You’ve done the modelling and know how the house should perform, but is it living up to expectations? A co-heating test can be useful to find what the as-built whole-house heat loss performance actually is.
IN A CO-HEATING TEST, a dwelling is heated to a reasonably high temperature and then held at this temperature constantly for several days or more. The energy required to heat the building is measured every 30 seconds, along with the conditions outside of the dwelling.
To get good data, the temperature throughout the whole house needs to stay even across the indoor space and throughout the whole test. This isn’t possible with normal heater thermostats. Instead, heaters in every room are controlled with a computer linked to temperature sensors set at multiple heights in every room. The computer turns the heaters on and off as needed, and fans constantly mix the air to make sure there are no cold or hot spots in any room. If it is a 2-storey dwelling, fans and ducting make sure that the air is circulated evenly between upstairs and downstairs.
Takes time to get good data
Co-heating tests take a few days or weeks. Initially, it takes time for the inside temperature of the house to reach the required equilibrium. Measurements during this stage aren’t used in the evaluation.
Enough data is needed to cover a range of indoor and outdoor temperature differences and wind conditions to get a reliable result from the analysis.
There are two types of heat loss involved. The first relates to the thermal performance of the building assembly. The second is due to the leakage of air through unintentional gaps and cracks in the building envelope (also called infiltration).
For the analysis, only data collected at night is used so heating from the sun doesn’t affect calculations.
Working out the heat-loss coefficient
Once enough data is collected, the relationship between the amount of power used for heating and the difference between the indoor and outdoor temperature can be worked out.
This relationship is used to calculate a heat-loss coefficient for the building – a measure of how strongly the house resists losing its heat (the thermal resistance for the whole house).
Figure 1 shows the results of a BRANZ co-heating test on a 90 m² house. The energy (heat) lost through the building fabric is shown in blue. In this building, the energy lost through infiltration (in orange) is much lower, which shows the building is fairly airtight.
At a temperature difference of 20°C, the whole-house energy loss is approximately 3.8 kW. This is the amount of energy that would be needed to keep the indoor temperature of this house constant at this temperature difference. The slope of the blue line is the estimated heat-loss coefficient for the whole building envelope, which in this example is 0.19 kW/°C.
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