With a super-airtight house, indoor air quality is a big concern. When the windows and vents are all closed, we get headaches, even though we do our best not to introduce VOCs into the house by buying a minimum of plastic, and off-gassing any new purchases before bringing them indoors.
In the summer at 9000 ft in Colorado, we simply keep the windows open, as long as there isn't smoke wafting in from a wildfire to the west of us. But in the winter, it gets very cold here, so leaving windows and doors open isn't an option.
Our house had two passive air vents when we bought it, one in the bedroom on the east end of the building, and one in the office on the west end. The little bit of fresh air that comes in through that vent in the bedroom is delightful, but we found that passive ventilation just wasn't sufficient to keep the air in the house fresh.
First we tested for radon, and found it was much higher than the recommended limit. We installed the smallest wattage radon fan we could find (50W), but running continuously it's a big load. With only 2.5 kW of generation before installing our 5 kW ground mount, we've had to turn it off for days at a time when it is cloudy.
Next, we added a pair of Lunos e2 heat recovery ventilators. It's ingenious technology. In the same size opening as our passive vents, there's a small round fan. Two paired units are installed at opposite ends of the house (we have them at the east and west ends of our large central room so they ventilate the whole house). One is pulling air in, while the other pushes it out, and they switch directions approximately once per minute. The warm house air heats the body of the fan as it exits, and then the fan heats the cold air coming in from outside. The result is that 85% of the heat energy is retained, while fresh air is introduced to the house, and the air pressure indoors remains the same as outdoors.
The Lunos HRV has the advantage over other HRVs that it requires no ductwork. If you have a larger multi-room house, you'll probably want a different type of HRV, but for our house, which is basically just one big room, this solution was ideal. We added the rotary switch made by 475 High Performance Building Supply, which also sells the HRVs, to make it easier to control the speed of the fans. This HRV solution uses much less power than the central units do, and is much less expensive. The pair use only 3W of power on low (twice that on high), and cost about $1K.
The fans are easy to install. You'll need a power drill with a hole bit the diameter of the fan to cut the hole in the wall (a little over 6"). Make sure to avoid the studs! Once you have a hole, the fan assembly simply slips into it, and the wires from the controller have snap connectors to attach to the fans. You'll need to run 3 wires (or telephone wire) from the controller to each of the pans. We employed the CAT-5 wiring already in the ceiling to route the power from the controller to the remote fan, to avoid a wire run across the ceiling. (If you do this, note that you'll need to use a twisted pair for each of the 3 conductors to carry sufficient current.)
The only downside we have found to the Lunos HRV solution is that the fans are not silent, nor is the sound white noise, since the fans switch direction every minute. On high, they are about as loud as the compressor on our refrigerator. On low, they are barely audible, but they don't give much improvement in indoor air quality. For now, we are running them on high in the daytime all winter, and turning them down at night. We haven't decided what our next step in indoor air quality improvement will be. We may add a second pair of Lunos fans on the north and south sides of the room, so we can keep them all on low, or we may add more of the passive vents.