Improving the Built Environment through Research, Design, and Technology

Connecticut Light & Power (CLP)

Heat Pump Water Heater Evaluation


Late in 2002, Steven Winter Associates, Inc. (SWA) was contracted by Connecticut Light and Power (CLP), a division of Northeast Utilities, to install and evaluate the performance of the WatterSaver heat pump water heater (HPWH). Unlike most previous HPWH products, the WatterSaver is a stand-alone appliance – not a retrofit to existing electric tank water heaters. The project consisted of finding twenty volunteers in CLP territory using conventional electric water heaters, replace the electric tanks with HPWH’s, monitor the performance for several months, and conduct surveys with home residents on the systems’ performances.

Over the first few months, SWA recruited twenty volunteers from around the state of Connecticut. Their water heaters were replaced, and SWA installed detailed monitoring systems to measure and record:

All sensors were connected to a Campbell Scientific datalogging system. Data were retrieved weekly via telephone modem connections. From the recorded data, SWA could determine the efficiency of the HPWH systems (effective COP), the effectiveness of the systems (adequate temperature and quantity of hot water), and the dehumidifying potential of the systems.

The overall performance of the WatterSaver units was good with average, effective COP’s of 1.67 (effective COP was calculated as thermal energy delivered to domestic water divided by total electric energy consumed). Customer satisfaction was fairly high with many participants noting the dehumidification benefits. As shown in Figure 2, a strong relationship existed between daily savings and daily hot water use.



Figure 2. Summary of savings from Heat Pump Water Heater installations in Connecticut.

The study also identified some consistent drawbacks with the systems and their daily operation. Many customers complained of water being too hot; SWA’s monitoring did indeed show that water temperatures near the tops of the tanks often reached more than 150°F. Part of this was because of excessive tank stratification – water temperatures near the top could be 50°F higher than temperatures near the bottom (see Figure 3). In fact, high-temperature switches in many of the systems shut down the water heaters completely (high-temp safety switches are designed to turn off water heaters when temperatures reach 170°F). SWA and CLP communicated this issue to the manufacturer, ECR International, who was able to largely correct the problem by adjusting the water heater controls.


Figure 3. Sample measurements from a Heat Pump Water Heater installation showing significant tank stratification.

Improving the built environment through research, sustainable design and technology. © 2008 Steven Winter Associates, Inc.