Large-eddy simulations (LESs) for two different T-junctions are performed for the prediction of thermal mixing loads on piping. In particular, the effects of wall treatment and mesh on temperature and wall heat flux fluctuations are studied. Wall-resolved LES shows good agreement with an experiment having adiabatic walls, but using wall functions shows deviations in root-mean-squared (RMS) temperatures and cross-stream mean velocities. The simulations show increases in peak RMS temperatures with local mesh refinement, and hence, too-low peak values are obtained with wall functions. The highest temperature fluctuations occur locally near the T-junction requiring a dense mesh. Wall functions are unable to capture high wall heat fluxes at a sharp corner, but otherwise, the maximum RMS value is close to a wall-resolved LES. For a T-junction having a round corner, higher RMS heat flux is obtained with wall functions compared to a wall-resolved case. Wall functions show lower instantaneous heat fluxes than wall-resolved LES, but the wall functions nonetheless result in higher pipe wall temperature fluctuations due to lower frequency content.