Ignoring any losses, estimate how much energy (in units of btu) is required to raise the temperature of water in a 90-gallon hot-water tank from 60°f to 110°f. the specific heat of water is approximated as a constant, whose value is 0.999 btu/·lbmr at the average temperature of (60 + 110)/2 = 85ºf. in fact, c remains constant at 0.999 btu/lbm·r (to three digits) from 60ºf to 110ºf. for this same temperature range, the density varies from 62.36 lbm/ft3 at 60ºf to 61.86 lbm/ft3 at 110ºf. we approximate the density as remaining constant, whose value is 62.17 lbm/ft3 at the average temperature of 85ºf.