What exactly is this for?

Saw this for the first time at an inspection today. Dual 40 gal water heaters. This device was on the cold water supply then below it the water line branched off to eat water heater. Everything appeared to be working fine, I just would like to know why this is there? The tag on it says “Thermal Expansion Control Valve” Rated 125 psi

Thanks in advance for any info, always tryin to learn !

They are a substitute for a bladder type expansion tank. They essentially “leak” water away to reduce the pressure.

Interesting. Thanks Charles!! Learn somthin new on here every day!!!:smiley: