Heat recovery and energy recovery ventilation systems remove stale and/or moist air from inside and replace it with air that has been warmed from heat recovered from the exhaust air.
They have three basic ingredients:
1) An air supply ducting system
2) An exhaust air ducting systems and
3) A heat exchange unit.
It is the heat exchanger which is the key to the system. Outdoor air is drawn into the ducts and passed through the heat exchanger, where heat from the outgoing air airstream is transferred to the incoming air. So long as there is a difference in temperature, heat from the warmer air will transfer to the colder air.
Note that some systems which operate in the ceiling space move warmer air from up there down into the house below. If these do not incorporate a heat exchanger, they should be more properly described as “forced air” or “positive pressure” ventilation systems, rather than “heat (or energy) recovery” systems.
HRV provides improved climate control, while also saving energy by reducing heating (and cooling) requirements. However, these units are primarily ventilation, not heating systems as no addition heating is added, beyond what can be recovered by the heat exchanger.
ERV stands for Energy Recovery ventilation. ERV units transfer both temperature and moisture. ERV strictly means using the energy contained in the air coming out of a building to “precondition” the air going in, which is why the term is sometimes used interchangeably with HRV. However there is an important distinction:
HRV systems recovers and transfers heat only, there is no direct transfer of moisture whereas
ERV systems recover and transfer both heat and moisture.
Somewhat confusingly, HRV systems are usually better at removing moisture, and ERV systems better at controlling humidity levels. Because here in New Zealand we have such naturally occurring high levels of humidity, ERV systems are less likely to be appropriate: they work well when the natural humidity levels are lower than is comfortable, by increasing it.
Designing an appropriate system requires a reasonable level of understanding, and is usually done by the supplier. If you would like to understand the principles in more detail and perhaps design your own, BRANZ has a Bulletin available: No 508 “Heat/Energy recovery ventilation systems” (link ) All such systems do require regular maintenance.
HVAC stands for Heating, Ventilation and Air Conditioning. HVAC system design is a subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer. Generally only used in medium to large industrial and office buildings.