Other kinds of ventilation system

There are two more kinds of heat recovery ventilation systems beyond the heat exchange and energy exchange cross flow or counterflow systems previously mentioned.

One is the enthalpy wheel. Enthalpy is not completely sensible. It is a measure of the total energy of a system, including latent heat, so they could probably have just called this an energy wheel. It is also called a thermal wheel, or a heat wheel. The wheel rotates with the incoming air going through one half of it, and the outgoing air through the other half, parallel to the axis of the wheel. If it's hot inside and cold outside, the exhaust air will warm up the part of the wheel passing through that side, then when it passes through the other side, the wheel will warm up the incoming air.  

These wheels have the advantages of energy recovery, potentially meaning more savings than a heat recovery system, and also reducing the risk of frost in the out-going air since the moisture is taken out of the air before it leaves the building. Also, the speed of the wheel can be adjusted to change the amount of energy recovery. This may be useful in seasons when you don't need to exchange much heat, or cooler nights in hot summers when you want to bypass the heat exchanger. 

Enthalpy wheels also have the disadvantage of cross contamination, as some of the exhaust is going to get back in again. Enthalpy wheels may be suited to large buildings which need constant temperatures and humidities, with relatively low ventilation rates. For example, the Passive House-certified Hereford archive and records centre uses one.

An even simpler heat exchanger is the single room energy recovery vent, or ductless vent.

This looks a bit like a regular extractor fan, but it both inhales and exhales air, and passes it through a ceramic core that stores and releases heat. Typically these systems inhale air for something like 70 seconds, pause, then exhale air for 70 seconds. 

More than one of these ventilators can be added in different parts of a building so that while one is blowing air in, another is sucking air out. 

This system has the advantage of not needing any ducts, and being very easy to retrofit, as Guy Marsden in Maine, USA explains. As it is recovering energy there is a potentially higher efficiency, better humidity control and lower risk of frost.

Also it has a remote control. I'm inclined to see this as a disadvantage, rather than an advantage, since we already have too many remote controls in our lives, and we really shouldn't need another one to breathe. Some people do like to have buttons to press though!

Another concern is that it's difficult to be sure that air blown out of the building is not going to be sucked straight back in again. Of course there is a potential problem with any ventilation system that badly positioned exhaust and fresh air outlets and inlets will just lead to a recirculation that makes irrelevant any worries of cross contamination within the system.  

Also, with the inherent problem of cross contamination in the energy exchange system, it's difficult to see how this would work with toilets, kitchens and bathrooms, where you would really only want to remove air, but not supply it. 

It's possible to imagine a configuration where the unit is placed next to an internal wall, and incoming air goes into one room, while outgoing air is expelled from another. I have no idea whether this is possible outside my imagination, but it may be worth trying!

Thanks to Devatech for the image of the wheel, which I shamelessly downloaded from your website! 

And to Nihon Stiebel who supply a "Twin air fresh" ductless, decentralised ventilation system with energy recovery. 

Air xchange.com has more technical considerations here about energy wheels.