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.

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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

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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