But if in summer you let the bath water cool to room temp before releasing it onto the loop field you are worse off as that heat is now in the building envelope and needs to be pumped out of the house. Worse yet is the increased latent load from the evaporation off the tub of water while it cools, although that probably displaces some of the sensible load. Even worse is the pushback you'll get from other family members about soap scum lines on the tub walls from the water sitting so long.

We are at a point of debating angels on pinheads...

Joe's remark about drain field repairs tearing up loop piping resonates - a perfect example of something looking good on paper(thus appealing to an engineer like me) failing miserably in practical application. I'll even sheepishly admit I've suffered unexpected plumbing repairs in the course of drain field replacement on a rental mobile home - prior owners had (in violation of code) run a water line atop the drain field.

Like I said, in the summer you can turn the functionality off by emptying it as fast as possible. Just pull up your 10 cent stopper as soon as the bath is done. Or in the summer make your children take cold baths. I'm sure they'll nominate your environmentalist ways for father of the year on that one.

...and if you don't use soap; no soap scum....;-)
Time spent here truley helps me build better mouse traps even if this one isn't the best idea.
J