[Richard]

The function of the two black relay cubes is now clear. The aquahot wants a 12 ground at it's terminal input strip to activate a zone. The PSC thermostats above the passenger and in the bedroom provide a 12+ signal. So what Newell did was use the 12V+ signal from the tstat to activate the relay. The contacts in the relay when closed, ground the signal. It's a little funky because the same color wire is used in and out of the relay. It isn't obvious what is happening until you match the wires on the relay to the function of the relay terminals.

When the tsat signal is now grounded to the living room input terminal, 12V positive now is present on the output side of the zone 1, topmost, clear relay cube mounted on the AH panel.