Using simulated co-heating tests to understand weather driven sources of uncertainty within the co-heating test method

Panel: 7. Monitoring and evaluation

This is a peer-reviewed paper.

Authors:
Samuel Stamp, UCL Energy Institute, United Kingdom
Robert Lowe, UCL Energy Institute, United Kingdom
Hector Altamirano-Medina, UCL Bartlett School of Graduate Studies, United Kingdom

Abstract

The so-called performance gap between designed and as-built building performance threatens to undermine carbon reduction strategies in the built environment. Field measurements to date have indicated that the measured as-built fabric heat loss of tested UK buildings is consistently higher than design values, often considerably so. Currently, our lack of knowledge over the extent of this gap, and the processes that cause it, is compounded by a lack of robust post-construction evaluation tools. Much of this post-construction evaluation work is based, in part, on the use of co-heating tests: a method utilising an energy balance to determine the heat loss across the entire building envelope, defined by the heat loss coefficient (W/K). However, the errors associated with co-heating are not well understood or typically addressed in the literature. Furthermore, the test procedure requires a building to be unoccupied for two to three weeks and is therefore often cited as costly and unsuitable both for developers and as a policy tool. In order to improve the application of this test method it is crucial firstly to understand the sources of uncertainty in co-heating tests and the ‘steady-state’ energy balance they are based upon. With a small database of tests performed to date it is difficult to discern these sources of error. This paper presents the results of a method using simulated co-heating tests to show how key weather variables influence the co-heating result. In particular the effects of short-wave solar and long-wave sky radiation are presented. Improvements to the co-heating method can be derived from this; in particular the need to consider when dwellings should be tested to avoid large solar-generated errors and the importance of an accurately calculated solar aperture. Recommendations also include the local measurement of sky radiation to avoid outlying data points, bias in the measurement and discrepancies when comparing design and as-built heat loss.