I have a 3 zone, three thermostat, hot water gas baseboard system. One zone doesn't work. Thermostat seems ok , can see small arc when mercury makes contact.

I read on internet that three zones each with own thermostat should mean I have three zone valves. I only see two ????? Where could third zone valve be ?

Anyway, last night I tried using the manual lever on the suspected valve (taco 555-102 24v ) serving that zone and it made the heat work in that zone. Would that likely indicate that the power head of that Taco valve needs replacing ? There are three screws on the valve. I would guess the red and white be the power source 24v. What is third (top) wire for ? Ground ?

How difficult is it to replace these. Someone told me they just twist off ? Do I have to shut system down at all, or do I just carefully remove and note position of wires, replace valve head and restore power ?

Any help would be greatly appreciated....I'm pretty handy electrically and mechanically, just never had to mess with plumbing or heating at all.

Thank You,,,