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Mitsubishi mini split questions

HeatPumpShopper | Posted in GBA Pro Help on

I am replacing an old heat pump in my Boston area condo unit (middle floor and attached units on each side). I wanted a hyper heat ductless mini split (Mitsubishi FE series) but it only goes to 18k btu. Would I be better off with this or with the GE series which is sized correctly (24k btu)???
The contractor wants to leave in the old (1985) heat pump as a back up. Does this make sense?

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  1. charlie_sullivan | | #1

    Has anyone done a careful heat-load calculation ("manual J"), or is the sizing just based on matching the existing unit? The existing unit is much more likely way oversized than correctly sized. 18 kBTU/h may be bigger than you need.

    Another option instead of a heat load calculation, which is actually more accurate when it's possible, would be to look at data on how much of the time the existing unit runs on a cold night. If it cycles on and off 50% of the time on and 50% off, it's 2X oversized.

    Backup heat is always a good idea, but a 1985 heat pump wouldn't be my first choice for that.
    1) It would be good to have the refrigerant removed and safely reclaimed. It's almost surely the now-illegal highly ozone-depleting type that is also a potent greenhouse gas. If it's a split system, a licensed refrigeration technician is equipped to do that and legally required to do so. Otherwise it should be taken to an appliance recycler who does the refrigerant reclamation. If you just leave it sitting for the next 10, 20, or 30 years, eventually the refrigerant will leak out.
    2) If it's for backup use, the week the refrigerant leaks out will probably be the week before you need the backup. Simple electric heat would be a more reliable backup.

    Note that if you need something a little bigger than 18 kBTU/h, you might be able to do some minor envelope upgrades on your outside walls/windows/doors to bring it back down under 18 (or under 12, or likely even lower).

  2. GBA Editor
    Martin Holladay | | #2

    Charlie gave you good advice. Start with a heat loss calculation. (This can be tricky, since many HVAC contractors who claim to be making heat loss calculations are actually using rules of thumb or sloppy input data.)

  3. HeatPumpShopper | | #3

    Thank you!! Great plan. I really appreciate all your input!!

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