IIRC, when the AH is turned on one of the pumps #2 (?) runs so as to circulate the fluid in the holding tank. That is, to keep the fluid in the tank from being a different temp from top to bottom. It mentions it in the service manual as I recall. Don't hold me to it, though.
Jim and Jennie, Cats=Bittles and Potter, 2000 Dynasty 350 ISC
2013 Silverado 4x4 Towed with R1200GS in bed.
PROV23:4 Do not wear yourself out to get rich; have the wisdom to show restraint.