What is a sensible heat exchanger?
A sensible heat exchanger is a component used in MVHR systems to transfer heat between the air extracted from indoor spaces and the fresh air supplied from outside, without mixing the two airflows. It mainly recovers the energy associated with air temperature, but not humidity. This distinguishes it from an enthalpy heat exchanger and means that, under certain conditions, condensate management may be required.
A sensible heat exchanger is one of the main components of a Mechanical Ventilation with Heat Recovery system. Its function is to recover part of the thermal energy contained in stale air extracted from indoor spaces and transfer it to the fresh air coming from outside, thereby improving the energy efficiency of the ventilation system.
Its operating principle is based on heat exchange between two separate airflows:
- indoor air, which has already been heated or cooled, is expelled outside;
- outdoor air is filtered and supplied to the rooms, passing through the heat exchanger before entering the building;
- the two airflows never mix, but pass through separate channels inside the heat exchanger;
- heat is transferred from the warmer airflow to the cooler one through the heat exchange surfaces.
During winter, the warm air extracted from indoor spaces transfers part of its heat to the incoming cold air. In this way, the fresh air is preheated before being supplied to the rooms, reducing the energy losses associated with air renewal.
During summer, the system can work in the opposite way: conditioned indoor air can help reduce the temperature of the warm outdoor air, limiting the impact of air renewal on indoor comfort.
It is referred to as “sensible” heat recovery because the exchanger mainly transfers sensible heat, meaning the energy associated with changes in air temperature. By contrast, it does not significantly recover the latent component, which is the energy associated with moisture content. This is the main difference compared with an enthalpy heat exchanger, which is designed to recover both part of the heat and part of the humidity contained in the airflows.
In MVHR systems with a sensible heat exchanger, the outdoor air is therefore tempered using the recovered heat, while humidity is not transferred to any significant extent between the extracted and supplied airflows. For this reason, under certain climatic and operating conditions, condensate may form inside the system, making suitable condensate drainage necessary in accordance with the product installation instructions.