Neutral hydrogen gas on large scales provides an excellent probe for various sources of heating. At low redshift (z < 0.1), I will discuss the prospect of detecting the intergalactic medium (IGM) in large-scale filaments through 21-cm emission with the upcoming Square-Kilometre Array (SKA). Such a detection would provide constraints on the strength of the cosmic UV background. Furthermore, the Lyman alpha forest is currently one of the best probes of the neutral gas in the IGM at high redshift (z ~ 2-3). Recently, the Lyman alpha forest has been used to generate 3D tomographic maps of the large-scale absorption field in surveys such as CLAMATO. Using simulations, we explore the potential of these maps for constraining the thermal and ionization properties of the gas through the distribution of Lyman alpha transmission as a function of reconstructed dark-matter density. I will show that this can potentially be used as a direct probe of preheating in proto-clusters. This will help to constrain the various models of galaxy and AGN feedback, both in the IGM and in clusters.