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Mitsubishi Heat Pump with PAC-US444CN and Nest

I have a Mitsubishi Hyper Heat MXZ-4C36NAHZ connected to two indoor units, including a MVZ-A30AA7 air handler. The air handler has a PAC-US444CN-1 that is connected to a Nest Gen 3.

The Nest is connected to the PAC with R, C, G, W1, Y1.

This is the first summer with the unit. After switching the Nest to "Cool" (rather than "Heat" or "Heat + Cool") I noticed that when the Nest would command cooling the blower would come on but the heat pump did not start and no cold air would come out of the registers.

Had my HVAC guys come out. They spoke to Mitsubishi who told them that there is some sort of issue with the Nest communicating with the PAC in cool mode. The Nest was sending the correct voltage signals but the PAC doesn't know what to do with it.

He disconnect the G fan wire from the PAC and the Nest worked in Cool mode. However, I now have no control over the blower only just to circulate air. He told me to keep the Nest on Heat+Cool mode to avoid the issue.

Has anyone ever heard of this issue with a Nest with a Mitsubishi PAC-US444? I also have a PEAD unit connected to a Nest with no issues.

Comments

  • ratioratio Posts: 2,075Member
    Try an interposing relay between the Nest and the PAC. A RIB would be a good choice.
  • IronmanIronman Posts: 5,166Member
    You really should ditch the Nest: it's logic is for single stage equipment. It fights with the logic of modulating equipment making it operate less efficiently.
    Bob Boan


    You can choose to do what you want, but you cannot choose the consequences.
  • tim smithtim smith Posts: 2,282Member
    I installed one of the 1st pacus to hit the USA, actually several cause the client wanted Nest. They worked but what I really did not like was if one wanted to go in cooling but the others had been in heating prior. The system would not switch. You manually had to switch all nest on that outdoor unit to cool mode and then cooling was energized and vise a versa in heating. Really funky. This was on a couple pumyp60 multi zone units. I new there were some hitches with these at that time. Maybe they corrected some but does not sound like all?
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