Never heard mine..but it was in 5vr basement compartment behind a false wall.
Just smelled it when if failed !!!!
I'm not following you about 'correct horizontal' install.
Horizontal to me is 'on the floor flat'.
Now vertical.....they do have cautions on how the relays are positioned when installing vertically (wall mounting) and that is the relays must not be situated up/down (one above other) they should be right/left (side to side orientation)
NOT Recommended install----wall mount

Correct wall mount
