Rheem XR90 Series Use and Care Manual - Page 9

Thermal Expansion Determine if a check valve exists in the inlet water line. Check with your local water utility company. It may have been installed in the cold water line as a separate back flow preventer, or it may be part of a pressure reducing valve, water meter or water softener. A check valve located in the cold water inlet line can cause what is referred to as a "closed water system". A cold water inlet line with no check valve or back flow prevention device is referred to as an "open" water system. As water is heated, it expands in volume and creates an increase in the pressure within the water system. This action is referred to as "thermal expansion". In an "open" water system, expanding water which exceeds the capacity of the water heater flows back into the city main where the pressure is easily dissipated. A "closed water system", however, prevents the expanding water from flowing back into the main supply line, and the result of "thermal expansion" can create a rapid and dangerous pressure increase in the water heater and system piping. This rapid pressure increase can quickly reach the safety setting of the relief valve, causing it to operate during each heating cycle. Thermal expansion, and the resulting rapid, and repeated expansion and contraction of components in the water heater and piping system can cause premature failure of the relief valve, and possibly the heater itself. Replacing the relief valve will not correct the problem! The suggested method of controlling thermal expansion is to install an expansion tank in the cold water line between the water heater and the check valve (see illustration below). The expansion tank is designed with an air cushion built in that compresses as the system pressure increases, thereby relieving the over pressure condition and eliminating the repeated operation of the relief valve. Other methods of controlling thermal expansion are also available. Contact your installing contractor, water supplier or plumbing inspector for additional information regarding this subject. IMPORTANT: Do not apply heat to Water Supply Connections the HOT or COLD Refer to the illustration below for suggested for servicing if necessary. The HOT and COLD water connections. typical installation. The installation of unions water connections are clearly marked and are If sweat connections or flexible copper connectors is recommended 3/4" NPT on all models. Install a shut-off valve are used, sweat on the hot and cold water connections so that in the cold water line near the water heater. tubing to adapter the water heater may be easily disconnected before fitting adapter to the cold water connections on heater. Any heat Typical Installation Union Heat trap 6" minimum Vacuum Relief Valve (Not Supplied) applied to the cold water supply fittings Hot water outlet to fixtures will permanently damage the dip tube and heat traps. NOTICE: The Water Heater Jacket Temperature and pressure relief valve Union If required, install per local codes and valve manufacturer's instructions. National Fuel Gas Code (NFGC) mandates a manual gas shut-off valve: See (NFGC) for complete instructions. Local codes or plumbing authority requirements may vary from the Manual gas shut-off To gas supply Ground joint union Sediment trap Cap Thermostatic gas valve Shut-off valve Heat trap 6" minimum To cold water supply Thermal expansion Shut-off valve tank (if required) Drain valve Relief valve discharge line to suitable open drain. instructions or diagrams provided and take precedent over these instructions. Jacket door Flammable Vapor Sensor Auxiliary catch pan Drain Pan Pipe to suitable drain. Combustion Air Inlet Openings 6" Air gap 9