Conventional gas and electric storage-type residential water heaters were operated at four different U.S. cities under accelerated test conditions to measure the effect of scale buildup on efficiency and to assess the benefits and limitations of common water treatment methods. The four Selected test sites had hard water supplies with expected scale-forming tendencies and were located in Columbus, OH; Lisle, IL; Roswell, NM; and Marshall, MN.

The main conclusions are as follows. After 60 lbs (27 kg) of scale buildup at two of the test sites (representing an estimated 20 years of equivalent scale buildup), the efficiency the gas water heaters gradually declined about 5%, while that of the electric water heaters remained constant. However, the buildup of scale in the electric heaters caused the electric heating element to fail periodically, and in the gas-fired heaters, it caused the tank metal temperatures near the burner to operate hotter. Treated water (either softened, softened plus polyphosphate, or hard plus polyphosphate) effectively reduced scale buildup and tended to reduce the corrosion rates of the metal test coupons in hot water. However, the sacrificial magnesium anodes were consumed at a much higher rate in the tanks using any type of softened water, and this could indirectly cause those tanks to experience corrosion sooner.

Several design recommendations resulted from this study that are expected to minimize scale buildup and anode-corrosion problems in electric heaters and in the newer, more efficient, gasfired water heaters that are being developed.

Units: Dual