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Community and Q&A

Ducted minisplit power consumption and operation

Chris Barnes | Posted in Energy Efficiency and Durability on

Hello,

I am currently designing a home in the 4C climate. We’re attempting to reduce our projected power consumption as we’re considering having the home off grid as it’s far away from the closest power line. I calculated that the home would need approximately 12,000 BTUs to heat the home (2100 sq feet, R35 walls, limited triple pane windows, air sealing, etc).

Can someone that is familiar with ducted mini-splits give me some insight into how they work and how much power they consume (during cooling, heating, and standby)?

Also, I had read that they the air handler typically runs continuously, similar to an HRV system. If so, it seems like the mini-splits could end up consuming quite a bit of power. Is this true, and if so, is there a method to turn this off so it operates more like a typical A/C system?

(I recently determined that a Zehnder HRV would require ~1.6 kWh per 24 hour period operating continuously at a medium speed. I was hoping to keep our total consumption at or below 8 kWh!)

Thank you,

Chris

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Chris,
    You are the second GBA reader this week who has asked a question about heating systems for off-grid homes. (The other thread is this one: Grundfos pumps in a radiant floor heat closed loop system. In my answer on that thread, I listed three options for heating an off-grid home.)

    A system that may make sense for heating a grid-connected home is often a terrible choice for heating an off-grid home. If your house is grid-connected, the output of your PV system is credited to your account with the local utility, so your PV-generated electricity is never wasted. If you're off-grid, on the other hand, you are unlikely to have more than three days of storage capacity in your batteries, so it's impossible to save up electricity generated in July for use in December.

    Here's the basic problem: we need space heating during November, December, and January, and those are the months when the sun doesn't shine very much. During those months, most of us who live off grid are getting our electricity from a gasoline-powered generator.

    The bottom line is that an off-grid house can't use a heating system that requires electricity. If you mistakenly install such a system, you'll find that you are running your gasoline-powered generator all winter long, and the fuel costs to feed your generator will quickly bankrupt you.

    So, here are your choices: a wood stove; a through-the-wall propane-fired space heater that requires no electricity; or an old-fashioned gravity-fed oil-fired parlor stove.

  2. Chris Barnes | | #2

    Thanks! That's a fair answer and something that I had not considered. I have sized the array and equipment to support 8 kWh in the winter months (i.e., worst-case) with a propane back-up generator. With that being said, what you're saying make sense. It seems to me that the most viable solution in your list is a wood stove; however, this doesn't provide any sort of cooling solution.

    Considering the worst-case power of 8 kWh, there will very likely be enough power in the summer months when cooling loads are required. With that being said, I suppose my questions still stands. I am pretty clueless on how mini-splits work and there doesn't seem to be much information about operating power consumption and how they work exactly.

    Thanks again,

    Chris

  3. Aj Builder, Upstate NY Zone 6a | | #3

    Curious to cost of running grid power to location? $10,000. If so, it would be worth the cost.

    Second idea... insulate off a small section of your rather large offgrid home so as to reduce the loads needed for HVAC at your desired temperature letting the larger area be a few degrees off desired temperature.

    Of course if cost is not an issue, have at it, install a 2000 gallon fuel supply for your generator and call it a day.

  4. Chris Barnes | | #4

    Cost to connect to the grid is about $32,000. Cost of parts to run off grid is around $30,000 before tax credits.

    On a side note: I am fine with a wood stove, however, it seems that the heating that the wood stove would supply is vastly greater than the required amount. It also seems like it would be problematic to modulate the heat coming from the wood stove. How is that typically done?

  5. User avatar GBA Editor
    Martin Holladay | | #5

    Chris,
    Q. "It seems like it would be problematic to modulate the heat coming from the wood stove. How is that typically done?"

    A. Experience. You pay attention to the weather; you learn how to operate your stove; you keep a selection of large-diameter wood and small-diameter wood; and you learn how to adjust the damper on the stovepipe and the air intake adjustment.

  6. User avatar
    John Semmelhack | | #6

    Regarding ducted or ductless mini-splits - though performance varies, with the best models and with your climate and house, you could probably expect a seasonal-average COP of between 3.0 and 3.5. On your peak day...if your heating load is 12,000Btu/hr for the entire day and assuming a COP of 3.0 on that day, then you'd use ~28kWh just for heating (12,000Btu/hr / 3,412Btu/kWh / 3.0 * 24 hrs).

    On the ducted models, the default setting for the fans is normally "on", but this can be disabled in a standard fashion using the thermostat control settings on most models I've worked with.

    Side note regarding storage - I recently estimated that I would need about 1,200kWh of battery storage going into the winter in order be off grid with my net-zero house and PV system. To put that into perspective...the high-end Tesla Model S comes with just 85kWh of battery storage and a price tag of about $80K...about half the cost of which is in batteries (give or take)! To sum up Martin's earlier comments: battery storage is a bitch, but the trees make good "batteries"...as long you don't use too many of them!

    There are several threads on GBA about using wood stoves in super-insulated houses. Lots of good discussion...

  7. User avatar
    Dana Dorsett | | #7

    Yes the air handlers of mini-ducted minisplits run continuously, but they modulate with load, and sip power quite conservatively at part load. When the load is below the minimum modulation of the mini-duct cassette/compressor combination's range it will cycle on/off at the minimum blower speed & power. At low speed the back pressure of the ducts are quite low, the equivalent of having oversized ducts. As a rule ducted minisplits will run lower efficiencies than non-ducted versions though, due to the duct impedance issues. This can be minimized by keeping the duct runs as short as possible.

    There no WAY that the 99% outside design temperature (the temperature at which the peak load is calculated) persists for anything like 24 hours, but it could easily average more than half the peak load. And the efficiency at the peak load won't be as good as the seasonal average either.

    But as others have noted, at the very high first-cost of power storage relative to the cost of low-temp thermal storage (or wood storage), off grid homes are still way better off using something that doesn't use electricity (or only very minimum electricity) for space heating.

    There are some pretty good wood stoves that put out 35,000 BTU/hr or less at their max burn rate, and most EPA rates stoves will still burn cleanly with the secondary -burners going at about 1/3 of the max rating. The house also has substantial thermal mass- it takes a sustained excess heat input over hours to turn it into a sauna. And if the woodstove itself has some thermal mass to it (small soapstone or ceramic stoves, as opposed to cast iron or steel) intermittent burns can average the output over periods of hours- it's not a tough thermal mass to manage in most homes.

  8. Chris Barnes | | #8

    Thank you very much for the help! I think I need to go back and determine how much power consumption a unit would take during the summer months. Based on the formula, it seems that I should be able to calculate the BTUs that need to be removed from the home assuming some exterior high temperature and then calculate the energy required to achieve that. During the evenings, the temperature typically drops down to the target interior temperature so there should be very little load at that time.

    What sort of COP should be expected in the summer months? How is COP estimated?

    Thanks,

    Chris

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