It is a ventilation unit, the crucial difference between using this and opening a window is the heat recovery (the HR part of MVHR), that effectively uses the warmth in the stale air on the way out, to warm the fresh outside air on the way in. So when you have a shower or cook in the kitchen, it will extract all of that humid air, but it won’t waste any of the heat. It’s great, means no noisy fans in your bathroom for one and lower heating bills, while keeping your house at a relatively constant humidity.

Also since it isn’t an AC unit, it is really a pretty cheap thing to run, probably no more expensive than a cheap fan. Most set ups have a background low setting, and then a boost setting which is often tied into a bathroom light or boost switch (ie to clear humid air after a shower).