A novel algorithm for demand-control of a single-room ventilation unit with a rotary heat exchanger

Energy renovations seek to improve the airtightness of dwellings and thus require ventilation and heat recovery to maintain or improve energy-efficiency, indoor climate, and durability. These ventilation systems often control the indoor air of an apartment as a single climate zone, which neglects the different demands of individual rooms.

Renovations result in greater retention of heat and air inside the building envelope, so rooms become especially sensitive to gains from solar radiation, occupancy, moisture loads and pollutants. Single-room ventilation units are able to provide balanced ventilation with heat recovery in individual rooms.

This provides a unique opportunity to meet the demands of each room with an appropriate ventilation rate, supply temperature and drying capacity. In prior publications, the authors described the development of a single-room ventilation unit with a rotary heat exchanger, which is commercially available in Denmark.

The unit includes temperature sensors at the inlet and outlet of the supply and exhaust airflows. At the exhaust inlet, a relative humidity sensor is standard and a CO2 sensor is optional. Together these sensors detect thermal comfort and air quality in the indoor environment. Based on these values, a demand-control algorithm varies fan speeds to change airflow rates and varies the rotational speed of the heat exchanger to modulate heat and moisture recovery.

The algorithm varies airflow rates to provide free cooling and limit CO2 concentrations and varies the coupled heat and moisture recovery to ensure the appropriate supply temperatures for heating or cooling and to modulate drying capacity. In the default setting, the algorithm is not aware of the heating set-point temperature in each room, so the algorithm decides when to bypass heat recovery without compromising efficiency.

Moisture control takes higher precedence in the algorithm and overrides temperature and CO2 controls. In previous publications, the authors demonstrated that modulating regenerative heat recovery could control relative humidities in ‘dry rooms’, so the algorithm first attempts to limit moisture recovery by varying the rotational speed and then safely unbalances airflows in a worst-case scenario.

In the algorithm, frost protection and minimum supply temperature take the highest priority and override other controls. This paper documents the proposed demand control algorithm and analyses its impacts on compliance of building regulations in Denmark. The paper presents an algorithm that manufacturers can program into their controls.

The commercially available single-room ventilation unit with a rotary heat exchanger uses this algorithm coded in the C language. Future work will document the effectiveness of the algorithm and how it behaves in a system.