It's a balanced mechanical ventilation system with a heat exchanger and located in the attic. It pulls in fresh air via an inlet on the roof and then distributes it via a set of tubes to the various rooms. Similarly, it extracts stale air via other tubes from other rooms to the outside through an outlet elsewhere on the roof.

Both airflows pass the heat exchanger so the outgoing air can heat up the incoming air to minimize heat losses. These airflows are balanced, i.e. the various inlets/outlets in the rooms are calibrated so the total inflow matches the total outflow without the fans in the device having to run at different speeds.

An alternative system that is frequently used here is mechanical extraction while fresh air is pulled in 'naturally' via slits at the top of the window frames. This is obviously cheaper but has some disadvantages:

- The incoming air is cold and thus your heating system needs to work harder to keep the temperature constant. - You can feel this cold air flowing in, typically around your feet and this can be uncomfortable. - Less control over how much air flows in: a bit of wind outside can mean more (cold) air is blown into your house. There are sliders on these slits but they are manual so people either keep them open (resulting in more incoming air than strictly needed) or they close them (resulting in too little ventilation).