In recent years, the inclusion of a thermal bypass device in Controlled Mechanical Ventilation units has become increasingly common. The purpose of the thermal bypass is to prevent heat recovery, either wholly or partially, in cases where this is not desirable. In this way, ideal indoor conditions are more easily maintained, without having to take into account heat exchange, which is not always beneficial.
How does it work?
In a ventilation unit with heat recovery, the indoor and outdoor air flows pass through the heat exchanger, transferring heat from one flow to the other. The bypass serves to prevent, either fully or partially, the exchange of heat between the two flows. To achieve this, the thermal bypass diverts the air around the heat exchanger rather than through it.
In summer, this is particularly useful when the outside air is cooler than the indoor air, for example at night. Thanks to the bypass, the indoor air does not transfer heat to the outside air, which is then introduced without raising the temperature. This process is known as free cooling.
What types of thermal bypass are there?
The most common solution is the automatic bypass. In this case, the control system uses information on the outdoor and indoor air temperatures – obtained via sensors – to determine when to open the bypass automatically. If the air conditions detected by the sensors are favourable and do not cause discomfort, the bypass opens and the air does not pass through the heat exchanger.
In the case of a manual bypass, the user intervenes directly to decide how to open it. Here too, temperature data obtained from the sensors is used, but the control system does not automatically manage the thermal bypass device. A total bypass occurs when all the air is diverted and there is no heat exchange. A partial bypass, on the other hand, occurs when only part of the air bypasses the heat exchanger. In this case, heat recovery is not zero but merely reduced.
Adding a thermal bypass device to a ventilation unit improves its performance by avoiding unnecessary heat recovery and making the most of favourable outdoor conditions for free cooling. The result is improved indoor air quality and a reduction in the energy required to maintain the temperature.
