The disclosed technology includes a water heater system having a blower, an igniter, a burner, a heat exchanger, and a refractory configured to attach to the burner and the heat exchanger. The refractory can include a unitary ceramic housing having a top and a plurality of sidewalls and be configured to retain heat from combustion gases. The top can have a burner aperture configured to receive at least a portion of the burner and the plurality of sidewalls can form a cavity configured to receive at least a portion of the heat exchanger. One or more of the sidewalls can additionally have a protrusion extending laterally along an inside surface of the sidewall that is configured to contact the heat exchanger to form a seal between the heat exchanger and the refractory. A sealing material can substantially create a seal between the refractory and the heat exchanger.