[ljosa]

I think it means that it's like [Lunos](https://www.lunos.de/en/for-heat-recovery). The unit alternates between exhaust and intake every couple of minutes. The air being exhausted heats up a core, which in the next cycle warms the air from the outside. Lunos e2 is advertised to recover 90% of heat and 20–30% of humidity.

[benj111]

yes i get that, but say you have an indoor temp of 30c and an outdoor temp of 0c. the average of this heat exchanger is going to be 15c. so on average youre only cooling the exhaust down to 15, and heating up the intake to 15c.

a counter flow heat exchanger can get the temperature higher than the average because 30c exhaust is meeting partially warmed intake, and 0c intake is meeting partially cooled exhaust.

Unless theres a phase change???

[ilyagr]

Perhaps https://news.ycombinator.com/item?id=42428537 or its parent will make sense to you: connecting 3 or more perfect constant-temperature regenerative heat exchangers in a series would make the ones at the ends work at a higher/lower temperature (of the constant-temperature heat sink for those exchangers) than the one in the middle, increasing efficiency of the overall system.

I'm not proposing this as a practical design, but it convinces me that 50% efficiency is not the limit.

[bittercynic]

Though the average temp of the overall core may be 15c, there may be a thermal gradient along the length, so maybe the inside end averages 28c and the outside end averages 2c, or something like that.