Heat Recovery from Exhaust Air: Why It Matters
The essence of ventilation is the removal of stale, contaminated air and the introduction of fresh air into a building. These processes are carried out by Air Handling Units (AHUs) or air conditioning units. To make outdoor air suitable for indoor delivery, it must be heated in winter and cooled in summer. When we exhaust this air after use without recovery, we lose all the heating and cooling energy previously consumed.
Why Recover Energy at All?
There are two primary reasons: economy and ecology. Firstly, we want to avoid paying exorbitant amounts for the same energy twice. Secondly, higher energy consumption leads to greater depletion of natural resources. The core of heat recovery ventilation is, therefore, to remove "dirty" air while retaining as much energy as possible within the facility—energy that has already been paid for.
Leading Recovery Technologies
In ventilation systems, specifically within AHUs and AC units, the following heat recovery methods are predominantly used:
- Rotary Heat Exchanger – recovers heat, cold, and moisture (latent heat).
- Plate Heat Exchanger – cross-flow or counter-flow.
- Run-around Coil (Glycol) Loop – for total separation of air streams.
- Heat Pump – for active thermodynamic recovery.
What Determines the Effectiveness of Heat Recovery?
Effective heat recovery in ventilation is defined by maintaining the highest possible efficiency throughout the entire lifespan of the AHU. Pay attention to the following aspects:
- Economics: Use heat recovery when it is economically justified; for example, utilize free-cooling technology to harness free outdoor cooling without engaging the heat exchanger.
- Optimal Selection: Properly size the recovery system. An experienced AHU manufacturer knows that systems that are too small or too large relative to the airflow volume will not achieve peak efficiency.
- Hygiene: Keep recovery systems clean. Contaminants reduce the active heat exchange surface area, while mechanical damage can cause air leakage and mixing, potentially dropping recovery efficiency to zero.
Best Practices
Through our collaboration with designers, contractors, installers, and investors—as well as our own observations as a manufacturer—we have identified several best practices for heat recovery in mechanical ventilation:
- Match Technology to Building Function: Recovery systems vary in capabilities; while all recover heat, some also recover moisture (enthalpy). They also differ in air-mixing levels—a critical factor when selecting units for specialized areas like commercial kitchens.
- Service Access: Although heat recovery systems are generally reliable, it is vital not to block service access. Keeping them clean is key to ensuring a long service life with maximum efficiency.
- Experience: Proper selection often depends on nuances. Rely on the knowledge of experienced AHU manufacturers who have a proven track record in implementing heat recovery across various AC and ventilation configurations.
Heat Recovery – Key Takeaways
With the rising demand for energy, recovery is now a necessity—but one we should embrace. A well-designed and operated system avoids high operational costs and reduces environmental degradation. Furthermore, high-efficiency heat recovery increases the chances of a building achieving prestigious BREEAM or LEED certification.
