Problem with Taco ESP zone valve on hydroair system

I have a three-zone hydro system, connected to two air handlers and the hot water.

The two air handlers are using Taco Sentry ZA024Q4A1 zone valves, and seem to work fine (they light up, turn properly, and fire the boiler).

The water zone is using a Taco ESP075C2-1 09-01 valve. When the water tank calls for hot water, the valve lights up and turns properly but does NOT fire the boiler.

Eventually the water tank will show an error code stating it "hasn't been able to get to temperature for over an hour" (unless of course one of the other zones is firing, which is my current work-around until I call the plumber on Monday).

We just bought the house a few months ago, so I'm new to the hydro game. Is anything I'm describing indicative of a specific problem (ie, bad Taco ESP)? Is there a simple way for me to test the ESP unit to make sure that's the problem?

Thanks!

PS: The zone failed two or three times since we moved in this past July; simply powering down the boiler via the boiler switch would automatically close the zone valve, and turning the switch back on would open it again and the boiler would fire. Only now does it seem to not be working at all, even when I try that trick.

 

UPDATE:
Here's a picture of the setup, if that helps: imgur: the simple image sharer

One thing I'm unclear on with my setup is "who" is calling the boiler for hot water. I understand the zone valve is opened by the water tank, but is the valve responsible for calling the boiler to fire or is there another connection from the tank making the call?

Also - I checked the voltage from the zone valves, all appear to be OK.

 

In case anyone was curious, the problem appears to be solved.

I bought a new Taco Sentry valve and replaced the old ESP. When I pulled off the ESP, you could see some pretty bad warping of the outer housing from all the heat cycles.

Anyway, after that two-minute fix we have not had any issues.