Option 1:
You can select a player to give spare heat to. Simple.

Option 2 (not so simple):

Heat Enhancement eXtractment Capsules aka HEXC

Spare heat is counted up in a bar on the results screen at the end of a level. When the bar is filled up, you get a HEXC.

On depths 4, 13, and 23, there is a machine that activates HEXCs, which upgrades your items for a depth.
Price: 10E and 100cr

after activating a HEXC, you may use one at the results screen.

Options:

1) upgrade 1 stat (Norm, Elem, Pier, Shad, Speed) by 10%

2) increase a variant by 1 degree (med>high). UVs can be upgraded as well.

You can do any of these options once on any item and lasts for 1 depth.