The focus of this research was on using the test cell building as a set of case studies for benchmarking via use of the co-heating, heat flux and blower door tests, and comparing these results with the decay method. This gave a practical, real-world proof of concept, taking on board a number of in-field realities the computer simulations could not provide.
Applying a new testing method is crucial to demonstrating its real world functionality compared to other testing methods, and to discovering potential flaws that computer simulations do not reveal.
This chapter presents the in-field testing carried out on the test cells during the course of the study. It details the layout of the test buildings and the testing apparatus, as well as the experiments carried out. This includes details of the specific equipment used and complications that arose during the experiments that may have impacted the results.
Finally, comparisons of the results of the experiments are discussed.
82 The approach taken for the field experiment side of this research was intended as a demonstration of potential only, and not a definitive, wide-ranging analysis of the decay method.
Experimental Method
Two methods for determining the HTC are to be compared with the results of the decay method. These are the co-heating test and the heat flux test combined with the blower door test. The co-heating test is designed to test the overall response of the buildingβs thermal shell, where the heat flux test takes spot readings of individual elements and combines them with the rate of air leakage determined by the blower door test. These methods are currently well understood and used to make judgements of the buildingβs in situ performance. These two tests should give the same figure for the HTC.
The Figure 4.1 illustrates the relationship between the building evaluation methods being tested.
Figure 4.1 Theoretical experimental relationships
Heat Transfer Coefficient Tests
Co-heating test
Heat flux testing + blower door
test
Decay Method
Relationship being determined
83 Experiments
4.2.2.1 Co-Heating test
The co-heating test requires setting up a steady-state thermal condition inside the building, and monitoring the energy required to maintain that state compared to the changing outside temperature. The suggested internal state is to create an internal temperature of 25 degrees centigrade, at a time of year where the external temperature will be cooler. The heat transfer through the thermal shell will therefore be from inside to outside. When this is averaged over a sufficient period, it is accepted as being equal to the rate of heat added to the building via a mechanical heat source.
This is expressed by Equation 3:
Equation 3 Building heat balance
πππππ’π‘+ ππ ππππ = [β{π Γ π΄} +1
3ππππ] βπ Where:
Qinput = power supplied by the heater, W Qsolar = solar gain, W
U = U value of thermal shell element (i.e. wall), Wm-2K-1 A = Area of thermal shell element, m2
n = ventilation rate at atmospheric pressure, ACH Vol = Volume of the building, m3
ΞT = difference between average internal and external air temperature, K
The result of the test is calculating the HTC represented by Equation 4:
Equation 4 Heat Transfer Coefficient
π»ππΆ = [β{π Γ π΄} +1 3ππππ]
This is determined through measuring Q and ΞT, and rearranging Equation 3.
84 The experiment procedure is set up so that the building is closed (that is, doors and windows are shut, but no attempts at additional weather sealing are to be made) and heated until the steady state is reached. The building should be held in this state for two to three weeks. The temperature difference between inside and outside (degrees Kelvin, K) and heat input rate (Watts, W) is averaged over a 24-hour period. The 24-hour period for this research is from 6.00am-6.00am so that the full solar cycle is included in the same averaging calculation. This is so that the analysis period mimics the natural cycle of the building responding to the surrounding climate. On completing the test, the daily (24 hour) average temperature difference is parameterised against the average heating rate give the HTC in W/K. This is a measure of the rate of heat lost by the building for every degree Kelvin of difference across the thermal shell. The average heating rate is adjusted for solar gains and wind speed using multiple regression analysis. Solar gains and wind speed is also the 24-hour average. Co-heating tests conducted on the buildings had to be interrupted fortnightly to allow for data to be collected.
4.2.2.2 Heat Flux Testing
Heat flux testing provides the means to calculate the actual R-value and U-value of the entire building element (for example, wall or floor). It requires a heat-flux meter affixed to the element, usually on the inside surface, and temperatures measured on the inside and outside surfaces. Over the experimental period, Equation 5 is used:
Equation 5 Average method for calculating material U-value
ππ‘ = 1
βπ=π‘0 βππ π
βπ=π‘0 ππ + ππππ‘ + πππ₯π‘ Where:
U = U-value
ΞTsi = internal surface temperature β external surface temperature in degrees Kelvin, K Qi = heat flux readings in W/m2
rint = Surface resistance due to internal air, assumed 0.13 W/m2K rext = Surface resistance due to external air, assumed 0.04 W/m2K
85 When the U-value is plotted against time variations it should dampen and approach an asymptote. After a week the U-value should stabilise to within 5% of the final value from four weeks of testing (Rye, 2010).
Note that these calculations used static R-values for surface air resistance. In reality, the surface resistance of the material changes with temperature and air flow. Using assumed static values simplifies the calculation. This approach was used by Baker (2011) and Rye &
Scott (2010) in their respective studies of in situ U-values. The values used are taken from ISO 6946:2007.
The different combinations of collection for the heat flux meters have been used. Set up 1 consisted of heat flux meters on the internal surfaces of the building, and a five second resolution. Set up 2 shifted the resolution to 30 seconds due to memory constraints of the data loggers, and kept placement of the meters on the internal surfaces. Set up 3 placed a heat flux meter on the inside, and a second meter on the outside of two walls, with a collection resolution of 30 seconds. Set up 4 placed heat flux sensors on both sides of the internal lining and external cladding (four sensors in total) in two different wall sections. In each case all heat flux sensors were accompanied by a thermocouple measuring surface temperature.
The first three combinations were applied to Test Cell 1 during the long term monitoring in the second year of this research. Set up 4 was applied to Test Cell 2 during the co-heating and decay tests. The co-heating test creates good conditions for the heat-flux testing to be conducted; however, the test is valid under any conditions.
4.2.2.3 Blower door test
The blower door test is used to measure the air change rate in, or infiltration rate of, the building. This is achieved by closing all doors, windows and other openings into the building shell, and creating a pressure difference between the internal space and the atmosphere, then monitoring the air movement required to maintain the difference. Two tests are conducted: one raising the pressure inside the building, and a second reducing the pressure until the air change at a difference of 50 Pa is measured or can be extrapolated. The average of the two tests is calculated and accepted as the air change rate.
86 The blower door kit used for this research is the Retrotec 3101 Hi-Power kit. The testing utilises the software βFanTestic Liteβ to carry the test out in accordance with Australian Standard ASTM E779-10. The software automates the process of powering the fan to incrementally change the pressure difference and take the required readings.
The results at 50 Pa are divided by 20 to give the infiltration rate, n (Kronvall, as cited by Stamp, 2010). This is used in the heat loss equation presented in the co-heating test Section 4.2.2.1. Infiltration heat loss (in W/K) may be then be calculated using Equation 6:
Equation 6 Infiltration heat losses
πππππππ‘πππ‘πππ= 1 3ππππ Where:
Vol = building volume in m3
n = infiltration rate at atmospheric pressure, ACH 1/3 = specific heat capacity of air
4.3 Experimental Set-up