The direction of flow is reversed every 30s. The cycle is short enough that due to the thermal mass there is thermal gradient within the heat-exchanger. So it effectively works as counter-flow heat exchanger. Same principle (but continuous flow) is used in rotor ERV: https://en.wikipedia.org/wiki/Heat_recovery_ventilation#Ther...
Heat exchanger there is usually an extruded ceramic grid (ERV) or rolled corrugated aluminum (functions closer to HRV than ERV)
Counter-flow heat exchangers. A parallel-flow heat exchanger would result in the average, as you say; but a counter-flow exchanger means that as the formerly-warm air gets progressively cooler, it is exposed to progressively colder air.
I've got a counter-flow heat exchanger, but it looks like they're using a different design:
> Each OpenERV TW4 module has a very quiet pair of fans, pointed in opposite directions, and a heat exchanger in a 6 inch pipe, that goes through a wall. The hot, polluted air from inside goes out for 30 seconds, and the heat from it is stored in the heat exchanger.
> Then, the fan reverses direction, moving clean air from outdoors to the indoors. On it's way in, it picks up that heat from the heat exchanger. This type of heat exchanger is called a regenerative heat exchanger, or less commonly, a regenerator. The kind shown in the video is a recuperative type, not regenerative. Recuperative types are what most people think of, consisting of a thin layer of material that separates two gas streams. Regenerative heat exchangers are different. They briefly store the energy while air flows in one direction, then release it when the air flow reverses.
> The OpenERV TW4 modules are made to always work in pairs. One always sucks air while the other blows air, synchronized over WiFi. This should be done, or hot air would be pushed out from the building through the walls during the ingress phase, causing heat loss.
the intuition: if the 2 colums flow in the same direction, the final temp is the average. but if the 2 columns flow in the oposite directions it is posible to fully exchange the temperature
Naively allowing the air columns to thermally mix would result in the average of the inside and outside temp. So how does this do better?