A Mathematical Model for Infiltration Heat Recovery

Infiltration has traditionally been assumed to affect the energy load of a building by an amount equal to the product of the infiltration flow rate and the sensible enthalpy difference between inside and outside. However, laboratory and simulation research has indicated that heat transfer between the infiltrating air and walls may be substantial, reducing the impact of infiltration. In this paper, two- and three-dimensional CFD simulations are used to study the fundamental physics of the infiltration heat recovery process and a simple macro-scale mathematical model for the prediction of a heat recovery factor is developed. CFD results were found to compare well (within about 10 percent) with limited published laboratory data corresponding to one of the scenarios examined. The model, based on the steady-state one-dimensional convection-diffusion equation, provides a simple analytical solution for the heat recovery factor and requires only three inputs: the infiltration rate, the U value for the building, and estimates of the effective areas for infiltration and exfiltration. The most difficult aspect of using the model is estimation of the effective areas, which is done here through comparison with the CFD results. With proper input, the model gives predictions that agree well with CFD results over a large range of infiltration rates. Results show that infiltration heat recovery can be a substantial effect and that the traditional method may greatly over-predict the infiltration energy load, by 80-95 percent at low leakage rates and by about 20 percent at high leakage rates. This model for infiltration heat recovery could easily be incorporated into whole-building energy analysis programs to help provide improved predictions of the energy impact of infiltration.