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Fujitsu Mini Split with Radiant Auxiliary

Hi everyone. I have a Fujitsu ducted mini split and a Rinnai gas boiler each on their own thermostats that I would like to combine. I'm hoping to use the Fujitsu as the primary heating and cooling source while switching to the boiler when outdoor temps fall below 40. As I'm sure everyone knows, mini splits have propriety controls. Fortunately, Fujitsu makes an adapter (UTY-TTRX) for standard thermostats. However, this assumes you're connecting the mini split as a conventional heating/cooling system and not the heat pump that it is. I've seen this asked a few times across the forums, but never found an answer, but is it possible to use a relay to simulate a heat pump's standard O/B wire, therefore convincing the thermostat that it's controlling a heat pump and allowing dual fuel switchover? I've attached my proposed setup below. Thoughts welcome. Thank you.

Comments

  • ratio
    ratio Member Posts: 2,936
    Looks about right. Watch it run through its paces once or twice though. ISTR that some thermostats will energize the reversing valve when they switch modes & not necessarily when they actually call for compressifacation.
    TheDreamX
  • TheDreamX
    TheDreamX Member Posts: 4
    Thanks, ratio. I'm thinking of getting an Ecobee, but their customer support is rather limited. As soon as I mention a mini split system, they run for the hills shouting incompatibility. I'll update when I get things setup. Does anyone know if these stats will keep O/B energized while calling for the compressor?
  • ratio
    ratio Member Posts: 2,936
    It'll keep it energized for the duration the the compressor call for sure. What ISTR is that some will energize it once when the mode is set to 'cool' & it'll stay that way until the mode is set to 'heat'. Mechanical thermostats with a physical switch for the mode come to mind, but there was a thread about this recently.
  • TheDreamX
    TheDreamX Member Posts: 4
    Just wanted to post an update. Been using this setup for a few days now and it works like a charm.
    ratio
  • spenser10
    spenser10 Member Posts: 2
    @TheDreamX Sorry for digging up an old thread. I stumbled across your issue and solution while trying to setup my system which is almost identical to yours. I am not well versed on most HVAC things but did recently install a ducted Fujitsu system with the UTY-TTRX as you have. I also have a boiler (oil) system. I just spent 3 hours on the phone with ecobee which was almost no help. Without an O/B wire, they scream incompatibility. Can you confirm that this setup is still working for you? Also would you mind sharing the exact relay you used. I would like to copy your exact setup to test out. Any help you can provide on install and setup with the ecobee would be appreciated as this is the last step in a very long install process. Thank you.
  • TheDreamX
    TheDreamX Member Posts: 4
    @spenser10 Yes, I've had this setup for months now and it has worked flawlessly. The diagram above is accurate and I used a White Rodgers 90-370 fan relay. Hope this helps.
  • spenser10
    spenser10 Member Posts: 2
    @TheDreamX Thanks for the quick reply. I will get a relay and try to get it setup. I may shoot you a few questions if I have issues. Much appreciated.