also with a single speed fan on a fan limit switch also how would I wire the fan centre in? diagrams would be helpful.

The circuit board controls it all. All the safetys are built into the circuit board. If you are trying to bypass the circuit board forget it. While I am sure it can be done I will not begin to post how to do it because it is a violation of the listing see UL/CSA for further info on tampering with approved circuits. Just use the HUM terminal on the board.