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Ecobee, Combi Boiler and Multi Head Mitsubishi with multiple PAC-US444CN-1s

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Ekkridon
Ekkridon Member Posts: 6
edited February 2022 in Thermostats and Controls
Hi - looking to determine if something is possible before I get a professional in to implement. I have a Navien combi boiler driving radiators and a three head mitsubishi air source heat pump ductless (MXZ-3C24NAHZ2-U1, MSZ-GL15NA-U1 and MSZ-GL06NA-U1 x2). Each indoor head has its own ecobee thermostat and its own PAC-US444CN-1.

Right now the contractor didn't set things up as was requested (so a change is needed one way or another) - right now the ecobee on the main floor is just connected to the mini-split and not at all to the boiler controller.

The PAC-US444CN-1 manual outlines a scenario (Example 4) that uses an alternate primary heat source by connecting the boiler (in this case) to W1 and the W1 terminal of the PAC-US444CN-1 to the W2 terminal of the thermostat. The instructions are then to turn off Switch2-6 to have everything run full bore all the time.

Is there a world where I could have my cake and eat it too - and turn ON Sw2-6 by connecting Y1 and Y2 to the indoor head (via PAC-US444CN-1), but connecting W1 to the boiler and W2 to W1 of the indoor head (via PAC-US444CN-1).

Ultimately the plan is to zone the heating system as well so each thermostat can call for heat for its own zone independently - but I figure getting the first one set up i the first step (right now in the winter we just set the 'other' two zones to off since running the boiler heat is significantly cheaper than the heat pump where we live).

I hope thats clear? Happy to answer questions and appreciate the help for a non HVAC pro.

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  • ratio
    ratio Member Posts: 3,652
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    It's possible, using relays to isolate the different units from each other. However, using the heat pump as second stage probably isn't going to help you—if it's cold enough that the boiler can't keep up the heat pump isn't going to be much help. What I'd do is use an outdoor air thermostat (Something like this:https://www.rancoetc.com/ranco-etc-111000-000-digital-temperature-controller) to change over from the HP to boiler at the balance point.

    BTW, there are a few threads regarding the PAC-US444CN here, google for it with 'site: heatinghelp.com' appended.

  • ayetchvacker
    ayetchvacker Member Posts: 63
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    I would recommend finding a Mitsubishi Diamond contactor first. Even then it’s not a guarantee that they will be capable of doing what you want. I believe that thermostat setup should have the ability to switch from the heat pump to the boiler automatically at a set “balance point” either by reading outdoor temp via weather app or outdoor sensor. 
    The Mitsubishi you have is a Hyper Heat unit which is rated down at full capacity heating below zero, however if your heat loss is greater than it’s output then you will need the Navien as back up. What area is your home in?
    Fixer of things 
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  • Ekkridon
    Ekkridon Member Posts: 6
    edited February 2022
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    Thanks to both of you. I'm in Toronto Canada. The only irritation is that the hyper heat heat pump was only set up with three heads and that leaves out entire rooms - my back-of-the envelope math shows that when cost for gas and electricity is taken into account the two systems are nearly identical in cost to run per BTU (where I am). From a heat distribution (and that I plan eventually to put some hydronic staple-up underfloor under my kitchen), I would actually prefer the boiler to be the primary and the pump to be the backup - or even for them both to just run together (or the pump on a delay to give the boiler a chance since it seems like it would be slower to distribute heat). The boiler is the only heat for the basement, bathroom and entryway and this winter my basement has been running at about 15 degrees since the only time it heats is when the buffer tank makes a heat call and the boiler pump 'by accident' circulates the system - the actual circulator pump never activates.
  • Ekkridon
    Ekkridon Member Posts: 6
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    Also worth noting - the average coldest day for the past 10 years has been around -4F
  • Ekkridon
    Ekkridon Member Posts: 6
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    More HVAC videos and reading tonight - here is something like what I was considering -


  • ratio
    ratio Member Posts: 3,652
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    Close, but not quite. The iso relay will need to be powered from the W(1 or 2, depending) terminal & the C(common) terminal on the stat. The normally open contacts of the relay will land on the R & W terminal on the zone board.

    I still don't think you'll see much difference using the heat pump as stage 2, unless it's warm enough outside that you won't actually need help, but I could be off the mark. Run out both ways for a few days/week to see how the different setups work.

  • Ekkridon
    Ekkridon Member Posts: 6
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    Ok - So if i'm using a RIB - which seems to be what others suggest for this kind of thing - i'm looking at wiring White/Yellow to the stat C and the White/Blue to the Stat W2 and the Yellow to the boiler R and Orange to the boiler W. Really appreciate the time and trouble of the responses.


  • Ekkridon
    Ekkridon Member Posts: 6
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    Also to your comment about not much difference - thats possibly true. I suppose what really happened is we upgraded both the boiler added the heat pump (for AC if i'm honest) - but then we realized that to qualify for some government incentives we needed to get the hyper heat model - so suddenly we found ourselves with two heating systems. Seemed a shame to not use either one at all - but I like the idea of radiators and future floor heat vs. heating air. I've also seen some discussion that hydronic heat 'feels' nicer at the same temp - but then again those were hydronic people speaking....
  • ratio
    ratio Member Posts: 3,652
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    WRT the wiring, that's about right. Cap off the white/black & blue with white nuts (I actually open them up & cut the wires off at the board. Makes it harder for the electricians to screw it up, but I suspect it voids the UL listing.)
    wmgeorge