Heat recovery ventilation captures energy from exhaust air that would otherwise be lost, transferring it to incoming fresh air. This technology dramatically reduces heating and cooling loads while maintaining required ventilation rates.
How Heat Recovery Works
All heat recovery systems exploit temperature differences between exhaust and supply air streams. In winter, warm exhaust air pre-heats cold incoming air. In summer, cooler exhaust air pre-cools hot incoming air.
Energy transfers through various mechanisms depending on technology type. Some systems transfer heat only; others also transfer moisture, affecting humidity balance.
Effective heat recovery can capture 60-90% of available energy, substantially reducing the heating or cooling required to condition fresh air to supply temperatures.
Plate Heat Exchangers
Plate heat exchangers pass supply and exhaust streams through adjacent channels separated by thin plates. Heat conducts through plates from warmer to cooler streams.
Cross-flow and counter-flow configurations offer different efficiency and size characteristics. Counter-flow arrangements achieve higher efficiency in longer units.
Advantages include no moving parts, no cross-contamination risk, and minimal maintenance requirements. Static design provides reliable, long-term operation.
Limitations include modest maximum efficiency and potential frosting in cold conditions. Frost protection measures may reduce effective efficiency during severe weather.
Thermal Wheels
Thermal wheels (rotary heat exchangers) use rotating cylinders containing heat storage media. The wheel rotates slowly between exhaust and supply streams, absorbing heat from one and releasing it to the other.
High efficiency—potentially exceeding 85%—makes thermal wheels attractive for applications prioritising energy recovery. Enthalpy wheels additionally transfer moisture alongside heat.
Careful attention to cross-contamination prevention suits most commercial applications. Purge sectors and appropriate speed control minimise contaminant transfer.
Maintenance requirements exceed static systems. Wheel drives, seals, and media condition all require periodic attention.
Run-Around Coils
Run-around coil systems circulate fluid between coils in separate supply and exhaust air streams. Heat transfers into the fluid from the warmer stream and releases into the cooler stream.
Physical separation allows heat recovery where supply and exhaust streams cannot be brought together. This flexibility suits refurbishment projects and complex building arrangements.
Lower efficiency than direct exchange methods—typically 40-60%—limits applications where maximum recovery is essential. The flexibility advantage compensates in suitable applications.
Selecting Heat Recovery Systems
Application requirements determine appropriate technology selection. Contamination sensitivity, space constraints, efficiency targets, and capital budgets all influence decisions.
Healthcare applications often prefer plate exchangers eliminating cross-contamination risk. Commercial buildings frequently specify thermal wheels for maximum efficiency.
Climate considerations affect technology suitability. Very cold climates may challenge plate exchangers with frosting; mild climates may not justify thermal wheel investment.
Integration with AHU Design
Heat recovery integrates within air handling unit configurations rather than standing alone. Position within units, bypass arrangements, and control integration all require considered design.
Bypass provisions enable heat recovery bypass when outdoor conditions suit direct fresh air supply. Unnecessary heat recovery when ambient temperatures approach supply requirements wastes energy.
Frost protection systems prevent ice buildup during cold weather. Various approaches including pre-heaters, face velocity reduction, and bypass arrangements address frosting risk.
Performance Verification
Installed heat recovery performance should be verified against design expectations. Commissioning measurements confirm actual efficiency achievement.
Ongoing monitoring identifies efficiency degradation from filter loading, wheel degradation, or other factors. Early intervention maintains designed performance.
Energy consumption comparison against baselines quantifies heat recovery benefits. This data supports business cases for future installations.
I-Flow Technologies designs air handling systems integrating heat recovery technology appropriate to application requirements. Our engineering expertise matches recovery technology to project needs, delivering optimal efficiency for your specific circumstances. Contact us to discuss heat recovery solutions.
