Techniques determine if an appliance having a fixed-rate of gas-consumption is degrading over time. In one example, a flowrate of gas at a service site is obtained. The flowrate of gas is disaggregated to obtain a flowrate of gas corresponding to an appliance having a generally fixed-rate of gas-consumption. The flowrate of gas of the appliance is compared to historical gas consumption by the appliance. Based at least in part on the comparing, it may be determined that performance of the appliance has changed over time. For example, the gas consumption of a hot water tank may increase due to mineral build-up in the bottom of the tank. Responsive to the determined degradation of the appliance, warnings may be sent, repairs may be made, and/or appliance(s) may be replaced.