Internal Check Valve in Water Heater?

[Rand Kocher]

For years I had a heat exchanging hot water tank fed from a propane boiler, and an electric hot water tank plumbed in parallel, that is the cold water 'T'ed to both and the hot water 'T'ed to both. Which ever tank was heating the water both tanks would get to full temperature because of, I guess, a slow reverse gravity flow.

I just replaced the electric hot water tank yesterday, plumbed in exactly the same as the old, but the new one now won't heat the other tank. Did Rheem change something, put in an internal check valve or something?

 

[breplum]

Mfrs do have heat trap devices that can be floating balls in the dielectric nipple or a rubber type waterway flap.
The dielectric nipple one is obvious, the other, not obvious w/o removing the dielectric nipple.