HVAC devices you may not know about
An economizer is a device that allows to shorten HVAC unit run time and thus reduce energy costs when the outdoor air temperature (more correctly, heat) is practical to use.
For example, imagine a situation (happens quite often in climates with sharp day/night contrast) when the outdoor air temperature is already low, and the indoor air is still hot. In this case, it would make perfect sense to pump outside air in and dump indoor air out, rather than run the air conditioner.
Reverse situation: imagine an old brick or stone house, which is still cold in the spring, when the outdoor air is already warm enough to be used.
An HRV (stands for Heat Recovery Ventilator) is a device that serves a purpose opposite of that of economizer: it allows to dump the stale indoor air and accept fresh outdoor air without losing as much energy as you would if you just opened a window. This is important in tight new houses to prevent indoor pollutants from reaching unhealthy levels.
Basically, an HRV is a fan with a heat exchanger: for cooling, the hot outdoor air will pass its energy to the air leaving the house, for heating, the hot indoor air will pass its energy to cold outdoor air entering the house.
HRVs only move the heat associated with air temperature difference. See the ERV section for more on this.
An ERV (stands for Energy Recovery Ventilator) is a device that does a similar job to an HRV, but handles both the heat from air temperature and moisture in the air (known as sensible and latent heat.) Because it exchanges both, they are usually more efficient and do not have condensation problems that can occur in a HRV. There are two types of ERVs, linear exchangers similar to HRVs and rotary wheel type systems where media moves between the air moving in and moving out to transfer the heat and moisture.
Note: Thanks to Jerry Scharf for his contribution to HRV and complete ERV section.