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Need help!! Use ecobee with Mitsubishi PAC-US444CN-1 and Boiler

Hi All!
I want to use nest/ecobee with mitsubishi heat pump and boiler as secondary heat . The thermostat is dual fuel so i can configure it so it switches heat source based on the outdoor temp so kinda use the heatpump till 0 degree and then move to baseboard. Which it says it can do in the heatpump + boiler setup.

Now the problem

The PAC-US444CN-1 does not provide a O/B and if thats not hooked to the thermostat it wont take it as a heat pump. Now i have a R and W coming from the boiler heat there is no C here . I have R , C , W1 , Y , G coming from the pac . Now what i am not sure is how to connect this , Would the thermostat send 24v only to O/B when it switches to heatpump heating . In that case i can just hook the W1 from the PAC to it . Is there more to it ? will it be sending 24v down Y as well as O/B in that case i would have to shutdown the Y and only send it to W.
So basically the question is to wire the PAC unit to appear as a heatpump to the thermostat instead of a standard heat and cool system.

Thoughts ??

Thanks a ton in advance for all the valuable ideas and suggestions.

Comments

  • ratio
    ratio Member Posts: 2,871
    The PAC interfaces with your thermostat as a standard heat/cool unit, not as a heat pump. I don't believe the Nest can be set up as a heat pump stat without something connected to the O/B terminal. I think the easiest thing to do would be to use an outdoor thermostat (Something like this) to switch the W call from the Mistubishi to the boiler. I'd personally prefer this setup, as I'm 99-44/100% sure that the Nest gets its outdoor temp from the internets. Wonder what it'd do when the cable goes out during a winter storm?
  • varunbtr
    varunbtr Member Posts: 3
    Thanks for the reply.
    So how would i be wiring this temperature control with the ecobee. You are right it gets its temp from the internet but lets assume it works correctly just hookin the W from the PAC to the O/B of the thermostat would not work ? Just trying to understand that approach and see if i can make it work somehow even by using some more relays if needed
  • ratio
    ratio Member Posts: 2,871
    I suppose you could do it with relays. The Y terminal calls for compressor operation, the O/B terminal picks the operating mode, the W calls for auxiliary heat (another stage if the compressor can't keep up, or to keep the air temp from cratering during a defrost cycle). There are two styles of reversing valves, one energized to select cooling mode, O, the more common around here; and one energized to select heating mode, B. You can pick the flavor in the stat configuration. I suspect that if you had a relay on the O/B terminal of the stat, it would allow itself to be set up for heat pump operation.

    Or you could set up a separate thermostat to handle the changeover & be done. You would loose remote access to the set point, but do you actually need to adjust the lockout point remotely?

    Also, you will need an isolation relay(s) between the thermostat and whichever unit is not powering the thermostat.

  • varunbtr
    varunbtr Member Posts: 3
    Sorry for the late reply due to the holidays. I am still a bit confused. What i am trying to achieve if you think of it this way. is to have a conventional system R,C,W,Y,G to hook up to a ecobee using R,C,Y,G,O/B . Can you explain how i can do that ?
  • ratio
    ratio Member Posts: 2,871
    The ecobee must be set up for a conventional system to operate the Mitsubishi, even though it's a heat pump. Therefor (I expect) the ecobee can't be set up for dual fuel. My suggestion is to use a separate outdoor thermostat (this or equivalent) to handle changeover from the Mitsubishi to the hydronic system.
  • tim smith
    tim smith Member Posts: 2,389
    Been there tried that, the pacus and a dual fuel set up would not work standalone. Ratio’s idea with outdoor stat might work.
  • pmorley
    pmorley Member Posts: 2
    I have a similar issue. I have Ecobees, boiler with C,R,W and just bought the PAC with Mitsubishi minisplit. We're in a cold snowy climate so I plan on powering the AC off and covering it up in the winter. Power can always be connected from the boiler, but I'll manually switch over the W1 contact from one system to the other. How best to set it up? What do I do with O/B and how does the Ecobee deal with two different setups?
  • ratio
    ratio Member Posts: 2,871
    From what I can remember, the PAC doesn't expose the reversing valve to the thermostat—it appears as a standard heat/cool unit. If it were me, I would add an outdoor thermostat to do the changeover automagically & forgo the cover. I think you'll run into trouble if you try to rewire it every season, as those terminals aren't engineered for that kind of continuous usage.
  • pmorley
    pmorley Member Posts: 2
    I agree the ecobee terminals are a bit flimsy. I was planning on wiring it permanently to a breakout box in the basement, maybe with a relay to switch between seasons.
  • ratio
    ratio Member Posts: 2,871
    Sorry to necro an old thread, but I just came across this sequence of operation for the PAC-US444CN-1 thermostat interface. It shows, on the last page, the correct, Mitsubishi-blessed, wiring between a heat pump thermostat (in this instance, a Honeywell TH8320) and the interface that allows the thermostat to operate in dual-fuel mode, including OAT lockout.

    It also states "We have tested the Thermostat interface with Nest®, Lyric®, ecoBee®, and
    INNCOM® thermostats."

    Canucker