Assessing the Relationship between Measurement Length and Accuracy within Steady State Co-Heating Tests

Abstract

Evidence of a fabric performance gap has underlined the need for measurements of in situ building performance. Steady state co-heating tests have been used since the 1980s to measure whole building heat transfer coefficients, but are often cited as impractical due to their 2-4 week test duration and limited testing season. Despite this, the required conditions for testing and test duration have never been fully assessed. Analysis of field tests show that in 12 of 16 cases, a heat loss estimate to within 10% of the result achieved across a full test period can be achieved within just 72 h. These results are supported by simulated tests upon a wider range of dwellings and across wider environmental conditions. However, systematic errors may still exist, even in cases of convergence and cases with significant uncertainties may never converge. Simulated examples of traditional dwellings and those built in line with current building regulation limits may be tested for more than half the year. However, even when simulated with reduced uncertainties, dwellings with low heat loss and high solar gains, such Passivhaus dwellings and apartments, could be successfully tested for just 22% and 12% of a year respectively, demonstrating the limitations of the co-heating method in assessing such dwellings.

Influence of Initial and Boundary Conditions on the Accuracy of the QUB Method to Determine the Overall Heat Loss Coefficient of a Building

“…The H-value estimated by using the co-heating method is considered accurate and is used as a reference value. Two co-heating tests conducted for the same building by two different teams have generated the same results [7].…”

“…It was evident that there was a slight variation ±0.8 °C in the The average outdoor temperature during the nights varied between 6 and 16 • C. The conditions in which some of the QUB experiments were performed were not optimal. A large temperature difference (e.g., 10 • C) between the indoor temperature (20 • C) and the outdoor temperature during the night is known to increase the accuracy of the method [7]. The conditions chosen in this paper were not optimal and, therefore, the performance of the method was expected to be better in many cases.…”

“…It is evident from the plot of error curves (Figure 3) that variation in input power changes the output of the QUB method. The optimum power for QUB experiment on any day with reference to the initial power can be estimated by using [7]:…”

“…Co-heating takes two to three weeks during the heating season, which reduces its practical applicability [7,8]. It is therefore important to search for methods that are faster, can be employed on a larger scale and, above all, are reliable enough [7].…”

“…The required measurements are internal temperature, heating power injected, air infiltration rate and external climate conditions. The test takes 5-15 days to be completed, depending on the thermal inertia of the building [7]. The method is sensitive to air infiltration rate and solar radiation.…”