Ventilation systems play a key role to maintain healthy and comfortable indoor air quality (IAQ) in almost all buildings. Adequate indoor air quality involves many factors, including heating or cooling, depending on the local situation and climate, humidity control, and filtration of pollutants. Health issues, such as breathing problems, can arise from air contaminated with dust, pollen, or other contaminants.Ventilation Guide - Improving Indoor Air Quality
Health issues, such as breathing problems, can arise from air contaminated with dust, pollen, or other contaminants. In respect to Covid-19, several organizations (e.g., ASHRAE and REHVA) are recommending boosting the ventilation rate to reduce the concentration of contaminants inside the building. Ventilation systems should be running longer and with a higher flow rate.
However, there are concerns around the leakage and recirculation of air and its impact on the number of harmful particles inside the building.
Even before Covid-19, ventilation systems and components were designed to eliminate carbon dioxide, smell, particles, and other contaminants in the air. But the recirculation of air (exhaust air reused as supply air) might still appear in two ways, through unintended leakages (e.g. in ducting and heat recovery units) or through intended recirculation (e.g. using recirculation dampers). Consequently, if the source of contamination occurs inside the building, it’s crucial to close the recirculation damper and reduce any leaks to avoid exhaust air from re-entering the building.
At the heart of a ventilation system, you’ll find an air handling unit and an energy recovery unit for heat and energy recovery. The two most used types are rotary heat exchangers and plate heat exchangers. If correctly installed and maintained, they have equally low leakage rates.
Rotary heat exchangers are often the preferred choice thanks to their low freezing risk (wheels by definition defrost themselves), compact size, and high sensible efficiency. The possibility of adding a coating to the wheel, which allows latent transfer, is another factor favoring these products.
If the heat recovery unit is switched off or bypassed, heating coils might not be able to handle the entire heat load, which leads to low inlet air temperature and an uncomfortable indoor climate. The reaction might be switching off the whole ventilation system, making IAQ even worse by preventing the dilution of airborne contaminants through fresh outdoor air supply. Consequently, it’s suggested to keep operating the heat recovery unit and continue to supply the building with fresh air before trying anything else. However, to ensure the best performance and cleanest air possible, it’s essential to understand the basics and best practices around rotary heat exchangers!
Outside Air Correction Factor (OACF)
To ensure clean and fresh supply air, higher pressure in the supply air compared to the exhaust air is needed. That pressure difference causes a gap in the seals between the air ducts and thus generating a leakage between the supply inlet and the exhaust outlet. This effect reduces the amount of supply air going through the wheel and entering the building. The scheme below shows the distribution of internal leakages if the pressure on the outdoor side is higher than on the exhaust side. By adding an optional special seal, OACF is reduced.
Exhaust Air Transfer Ratio (EATR)
Due to the wheel rotation, some air gets trapped inside the matrix during the rotation from one air duct to the other. The air amount trapped in the wheel is transferred and mixed with the next airflow. If the air transferred is exhaust air into the supply air, the result is the contamination of the supply air. This effect is called carryover or Exhaust Air Transfer Ratio (EATR onwards) and is expressed in percentage (%) of the total airflow. By adding an optional purge sector, the carryover is reduced. To examine if there is any leakage connected to the rotary heat exchanger, measure in the air duct around the rotor.
The most critical step to prevent exhaust air from mixing in with the supply airflow and reentering the building is the correct distribution of pressure between the separate airflows. Air pressure should always be higher on the supply side compared to the exhaust side. This minimizes any leakage from the exhaust air into the fresh supply air. This applies not only to the heat recovery unit itself but to the building as a whole and most of the parts constituting the ventilation system (e.g., ducting and recirculation dampers).
The combination of higher pressure in the exhaust airflow versus the supply airflow affects the rotary heat exchanger by disabling the purge sector, which leads to a carryover effect through the matrix. Turning off the rotor will only stop the carryover, but leakage through the sealings might still appear.
Different fan positioning has different advantages. To minimize EATR and OACF values the combination of two pulling fans for both the supply and the exhaust airflows has proven to be a favorable solution, although it leads to negative pressure on both sides. Yet to compensate, slightly increase pressure on the supply side.
The purge sector is optimized to reduce carryover or EATR. It will stop the inlet of exhaust air in the small area right before airflows switch, thus avoiding exhaust air to get trapped into the matrix. A small amount of the supply air is used to blow out the minor amount of exhaust air that might have been trapped to ensure a fresh and clean supply air. Note, Heatex provides modular purge sectors to be assembled at a later stage.
Heatex does not support lowering the flow rate to reduce the leakage rate because the flow rate induces a pressure drop required by the purge sector to function. We recommend therefore running the rotor at a nominal (or if possible) higher flow rate.
By adding an optional Heatex “Special Seal” made of polymer on the middle beam and across the purge sector, leakage through OACF is reduced by 50% compared to standard seal.
”It is shown that rotary heat exchangers, which are properly constructed, installed and maintained, have almost zero transfer of particle-bound pollutants (including air-borne bacteria, viruses, and fungi), but the transfer is limited to gaseous pollutants such as tobacco smoke and other smells.”– REHVA COVID- 19 Guidance Document, April 3, 2020.
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