Quantification of the Impact of Indoor Temperature Gradients in Dwellings on Useful Recovered Heat of Ventilation Systems

Ventilation in buildings dilutes the indoor air pollutants by replacing part of the air with outdoor air to guarantee an adequate indoor air quality (IAQ). In heating-dominated climates, the exchanged air has a lower mean temperature than the indoor air, which leads to a surplus heating demand in the building. A heat exchanger recovers part of the heat from the expelled air, contributing to the reduction of the extra heating demand. Smart ventilation systems work with reduced airflows, without compromising the IAQ and lowering the heating demand. A simplified way to calculate the heating demand reduction is possible considering uniform indoor temperatures and knowing a few parameters such as the outdoor air temperature. However, the in a real building this is not the case, and the calculation neglects the different temperatures in the zones. When using a central heat exchanger, the warmer airflows from heated zones are combined with colder airflows from unheated zones, reducing the potential of the recovered heat. Moreover, the recovered heat can be distributed among zones that may not need heat, to detriment of the zones that demand it. The useful heat that can reduce the heating demand is then reduced compared to the scenario that uses uniform indoor temperatures. Besides, the internal airflows between zones caused by the ventilation system can affect the heating demand when transferring heat from warmer zones to the colder ones, and vice versa. Smart ventilation systems reduce the latter effect and prevent some heat to be released into the atmosphere, which results in an equivalent effect as the recovered heat. The purpose of this work is to investigate the mentioned effects and determine their importance using building energy simulations. To do so, six typical dwellings were modelled combined with ten ventilation systems representing various commercial solutions and six extra non-smart ventilation systems used as reference. The study calculates three heating demands: when the buildings do not have a ventilation system, when the ventilation system is connected and when the ventilation system works without a heat exchanger (if present) or at maximum design airflows (neglecting the smart controls). On top of that, two heating scenarios are investigated: uniform heating and non-uniform heating. For each, the useful heat can be calculated and expressed as a reduction in the surplus heating demand. Finally, a coefficient is obtained that relates the performance between both heating modes. The results show that, under non-uniform heating conditions, the energy performance of the ventilation systems is typically from 5 % lower to 1 % higher than the energy performance under uniform heating conditions. Furthermore, apart from the ventilation system, the dwelling envelope appears to be the most influencing factor for the energy performance under both conditioning strategies.