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Master valve is my terminology as it governs the subordinate vavles, look at the diagram, it's in series with the manifold valves. The manifold valve triggers the boiler when any of the thermostats call for heat. All the thermostats are wired into a relay for this purpose.I am confused by what you mean by a master valve? Are there just 2 zone valves. If so you have to have these otherwise the under floor would only come on with the water and they will want the underfloor on by itself .