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I sooo love the classic nest. This is above the boiler we replaced today. Of course I clipped the zip ties and Re zipped when I was done. The budget called for a new boiler only. The basic slip in. The only thing that required expertise was jockeying the gas line and figuring out the nest. The new boiler reconnected to circ flange and we chopped in a union on the supply riser
So, 3 HW zone valves, previously installed. Two heat and one indirect. The Two warm air AHU units, out of site and Two ac units have their control wiring in this nest too.
The original installer has the common wire on the two ac’s tied to the common Yellow on the zone valves.
Anyway, here’s to you Mr Low Voltage Control Guy
Wilson Services, Inc