Analysis of a reheat system showed that just 17% of the heating energy cost was useful reheat.

Status: Completed

Funding Sources: California Energy Commission PIER Program CBE Industry Consortium

Project Objective

Previous research showed 65% energy losses in domestic hot water systems in 28 multifamily buildings. This led CBE’s research team to investigate whether hot water reheat systems in commercial buildings have similar poor performance.

Project Results

The results were very surprising: Just 17% (!) of the heating energy cost was useful reheat. The rest was wasted due to a combination of boiler inefficiency at ultra-low part loads, distribution losses, and pumping power. Distribution losses alone were 44% of the heat output from the boiler. In this building, and likely in many others, a modern electric reheat system supplied by a small solar photovoltaic system would have a lower operating energy cost and a lower initial cost than the hot water reheat system.

Significance to Industry

Designers typically choose hot water reheat instead of electric reheat, justifying the increased first costs based on assumptions that it is more efficient and costs less to operate. This study raises questions regarding the trade-offs between hot water systems, which have significant boiler and distribution losses, and electric resistance systems, which effectively have none. These findings are relevant to designers and standards developers in deciding between electric and hot-water reheat, especially given the current move towards electrification and de-carbonization.

Research Approach

We developed a new method to estimate useful versus wasted hot water reheat energy using data obtained from typically installed instrumentation that applies to all pressure independent VAV terminal units with discharge air temperature sensors. We evaluated the method using a year of one-minute interval data for a 120,000 ft2 building with 98 terminal reheat units, and found a 14% upper bound for the uncertainty associated with this method.

Publications and Reports

  • P. Raftery, A. Geronazzo, H. Cheng, and G. Paliaga, 2018. Quantifying energy losses in hot water reheat systems, Energy and Buildings, Nov, Vol 179, Issue 1, pp. 183-199.

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