Thermal wheels, also known as rotary heat exchangers, are commonly used in HVAC systems for their high energy recovery ventilation performance. But their application in sensitive environments like operating theatres, isolation rooms, and clinical laboratories raises concerns due to strict air quality and infection control requirements.
Even though standards like HTM 03-01 may approve the use of thermal wheel heat recovery units in applications with filters designed to prevent microbiological contamination, there are important reasons why their use might still be discouraged or avoided in certain healthcare or sensitive environments.
Reasons to avoid thermal wheels in clinical settings
1. Regulatory and safety considerations
Healthcare facility guidelines often recommend against the use of energy recovery devices that allow for any potential cross-contamination. For example, NHS Scotland guidance permits thermal wheels in general wards but advises against their use in areas treating vulnerable patients.
Additionally, future changes in room usage—from general to specialised clinical functions—could necessitate the costly removal or replacement of thermal wheels to comply with stricter air quality standards.
To better understand the structure and potential leakage paths in thermal wheels, consider the following schematic:
Figure 1: Thermal wheel components
For robust compliance with evolving requirements, many designers opt for safer, more controlled air handling units for healthcare sites that ensure long-term flexibility.
Explore air handling units for healthcare sites.
2. Risk of cross-contamination
Thermal wheels inherently allow a degree of air leakage between the exhaust and supply air streams. This leakage can occur through:
- Carryover: Residual exhaust air retained in the wheel matrix and transferred to the supply air stream.
- Seal Leakage: Imperfections or wear in the seals separating the two air streams.
Even with mitigation measures like purge sectors, studies have shown that cross-contamination can be reduced but not eliminated. For instance, a purge section can lower carryover contamination from approximately 2–4% to about 1% but cannot address leakage through peripheral and transverse seals.
Figure 2 Microbiological contamination
In clinical environments, where patients may be immunocompromised or exposed to airborne pathogens, even minimal cross-contamination poses unacceptable risks to health and safety.
3. Reduced effectiveness of the purge sector
Issue: The thermal wheel relies on a specific airflow pattern to exhaust contaminated air during purge cycles. The purge sector uses a small stream of clean supply air to flush or “purge” the matrix before it rotates into the supply air stream. This purge air displaces trapped contaminants in the matrix and redirects them back into the exhaust airstream.
Impact: If the extract fan is not correctly set or if airflow is pushed in the wrong direction, the purge sector may be bypassed or rendered ineffective, compromising the system’s ability to flush out microbiological contaminants.
Figure 3 Specific airflow pattern for Purge
Figure 4 Possible Airflow Patterns
P21 = Static pressure, outside air
P22 = Static pressure, supply air
P11 = Static pressure, exhaust air
P12 = Static pressure, outlet air
This reinforces the importance of consistent and professional air handling unit maintenance across all HVAC systems in clinical settings.
4. Incorrect fan and exhaust configuration leading to cross-contamination
Issue: If the extract fan is positioned or controlled improperly—such as blowing air through the thermal wheel—it can disrupt the intended airflow patterns.
Impact: This may prevent the purge sector (the part of the wheel that exhausts contaminated air) from functioning effectively, leading to recirculation of contaminated air or backflow into the supply airflow. These risks violate core HVAC infection control standards.
5. Complexity in design and control
Issue: Thermal wheels require precise control of airflow rates, bypass dampers, and fan positions to operate correctly.
Impact: In complex or poorly designed systems, misconfiguration can lead to ineffective heat recovery and contamination risk, especially if staff are unfamiliar with the specific control strategies needed. This makes correct integration with air handling units crucial. Learn more about air handling units.
6. Challenges in cleaning and maintenance
The structure of thermal wheels includes a matrix of narrow channels designed for heat exchange. These channels can accumulate dust, biological contaminants, and other particulates over time. Cleaning these intricate components is labour-intensive and may not achieve the level of decontamination required in clinical settings.
Moreover, certain contaminants can adhere to the wheel’s surfaces, reducing its effectiveness and posing additional cleaning and maintenance challenges. This emphasises the importance of proper HVAC system maintenance in hospitals.
Figure 5 Thermal Wheel Storage Mass (Matrix)
Alternative solutions
For specialised clinical areas, alternative heat recovery methods that ensure better separation of air streams are recommended:
- Run-Around Coil Systems: Utilise a closed-loop fluid circuit to transfer heat between supply and exhaust air streams without mixing the air itself.
- Plate Heat Exchangers: Employ solid barriers to separate air streams, reducing the risk of cross-contamination.
These systems, while potentially less energy-efficient than thermal wheels, prioritise patient safety and compliance with healthcare air quality standards. In the run-around coil vs thermal wheel debate, clinical safety often tips the balance in favour of separation-focused solutions.
Summary
While thermal wheels offer energy efficiency benefits, their potential for cross-contamination and maintenance challenges make them unsuitable for specialised clinical environments where air purity is paramount.
Although HTM 03-01 may approve thermal wheel heat recovery units with appropriate filters, the practical risks—such as incorrect extract fan positioning, improper airflow control, and potential for cross-contamination—can undermine their microbiological safety.
In critical healthcare environments, these issues outweigh the theoretical benefits, leading to a preference for alternative heat recovery methods and a more robust filtration strategy.
Figure 6 General principle of rotary heat exchangers
