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ERV-Minisplit combo vs. Magic Box

user-705006 | Posted in Energy Efficiency and Durability on

We are conducting a deep energy retrofit on our two unit building (each unit 1,500 sf) in Chicago (zone 5a). I am pretty much done with the first floor and chose the ERV-Minisplit combo to primarily address our ventilation needs, cooling needs and summer dehumidification. And it appears to work well (Recoupaerator and one 12,000 Btu Fujitsu Minisplit): Winter time humidity around 40%, summer time humidity around 55%.

What has always bugged me is that the ERV duct work serves one purpose only: to supply fresh air and exhaust stale air. Wouldn’t it make sense to also use the duct work to distribute cooled (or heated) /conditioned air?

Well, that now seems possible with the emergence of the Magic Boxes (i.e. the Boreal 12000). If I believe the advertised cooling capacity of 8,700 Btu, the Boreal could potentially cover almost all of my cooling needs for the 2nd floor, and I am wondering if this would be the way to go.

But I can’t wrap my head around is this: Which one is more efficient? The Magic Box (i.e. Boreal 12000) – or – the ERV-Minisplit combo. How do you even begin to compare? Should I compare?

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  1. Expert Member
    Dana Dorsett | | #1

    This comes up a lot- the short answer is:

    No, ERV/HRV air volumes are too low, with temperature differences too small to make them useful for distributing heat/cool.

    The Boreal 12000 is only delivers SEER 11 cooling efficiency, which doesn't even meet code min in many places, WAY below the SEER 33 efficiency of a 3/4 cold-climate Fujitsu:

    The HSPF heating efficiency numbers are worse too, but at least it's not by a factor of 3... :-)

    The combo only makes efficiency-sense if you have nearly no cooling load to worry about, or you are really cramped for space.

  2. user-705006 | | #2

    Dana, thank you for your input! The performance numbers on the magic boxes prompted me to post this question. Good to know that I am not the only one looking at those numbers and wondering.

    I agree that ERV/HRV air volumes are too low to distribute conditioned air - in conventional homes. Not so sure if that still holds true if we talk about efficient homes (well insulated envelope) with low ACH numbers...

    You say: "combo only makes efficiency-sense if you have nearly no cooling load to worry about." Not sure if I understand. What do you mean by "nearly no cooling load", and what would be an alternative to the combo?

  3. GBA Editor
    Martin Holladay | | #3

    You wrote, "What has always bugged me is that the ERV duct work serves one purpose only: to supply fresh air and exhaust stale air. Wouldn’t it make sense to also use the duct work to distribute cooled (or heated) /conditioned air?"

    In fact, keeping ventilation ducts separate from the space conditioning system is desirable. Attempts to design a duct system to perform both functions -- delivering ventilation air and providing space heating and cooling -- always results in compromises and performance problems (due mainly to the fact that air flow rates for ventilation are so much lower than air flow rates for space conditioning, but also due to the fact that ventilation needs and space conditioning needs are often non-simultaneous). So separate ducts are a virtue, not a drawback.

    Moreover, the beauty of a ductless minisplit is that it has no ducts. Yay! That means much higher efficiency than a system that requires ducts.

  4. charlie_sullivan | | #4

    As best I can tell from the datasheet, when the Minotair says 12000 BTU, that counts all the heat that is delivered to the incoming air, including what's needed to get the incoming air up to room air temperature, plus the additional heat it delivers. On the energy recovery performance chart, at 250 CFM, which is where the COP is rated, the recovery is 110%. That means that about 90% of the 12000 BTU/hr goes to reheating the incoming air to room temperature, and only 10% to delivering additional heat to the room. So in that mode, it's like having a perfect-recovery HRV system plus a 1200 BTU/hr heater. But the COP is figured based on the full 12000 BTU/h, so the electricity cost for that additional 1200 BTU/h is actually huge. Ignoring the benefit of the ventilation, the effective COP is less than 0.5. The electricity consumption is similar to running a conventional HRV plus an electric heater for the same overall effect.

    It's probably better to run it 100 CFM, where the recovery is 210%. Now the net heat delivery is around 2400 BTU/h, i.e. 700 W. But we are spending more than 700 W to get that, so the effective COP is less than 1. We could ventilate at 100 CFM with 50 to 100 W for the fan, and then separately provide the heat with a minisplit for about 250 W. The total energy consumption is then about 1/3 of that the Minotair uses.

    I've convinced myself that even in Dana's scenario where you don't have any significant cooling load, the Minotair doesn't make sense.

    So my only suggestion, if you don't want single-purpose ductwork, is that you could complement ductless mini-splits with ductless HRV--the Lunos system.

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