Efficiency Calculation Comparison – CERV or Minotair “Magic Box” HRVs

| Posted in General Questions on

Maybe some members can help shed some light here.  Trying to come up with a way to look at the Minotair, CERV, or similar HRVs with built in heat pumps from an efficiency standpoint.  These don’t fall into the normal methods of an HRV, Minotair claims >100% efficiency, but this isn’t without energy input, unlike a traditional ERV.

Math is attached in a screenshot.
Here’s my assumptions and notes
1. Outside of the heat transfer of the “generic ERV,” the rest is made up by a Mitsubishi 12k mini split sitting in the 5*F outside air.
2. The COP of the heat pump HRV is a guess, it’s operating in the outgoing air stream, so the COP should be fairly high, but it isn’t published.
3. I’m calling the efficiency of the Heat pump HRV 0%, this is for calculation reasons since it doesn’t have a core that transfers heat
4. Not counting the fans/ducting losses – these we are saying are equal in both units
5. And this is a big one – I didn’t account for any preheaters, etc in the generic heat pump.  But even if the COP of the make up heat pump was 1 (resistance heat), the combo is STILL more efficient

So what I come up with, the heat pump HRV uses more energy unless your traditional HRV is less than 50% efficiency.  Even then, we are only looking at cold temps where the outdoor heat pump is operating at ~2 COP.

None of this is to say these aren’t great units, they particularly seem to have very nice control systems.  Maybe I’m missing something with my math, but according to this, the heat pump HRVs use double the power of a normal HRV/ERV.

Thoughts?

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Replies

1. | | #1

I found on the Build Equinox website, they do list the efficiency of the CERV in ventilation mode. It's quite high, COP is 6.2 at outdoor temperature of 17F (they don't have the 5F rating). And in cooling, the COP is 7.6 at 95F outdoor.

Assuming the 6.2 carries to 5*F (in ventilation mode, it actually gets better as the outdoor temp drops), it puts the comparison at 0.34kwh for the CERV and 0.23kwh for the 75% efficient generic HRV.

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