Performance Comparison of Residential Hot Water Systems

This report by Home Innovation Research Labs is a continuation of past Renewables and Energy Efficiency Program (REEP) technical efforts sponsored by the National Renewable Energy Laboratory (NREL) through 1999 and 2000. This work was undertaken to verify the estimated energy savings for hot water systems. The test results presented here support water heating energy savings reported in 2001.

Results of weekly performance testing and annual simulations of electric water-heating systems are presented. A laboratory test experiment was conducted to measure the energy performance of two different types of water heaters - electric storage tank and demand (tankless) - in two types of plumbing distribution systems.copper piping in a tree configuration and cross-linked polyethylene (PEX) piping in a parallel configuration. Two water-usage patterns were used in the week-long experiments and in the annual simulations: one representing a high-usage home and the other representing a low-usage home.

Using the Transient Energy System Simulation Tool, TRNSYS1, a simulation model was developed to estimate energy consumption for each hot water system and to further simulate other system design options. The simulation model was calibrated with heat-transfer coefficients determined by experimental results. Annual simulations showed an increase in overall system efficiency of 12% for the demand water heater with a parallel piping distribution system over the storage tank water heater with copper piping for the high-use home and an increased efficiency of 26% for the low-use home. When normalizing the total output energy for each system, the electrical energy savings of the demand water heaters with a parallel piping system over the standard tank with a tree-piping system (tank/tree system) was 34% for the low-use home and 14% for the high-use home.

A point-of-use model was subsequently developed to simulate a hot water system having multiple demand heaters distributed at the outlets and served by a tree-type supply piping (cold only). Since the heaters are located at the outlets, lower delivery temperatures are required. Using the point-of-use model, simulations show that the system efficiencies are nearly 100% and annual energy consumption can be reduced by almost 50% for the low-use home and 28% for the high-use home over a storage-tank water heater with a tree-type distribution system.

When improving the energy efficiency of the overall water-heating system, especially in the reduction of piping losses, the environmental benefits extend beyond those of reducing use of electricity or other fuels. Reductions in water use, often significant, may be obtained if the period of time to wait for hot water to arrive at the outlet is reduced as with the parallel piping system or even eliminated as with the distributed-heater system. Other energy benefits occur when low, but frequent, unintentional uses of hot water, such as a single-handle kitchen faucet set near the cold-water position, are eliminated with demand heaters that do not activate at low flow rates.

1 University of Wisconsin-Madison, Solar Energy Lab,

Performance Comparison of Residential Hot Water Systems
(669 Kb)