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I would like to use nest to control a "Marine Air Systems Vector" with Mechanical controls. I have an idea using Honeywell relay(s). Please review the wiring diagram attached and provide any feedback. I am trying to replace the MCP in the diagram. The variable speed fan control is unnecessary. RV below is the reversing cylinder. The MCP uses a steel plate thermostat.

enter image description here

enter image description here This is my first draft.

Please limit comments to Technical Facts about the electrical circuits and relays.

Thank you in advance.

  • Hello, and welcome to Stack Exchange. First, it's difficult for us to answer this; it looks like you're asking us to design a wiring system for you. Second, I'm guessing that the Nest isn't meant for marine applications; it may not be able to handle the surprise changes in temperature you could expect in a boat. – Daniel Griscom Dec 25 '16 at 15:02
  • I am asking for input on a project I am designing. If anyone has real world experience using the Honeywell RC840T relay, your experience would be appreciated. The Nest Thermostat will work fine in this application. – J Sha Dec 26 '16 at 13:46
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Not enough grunt

The RC840T only has a 1.2VA transformer built-in, which isn't much, and likely not enough to run the power-hungry Nest. I'd use another RC840 instead of the RC840T and an external 24VAC/40VA transformer to power the Nest and the relays.

Also, I would keep the C terminal wiring isolated from the boat -- this is done to avoid putting 24VAC return currents into the boat hull and perhaps the DC ground system as well.

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