Technologies to overcome effects of condensation in exchangers of ventilation units - analysis of monitored field studies

Balanced ventilation with heat recovery is an efficient way to maintain low heating demand for ventilation in residential buildings. Laboratory measurements of today’s heat recovery ventilation units show high temperature recovery efficiency during standard conditions. In practice, however, the recovery efficiency may decrease due to circumstances that deviate from the standard laboratory conditions.
The present study shows detailed measurements of two field tests with similar balanced ventilation systems installed in houses. One of the houses is in The Netherlands and has a heat exchanger installed (for heat recovery). The other house is in Austria and has an enthalpy exchanger installed (for heat and moisture recovery). During the same period in October and November 2016, various parameters were monitored among which temperatures and humidities of the air streams, extract and supply fan speeds and their corresponding flow rates. Outdoor temperatures vary in the range of 22 °C at the start of the observed period until -2 °C at the end of the observed period. Data is monitored on a 5-minute interval time, and later analysed using the hourly averaged values of the parameters.
From the monitored data, it is shown how the thermal recovery efficiency correlates with other parameters as outdoor temperature, and the combination with the dew point of the extract air. The effects of condensation on fan behaviour is shown for a mass flow balance correcting algorithm, and compared to measurements without correction of mass flow balance. Moreover, the thermal recovery efficiency is shown for the ventilation systems with enthalpy exchanger to compare them with the systems with heat exchanger.
Conclusion of the present study is that a mass flow balance correcting algorithm maintains a high thermal recovery efficiency for a balanced ventilation system, even when condensation occurs in the extract channels of the heat exchanger. Because of moisture transfer, enthalpy exchangers experience no condensation, and therefore show no change in thermal recovery efficiency than heat recovery systems, even when outdoor temperature drops below the dew point of the indoor climate. For the enthalpy recovery units, the thermal recovery efficiency is only a function of ventilation air flow rate.