How to Install a Rheem Classic 90

Select a site with electric, gas and duct connections, where vent pipes will have the shortest possible length and fewest elbows. If you're replacing an existing furnace, shut off the electricity and gas. Disconnect and remove the old furnace. Any furnace enclosure must be at least 26 inches wide, 58 inches high and 52 inches deep. Allow space for electric and gas lines, vents, piping, ductwork and access to air filters. Uncrate the furnace.

Cut a rectangular return-air duct opening in the sheet metal side of the furnace 15 inches high and 23 inches long. Cut with a reciprocating saw within the embossed markings. Attach a duct connection flange to the opening with sheet metal screws. Slide a 15-inch by 23-inch rectangular connecting duct from the house return air ductwork over the side connection flange. Fasten it with sheet metal screws then seal it with tape. Slide a 17.5-inch by 20-inch rectangular connecting duct from the overhead supply ductwork down over the flanges of the supply duct opening at the furnace top. Fasten it with sheet metal screws and seal it with tape.

If you're replacing a high-efficiency furnace, connect the existing air intake and exhaust vents to the openings on top of the new furnace with compression fittings. For a new installation, run in 3-inch PVC pipe vents from an exterior wall or the roof. Terminate horizontal vent pipes at least 12 inches above the ground or roof. Install a 180-degree elbow or other approved termination fitting on vertical vent pipes. Secure joints with PVC cement.

Cement a piece of ½-inch PVC drain pipe into the condensate-trap drain elbow with PVC cement. It should extend outside the furnace casing. Cement a ½-inch, vertically-oriented PVC pipe tee to the trap drain pipe's outer end. Cement a 5.5-inch open-ended PVC riser to the upper tee opening as an alternate outlet in case of drain blockage. Cut a PVC pipe section 2 inches shorter than the distance from the tee to the floor. Cement a 90-degree elbow to one end. Cement the other end to the lower tee opening. Cut a PVC pipe section long enough to reach a house drain and cement one end to the elbow. Prime the condensate trap by pouring in a cup of water through the hose from the secondary furnace coil.

Screw a threaded piece of 1/2-inch iron gas pipe into the burner control valve’s threaded supply opening. The pipe must extend outside the furnace casing. Thread a pipe tee onto the outside end of the pipe, and orient it vertically. Thread a 6-inch capped pipe drip leg into the lower tee opening. Thread a 6-inch gas pipe section into the upper opening. Thread a shutoff valve onto the other end of that section. Thread an approved connector from the house gas line into the shutoff valve. Seal the connections with petroleum-resistant joint compound. Gas hookups must conform to local codes and regulations.

Connect household AC wiring to the AC power connection terminals on the furnace controller. Run the furnace from a separate 15 amp grounded circuit. Ground the metal casing. Electrical connections must conform to local codes and regulations. Connect the wire from the house thermostat to the 24-volt terminals on the furnace controller. Close the furnace casing.