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Mitsubishi Ducted mini split

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I have a 900 sq ft house built in 1949. Oil boiler for dhw and radiant floors. Future plans are to expand the house by maybe 300-400 sq ft. Rough manual j shows around 25k btu loss. Looking to replace current 20 year old ducted ac. Very interested in the Mitsubishi hyper heats but would still use oil below 25-30*. Just like having the HP as a backup below that. Degree design day is 10*

Had 2 contractors come in. One quoted a 36k btu unit the other came in with a 30k btu. Looking at the spec charts it looks like the 36k btu can modulate down to 15k btu and the 30k can modulate down to 10k btu.

My thoughts are the 36k is too big right now. The 30k may be too small in the future. Which would you pick? Thanks

Comments

  • ratio
    ratio Member Posts: 3,649
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    If it's not going to be used at design conditions, there's no reason to size it to design conditions. It'll still be adequate as a backup & if the heat loss is too pessimistic it may be plenty big enough anyway.
  • HVACNUT
    HVACNUT Member Posts: 5,861
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    Make sure you set a minimum floor temperature because Stage 2 radiant from a cold start won't do much good. 
    I have a client with 6 Mitsubishi City Multi Ducted air handlers and 2 ductless heads for the finished basement. Also radiant throughout. But the air is controlled by room sensors and the radiant by floor sensors by different thermostats. They work in conjunction with each other, rather than staged.
    The radiant is fed by a mod con, not oil, so it makes it more feasible. 
    What make and model boiler? How old? If piped and mixed correctly, and with outdoor reset, you could do stage 1 radiant and stage 2 heat pump. The more efficient, properly piped and controlled the boiler, the more sense it makes.



  • gaabbee
    gaabbee Member Posts: 43
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    HVACNUT said:
    Make sure you set a minimum floor temperature because Stage 2 radiant from a cold start won't do much good. 
    I have a client with 6 Mitsubishi City Multi Ducted air handlers and 2 ductless heads for the finished basement. Also radiant throughout. But the air is controlled by room sensors and the radiant by floor sensors by different thermostats. They work in conjunction with each other, rather than staged.
    The radiant is fed by a mod con, not oil, so it makes it more feasible. 
    What make and model boiler? How old? If piped and mixed correctly, and with outdoor reset, you could do stage 1 radiant and stage 2 heat pump. The more efficient, properly piped and controlled the boiler, the more sense it makes.



    It’s a 17 year old crown unit. 91k btu. I have a 26 gallon turbomax reverse indirect for dhw and buffer. I-series r mixing valve on the secondary piped radiant side. Taco 406-exp controller. 

    Trying to wrap my head around what you are saying here. So you say keep radiant on low temp say 80* swt and then heat pump kicks on when the room temperature cant be maintained by the low radiant?
  • GW
    GW Member Posts: 4,700
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    You told the contractors that you’re keeping your oil for midwinter weather, and they still quoted you systems that large? That is wild. Go Smaller 
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    gary@wilsonph.com
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,053
    edited March 2021
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    Select the ducted heat pump based on the cooling needs of your home. FAGADABOUDIT for heating. You want the correct size for Air Conditioning.

    I have a home with Radiant Floor heat that is adequate for most of the time. Existing gas heat with AC ducted in the home. Added the Radiant in an addition with a concrete slab floor. In the existing section of the home, I did a staple up on hardwood floors. When the temperature gets really cold, the floor is not enough so I bump that thermostat up to 72° to be sure the floor keeps flowing, then I set the furnace thermostat to 68° in order to compensate for the difference. This works out great.. as the outside temperature rises above 28° the floor is enough the do the home, I just set the thermostats for the Furnace OFF and floor heat to 66°.

    The floor heat keeps the house comfortable at 66° and I'm happy with the minor adjustment for a few very cold days each year.

    I do this with 2 Nest thermostats. It is easier than trying to set up one 2-stage thermostat on both systems and get the results I wanted. Too many relays and algorithms to make that happen.

    So @gaabbee, keep the floor operating all the time and add the HP only as needed. Use two thermostats. One for the floor heat and one for the heat pump. Don't try to over-engineered it because once the fan starts blowing, the desired temperature setting in your home will be higher than with floor heat alone.

    My recommendation and opinion, based on experience with my own home.

    Yours Truly,
    Mr.Ed

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

    gaabbee
  • gaabbee
    gaabbee Member Posts: 43
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    @EdTheHeaterMan Now wouldn't this cause the heat pump to short cycle? If the floors are putting out say 5k btu at low temp and the HP can only modulate down to 15k btu. Wouldn't I just be better off letting the HP do it's thing without the added btu's from the floors?
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,053
    edited March 2021
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    I believe your Mitsubishi heat pump will be variable input. So the short cycling will be minimized. In my case, I want to keep the floor heat on, so I need to keep the floor temperature thermostat set above the gas furnace temperature setting. If I put the furnace only on and shut off the floor heat, it gets very uncomfortable. I need to set the furnace temperature to 72 or higher to get the comfort I want. My duct system is in the attic so the floors were very cold the first few years we lived there. I have had 5 years since the floor heat commissioning to play with all the different scenarios. The one I described above works best for me.

    I even had a 2 stage Nest thermostat with several relays to try and get the system to work the way I wanted. I finally came up with 2 separate thermostats and keeping my Mod Con floor heat boiler on a separate thermostat and for most of the winter that is all I need.

    Eventually, I will be removing the gas furnace from the duct system. I will be getting a variable speed heat pump (like the Carrier Infinity series or something equal. I will size based on the cooling load. The heating cycle will only be used when the Outdoor temperature drops below 27° (for a few days in a row). The modulating feature of the heat pump will reduce short-cycling.

    So I hope my experience with a system similar to what you are contemplating will give you some insight. You will need to experiment with your system as the weather conditions change. Good luck with your project.

    Yours Truly,
    Mr.Ed

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

    gaabbee