[LukeShu]

Counter-flow heat exchangers can be very efficient, without a heater or cooler attached. That said, I don't think I've seen a commercial ERV claim to be more than 80% efficient, so I'm skeptical of the 90% measurement.

(I've seen ERVs with heaters attached; but for the purpose of avoiding frost buildup when it's below freezing outside.)

[cyberax]

Commercial HRVs often use rotating disks, not counterflow heat exchangers. Disks are freeze-proof, but they need to be powered.

[cheeseface]

In my experience 80% plus is quite common in the models sold for colder environments. E.g. Mitsubishi electric Lossnay advertises 86%.