Date of Award

Spring 1-1-2011

Document Type


Degree Name

Master of Science (MS)

First Advisor

Jay Burch

Second Advisor

Moncef Krarti

Third Advisor

Michael Brandemuehl


In recent years, solar combisystems have received an increasing amount of attention in both the European and U.S. markets due to their ability to increase the energy savings provided by residential active solar water heaters. However, since the extra savings are accompanied by extra installation costs, it is not trivial as to whether or not these systems are a worthy investment, especially when compared to solar water heaters (SWHs). To help answer the question of whether or not these systems are cost-effective, the annual performance of these systems, as a function of location, size and load, was simulated using a TRNSYS model of a typical combisystem. The model was validated using data from a residential combisystem installed in Carbondale, Colorado, which was monitored as part of a Building America research project.

The TRNSYS model was then used to study the annual performance of combisystems for residential applications in six locations within the U.S. The six locations are Phoenix, Atlanta, San Francisco, Denver, Boston, and Chicago. For collector area sizes of 96 ft2 or smaller, the performance of these systems is measured by the incremental energy savings it yields in comparison to a SWH of the same system size. Additionally, the combisystems' energy savings are evaluated based on the reduced auxiliary energy required to meet the thermal loads as compared to a reference system without any solar.

This study found that combisystems are able to provide significant energy and cost savings relative to both small SWHs and reference systems. In terms of incremental savings from the combisystem as compared to a SWH of the same size, the largest incremental savings will occur when DHW loads are small and space heating loads are high. The economic analysis revealed that electric combisystems in the locations of Denver, Boston, and San Francisco yield the highest incremental cost savings and highest incremental breakeven costs relative to a SWH.

Regarding the cost-effectiveness of combisystems relative to a reference system, the analysis reveals that the economics could be favorable for combisystems in the locations of Denver, Boston, and San Francisco, provided that these systems are electric and the thermal loads in these homes are high. However, if evaluating the economics of combisystems in applications for lower space heating loads due to more efficient construction, combisystems--given their current high installation costs--are not a cost-effective option in any locations evaluated.