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