Why Pay for Two Condensing Systems When One Will Do?

This report was created for the 2014 ACEEE Summer Study on Energy Efficiency in Buildings. 

Abstract

As construction practices improve and people become more conscious of energy usage, space conditioning and water heating bills have declined in many residential homes. Under these circumstances it becomes harder to justify the added cost of two high efficiency mechanical systems. One solution is to use a single water heater or boiler for both space conditioning and water heating. While combined systems have been available for years, the design and installation must be optimized to achieve the expected efficiency for the new generation of systems that use condensing equipment. Installed efficiencies of greater than 90% and energy savings of 20% have been shown with combination systems. However, testing has shown that poorly chosen set points or undersized hydronic air handlers will reduce the system efficiency from 90% to 77%.

Guidelines for the implementation, installation and operation of these systems were generated from laboratory tests, a 200 home implementation project, and detailed field monitoring in 20 homes. An additional hurdle to implementation is the lack of a common rating method and or performance metric. A simple strategy has been developed to ensure quality installation and high performance of these systems, allowing for improved implementation. Following these guidelines and strategy would allow many homeowners the benefits of high efficiency space conditioning and water heating systems using only one heating plant.