Comparison between Heat Flow Meter (HFM) and Thermometric (THM) Method for Building Wall Thermal Characterization: Latest Advances and Critical Review

Abstract

It is well-known that on-site measurements are suitable for verifying the actual thermal performance of buildings. Performance assessed in situ, under actual thermal conditions, can substantially vary from the theoretical values. Therefore, experimental measurements are essential for better comprehending the thermal behavior of building components, by applying measurement systems and methods suitable to acquire data related to temperatures, heat flows and air speeds both related to the internal and external environments. These data can then be processed to compute performance indicators, such as the well-known thermal transmittance (U-value). This review aims at focusing on two experimental techniques: the widely used and standardized heat flow meter (HFM) method and the quite new thermometric (THM) method. Several scientific papers were analyzed to provide an overview on the latest advances related to these techniques, thus providing a focused critical review. This paper aims to be a valuable resource for academics and practitioners as it covers basic theory, in situ measurement equipment and criteria for sensor installation, errors, and new data post-processing methods.

Surface heat transfer coefficients in building envelopes: Uncertainty levels in experimental methods

“…A general review of many works in the 2000-2017 period presented typical discrepancies in the range of 20%-30% (Teni et al, 2019). Evangelisti et al (2022) performed a bibliographic research on HFM and THM methods and identified a number of sources of uncertainty and limitations for milder climates. ''Inaccurate installations cause uncertainties ranging from 26% to 30%; poor contacts, 2% to 5%; non-one-dimensional heat flux, 1% to 5%; wall orientation, 37%; moisture up to 71%; and different heights among sensors, 17% to 22%.''…”

Combining Deep Learning and the Heat Flux Method for In-Situ Thermal-Transmittance Measurement Improvement

“…Policy measures often limit the U-values of building elements, but when we solve a problem of great importance, it is better to analyze the problem with numerical methods, since dynamic parameters such as heat storage are included in the analysis, so that we can, for example, optimize the thickness of the insulation layer so that we have a larger U-value but better energy efficiency when considered all year round [44]. All results are compared with the U-value defined in this subsection, since the analyzed specimens from the experimental research have known layers and it is common in the literature ( [23,27,33,[45][46][47]) to compare the analyzed solution with the theoretical U-value defined in this way.…”

“…The HFM measurement was continued after 72 h to compare the resulting U-value from the HFM measurement (U HFM ) with the U-value from the heat flux predicted by the deeplearning model (U DL ). Both values were compared with the theoretical U-value according to ISO 6946 (U ISO ) [30], so that the results of this research can be compared with related papers [23,27,33,[45][46][47]. This research methodology is clearly presented in Figure 4.…”

“…In the case of such an assessment for existing buildings, one of four experimental in-situ methods should be used to determine the U-value: the heat-flux method (HFM), the thermometric method, the HFM with hot box, or quantitative infrared thermography [17,20,21].There are two main reasons why it is important to estimate the U-value as close to reality as possible-we can decide on measures that lead to poor energy efficiency (optimistic U-value estimation) and those that lead to unnecessary measures, which are usually quite expensive (pessimistic U-value estimation). Of the listed methods, only the HFM method is a standardized method according to ISO 9869-1 [22] for the in-situ determination of the U-value, and is widely used not only for engineering purposes but also in academic research [20,23], as shown in Figure 1.…”

“…Figure 1. Distribution of research articles for each defined in-situ method for U-value determination according to [23].…”