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You can't power the zone valves with the transformer on the boiler you need a separate transformer. A standard 40va 24 volt transformer will provide enough power for at least 3 zone valves.
Do you have the wiring diagram that should have come with the new valve(s)? It shows you how to wire them to a separate transformer. Let me know if you have more questions.
Perhaps you're skeptical. If you connect the zone valves to the boiler transformer you'll burn it up. It can't handle the load.