Design masterclass 2 Thermal response
composite waveforms when multiple periods are involved, such as diurnal and seasonal variations in temperature. The effect of thermal mass on these analogue waveforms is essentially the same, but the analysis is more complicated and we’ll come back to this subject in the future. For now I want to stick with simple step change inputs, such as the start and end of daily occupancy gains or switching cycles of HVAC plant. The crux of the matter when it comes to designing
HVAC systems is: if the natural response of the system is of the same order as the period of the change in the heating or cooling input, then the inertia of the thermal mass will act as an integrator. An integrator is a fundamental part of most HVAC controls but, if
We must ensure that we understand the natural response of the building fabric when designing any heating or cooling system
we don’t allow for the natural integral effect of thermal mass, the combination can lead to the accumulation of very large errors and the loss of control. This can manifest itself in many ways, from large
temperature overshoots in underfloor heating systems, to the increase in overheating over a period of days with TermoDeck-like systems. Some of the most difficult issues to identify often
Figure 2: As the time constant (T) of the system increases the response to the input signal becomes slower. The system acts initially as an integrator and then as a low pass filter, attenuating signals of less than 5T duration
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circuit. At extreme the output voltage will become the time average of the input, a property that is used to smooth out ripples on DC power supplies. We see exactly the same behaviour in buildings
when we increase the thermal mass as we do by increasing the capacitance in the RC circuit. The natural response of the building begins to act as an integrator, delaying the rise in temperature. Should we be able to incorporate sufficient thermal mass, then we can not only reduce the peak temperature, but also delay the occurrence of the peak until the end of daily occupancy, thus further reducing the temperature rise experienced by occupants. Ultimately, as we continue to increase thermal mass,
the temperature of the building structure would tend towards the diurnal average of the space temperature as it filters out short-term changes. Now, few things in nature produce square waves; furthermore, we may need to consider complex
48 CIBSE Journal August 2010
occur when refrigeration plant is coupled to high thermal mass systems, such as in ground-source heat pumps (GSHP) or embedded slab cooling systems. I am often asked to diagnose apparently intractable problems with the operation of chillers and heat pumps. GSHPs in particular are now sold almost routinely as plug-and-play devices. However, the step control nature of most refrigeration
machines can cause great problems if the hydraulic systems are not designed to account for the transients caused by the delay in heat transfer into the ground or into the slab. In an underfloor heating system the primary heating
circuit is highly tolerant to varying return water temperatures that result from the slow response of the thermal mass to a change in flow temperature. Refrigeration machines, on the other hand, are not at all tolerant to the spikes in return temperature that occur before the thermal mass has time to soak up the change. If the natural response of the building significantly exceeds the periodicity of the control changes, then it will act as a low pass filter and we may not be able to control the temperature at all. This is not common, but can occur in extremely heavy buildings like churches, or where the heating or cooling system is actively coupled to the thermal mass as in TermoDeck or embedded slab heating or cooling.
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