Heat actually doesn’t seem to affect my flaccid size much. And water of any kind — including hot tubs and hot showers — tends to cause shrinkage for me.
The only thing I’ve found that has a big impact on my flaccid hang is how much I’m using my dick, whether through sex or masturbation or PE. If I’m using my dick a lot, it tends to hang big and heavy, and spontaneous erections become more frequent.