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Can a multi-head minisplit be electrically separated to two different electrical meters?

Brad Hardie | Posted in General Questions on

I’m wondering if single outdoor mini-split units, that operate two or more interior wall units, can have the electrical demands of the interior units metered separately?

I have spec’d a MUZ/MSZ-FH12NA Mitsubishi H2i Hyper Heat Pump for an apartment above a garage, and was trying to avoid installing two outdoor units, when I can buy one outdoor unit that would heat both the apartment and the garage/workshop and bathroom below. The apartment is on it’s own electrical panel, but shares the same electrical meter. I’m using a wattstopper like device to monitor the apartment’s electrical use (for billing purposes), that way we don’t incur an additional $30+ charge a month from our utility just to have a separate meter.

I’ve attached a plan that shows the wall makeup of the garage/workshop/bathroom, and includes window, door, and garage door R-values, and sizes ( for heat loss calcs). I’m pretty certain I can go with a FH09NA mini-split for the garage, but that means having two outdoor units, which increases my costs quite a bit – about $2500 more than going with radiant ceiling panels – obviously the mini-splits are more efficient. I also have lots of PV, because I’m hoping to have a zero-energy property.

The other concern I have is that the apartment (which are totally isolated from the lower garage), has it’s own entrance (with small vestibule), which provides a small mudroom type area for the NH winters we have. The apartment stairwell, which is accessed via the vestibule is open all the way up to the apartment. The apartment has exposed rafters and barn board, so it is a cavernous space. Should I assume that since the vestibule is just a entry vestibule, that the passive heat it collects from the mini-split above in the apartment will be sufficient? Do you think I should add electrical resistance heat there? OR should I figure out a way to isolate the vestibule at the stairwell (like an airlock persey?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Brad,
    I'm not sure of the answer to your question about power supplies that are separately metered. If I were you, I would call up Mitsubishi and talk to their technical help people. My guess is that they will tell you that you can do it, but I'm not an electrician.

    If you keep your garage heated all winter, then the apartment vestibule won't need any supplemental heat. If you are going to allow your garage to cool down, then a small electric-resistance heater in the vestibule would probably be a good idea. If you don't end up needing the heater, it's easy enough to turn it off.

  2. Hobbit _ | | #2

    Since the outdoor unit likely has only one compressor, the
    answer is probably no. Two separate units with separate power
    feeds would be needed.

    _H*

  3. D Dorsett | | #3

    Not possible to separately meter, due to the fact that both heads are being served refrigerant from a single compressor, as noted by Hobbit. You could separate out the control & blower power for the heads (though it would be awkward- requiring cutting into the control cable), but there's no way to proportionally apportion the power used by the outdoor unit.

    In recent experience rather than being more expensive, it's usually cheaper (and definitely more efficient) to go with a pair of 1-ton or 3/4 ton Mitsubishis than a multi-split of similar capacity. The modulating range at the low-end is also more optimal with a pair of small units rather than a multi-split, which is critical for reaping the potentially very high shoulder season efficiency.

  4. Charlie Sullivan | | #4

    The only way I can think of that separate metering could be achieved would be if Mitsubishi built something into their software that would keep track of which head was doing what and provide an output directly. Just measuring electricity flow doesn't do it because the electric power is going to the common compressor, and it's the refrigerant flow that is split between heads.

    On the vestibule, opinions vary. Personally, I like having an area like that float between indoor and outdoor temperatures. It gives you a sense of how cold it is outside, to better guide your choice of which jacket to put on. But it's really a matter of taste.

  5. JAMES KREYLING | | #5

    The single zone units also have higher cooling SEER and heating HSPF ratings than the multi-zone units, so using individual units give you the potential for significantly higher operating efficiencies.

  6. Reid Baldwin | | #6

    I don't know NH law, but in some states it would be illegal to charge your tenant for electricity as you are proposing. The utility has a monopoly on retail sales of electricity. Providing it as part of the rent is ok as long as the rent doesn't vary based on electricity use. Also, some state's net metering laws would not allow the PV to serve two residences.

  7. Brad Hardie | | #7

    Gents,

    Thanks for all the input. I figured with a single compressor there would not be an "easy" way. I understand two smaller units will be more efficient too. Changing the wiring in either unit would void the warranties too.

    Two units it is.

    I had the electrician wire for the baseboard in the vestibule, and blank cover it till I see how it gets in the winter. If need be it will be easy to install.

    Reid - thanks for raising concern on both those issues. As far as NH goes, if you spell out the rental agreement in writing, and provide written documentation, along with a way to record the data - you can charge your tenant separately without needing a separate meter. It's simply about being upfront, with full disclosure upon entering into the rental agreement.

    As far as group net-metering - NH adopted it last year, but this isn't group net-metering, because I own the building and it is on the same property as my residence.

  8. Brian Larsen | | #8

    Each units thermostat could be monitored for how long its on and the amount split by the ratio....

    But not really practical unless the dollar amounts are fairly large.

  9. Expert Member
    Dana Dorsett | | #9

    Brian- These are fully modulating systems with significant turn-down ratios. If properly sized they should be "on" almost constantly over the winter, but the amount of heat being delivered and the amount of power being used is a function of both the outdoor temperature and the modulation level of the individual head. One could easily be delivering 2x the heat and responsible for 3x the power use as the other in a 2 head system, even if they were both "on" the same amount of time. Raising the modulated output of one head lowers the net efficiency for the whole system, so if all else was equal somebody keeping it at 73F indoors would cause a disproportionate amount of power use relative to the difference in heat being used. Raising one half's temperature causes the other side to run less efficiently.

    Theoretically with a great deal of software development and monitoring the control signals between the heads and the compressor unit you could probably separate it all out, but there's no simple way to get there.

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