The issue of 80 degree feed water needs special consideration because it has, by far, the greatest impact on published efficiency. Unless an appliance is specifically design for continuous condensing, it cannot accept 80 degree feed water in normal operation. It is, in fact, usually a breach of the manufacturer's warranty to install NON-condensing equipment in an application that will supply feed water in the 80° temperature range.
But what is the effect on published efficiency of using 80° feed water? Conventional water heater and water boiler heat exchangers are usually designed for 130° to 140°F and higher entering water temperatures and a 20°-30°-35° temperature rise. If water arrives at the equipment at 130°F, for example, it should leave at 150° or 165°F (depending upon the specific appliance design). By feeding the non-condensing equipment 80° feed water instead of the 130°-140° feed water for which it was designed, the water heater or boiler becomes a condensing design for the purpose of the efficiency test.
Adding 30°-35° to the 80° feed water generates an exhaust temperature below the dewpoint and condensing occurs in the equipment regardless of the fact that the equipment is a non-condensing design. Operation in this manner yields a test efficiency that is significantly higher than the equipment would generate in normal operation with 130°-140° return water. This situation raises another important question. What is the efficiency of a conventional non-condensing water heater or boiler design with 140° feed water?