what kind of T-stat are you using?, the size of the probe tip i have seen are not all one size, i got a VE-200 and it fits easily under the center tub, without issues, my accurite is taped just above it in the tub. VE-200 set to 92 deg's, and accurite reading 89 deg. my cheaper T-stat has a huge probe tip i couldn't fit under the tub.