The principle and practicality of heat recovery in HVAC systems
I. Core Principles of Heat Recovery
In short: the energy (heat/cooling, moisture) that would otherwise be wasted in the exhaust air and sent outdoors is transferred to the fresh air through a heat exchanger. The fresh air is pre-cooled/pre-heated, pre-humidified/dehumidified before entering the air conditioning unit for processing; the supply and exhaust air are physically separated to prevent air mixing.
II. Divided into two main categories:
1. Sensible heat recovery (HRV, which only transfers temperature, not moisture)
It relies on metal plates, heat pipes, and intermediate water coils for heat exchange, transferring only sensible heat and not exchanging humidity.
In winter: The indoor warm exhaust air transfers heat to the cool fresh air, causing the fresh air to warm up;
Summer: The indoor exhaust fan transfers its cooling energy to the hot fresh air, thus cooling the fresh air.
Efficiency is generally 60%–80%;
Advantages: No moisture migration, no fear of cross-contamination;
Disadvantages: In hot and humid areas, the dehumidification load of fresh air cannot be reduced.
Representative equipment: plate aluminum core, heat pipe, ethylene glycol coil (winding type).
2. Total heat recovery (ERV, simultaneous transfer of sensible heat + latent heat = enthalpy recovery)
The core is made of a breathable polymer membrane/special paper material, which allows water vapor to pass through but not air, and allows for the exchange of temperature and humidity.
Summer: Exhaust ventilation removes moisture, and fresh air is pre-cooled and pre-dehumidified;
In winter: The dry fresh air system recovers moisture from the exhaust air, preventing excessive dryness;
Efficiency 60%–85%; suitable for humid, rainy season, and high-humidity areas;
Risks: Long-term membrane clogging and trace permeation risks necessitate regular maintenance.
Representative equipment: total heat plate fins, rotary total heat exchanger (enthalpy wheel).
Another type of heat recovery is chiller condenser heat recovery (not fresh air exhaust heat exchange), which recovers heat from the compressor condenser side to heat domestic hot water. This is commonly used in hotels and hospitals.
III. Comparison of Commonly Used Structures
Plate type (static): Simple structure, no rotating parts; relatively high resistance, attention should be paid to condensation and icing of the core in winter; most common in household and small units.
Rotary type: highest efficiency; has a motor and seals, but there is a small amount of air leakage; suitable for office buildings and workshops with large air volume.
Heat pipe type: no power supply, low temperature resistant; can only recover sensible heat, suitable for cold and dry regions.
Intermediate refrigerant coil (ethylene glycol): Supply and exhaust ducts can be laid out separately, making it friendly to retrofit projects; however, it has sensible heat, low efficiency, and requires additional power consumption from the fan and water pump.
IV. Practicality (Advantages, Limitations, Applicable Scenarios)
1. Profits
Significantly reduce fresh air load: Fresh air is one of the largest loads on air conditioning. Heat recovery can eliminate most of the energy consumption for heating, cooling, and dehumidifying fresh air. Buildings with high air exchange rates have a very significant energy-saving effect.
Balancing ventilation and energy conservation: Meeting CO₂ control, fire protection, and cleanroom ventilation requirements, eliminating the need to choose between "saving electricity" and "opening windows for ventilation"; airtight passive houses and low-energy buildings are almost standard features;
Improved comfort: In winter, the fresh air system won't blow cold air directly on you; in summer, it won't feel hot and humid.
Bonus points for green buildings: national standards, green building, ultra-low energy consumption buildings, and in many places, energy conservation reviews mandate or encourage exhaust heat recovery.
2. Shortcomings (many problems encountered in engineering projects)
Additional resistance and increased power consumption of the fan: Don't just look at the heat exchange efficiency, look at the net efficiency; a low-efficiency, high-resistance core means the recovered energy is less than the extra power consumed by the fan, making it a waste of energy or even more wasteful.
It's not suitable to run it all year round: When the outdoor temperature and humidity are suitable in spring and autumn, the bypass should be turned on so that fresh air can enter directly without passing through the heat exchanger; running it continuously will only waste the fan's electricity.
Maintenance costs: Dust, oil, and mold accumulate in the filter and core; once the thermal membrane becomes moldy or clogged with oil, its performance drops drastically; thermal heating is not suitable for kitchens or spaces with high oil and dust levels.
Freezing prevention: In cold regions, condensation and ice easily form on the exhaust side during winter, requiring preheating or freezing control.
Investment recovery period: Small residential apartments with low ventilation have a long payback period; buildings with high traffic and 24-hour high fresh air volume (hotels, conference rooms, hospitals, laboratories, shopping malls) have extremely high cost performance.
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