Steady-State vs Dynamic Heating Loads: Why It Matters.
Plant sizing: steady-state vs dynamic
During the design of new and refurbished buildings, a series of calculations are undertaken by engineers when designing their heating systems. The very first of these calculations is to establish the heating energy demands (or loads) of the building and the rooms within. These results are then used to size the emitters, pipework, ductwork, valves, pumps and heat generation plant (boilers, heat pumps etc.).
Knowing this, it goes without saying that accurately calculating the heating energy loads is key for maximising building performance. Get them wrong and the systems will operate at conditions they weren’t designed for, resulting in inadequate levels of comfort, and increased energy consumption, fuel costs, and carbon emissions. The magnitude increases with building size and complexity.
This is also the case for cooling systems, but this article will focus on heating.
Steady-state heating loads
Traditionally, heating loads are established using the steady-state method. This approach calculates heat transmission through the building fabric and by air exchanges, and assumes constant internal and external temperatures during the depths of winter. Thus, steady-state heat loss calculations simplify as well as ignore many key aspects of heat transfer. Some of the main exclusions include solar heat gains and internal heat gains. Excluding these mechanisms and using simplified building physics means that the emitters, pipework, ductwork, valves, pumps and heat generation plant will all be sized on fictional heat loads.
What’s the alternative?
‘Dynamic’ thermal modelling offers significant advantages over steady-state calculations when determining building energy loads. Firstly, dynamic thermal modelling provides a more accurate representation of a building's thermal performance throughout the year, considering variations in external conditions such as temperature, wind speed, and humidity.
Accurate Sizing: In the UK, where weather patterns can change rapidly and dramatically, dynamic modelling captures these fluctuations, leading to more precise estimations of heating requirements. Dynamic heating load calculations also account for occupancy patterns and internal heat gains, providing a more accurate estimation of the actual heating demand. This ensures that the heating system is appropriately sized to meet the varying needs of the building throughout the day and across different seasons.
Energy Efficiency: By considering dynamic factors such as solar gain, occupancy variations, and thermal mass effects, dynamic load calculations help optimise plant sizing for better energy efficiency. Oversized heating systems, common with steady-state calculations, can lead to energy wastage and inefficiencies during periods of low demand.
System Responsiveness: Dynamic load calculations enable heating systems to respond more effectively to changing conditions, such as sudden temperature changes or unexpected spikes in occupancy. This enhances comfort levels within the building while minimising energy consumption by ensuring that the heating system operates at the appropriate capacity when needed.
Cost Savings: Properly sized heating systems based on dynamic load calculations can result in significant cost savings over the system's lifecycle. By avoiding the expense of oversized equipment and reducing energy consumption through optimized operation, building owners can enjoy lower installation, maintenance, and operating costs.
Improved Comfort: Dynamic load calculations take into account factors like thermal lag and building usage patterns, resulting in a more comfortable indoor environment. By accurately sizing heating equipment to match the actual heating requirements of the building, occupants experience consistent and reliable comfort levels without the discomfort associated with temperature fluctuations common in steady-state calculations.
Adaptability to Building Changes: Buildings are not static entities; they undergo changes in occupancy, usage patterns, and internal configurations over time. Dynamic load calculations provide a framework for adapting heating systems to accommodate these changes efficiently, ensuring continued performance and comfort without the need for costly retrofits or upgrades.
Example
A series of simulations were performed on some of our recent projects, comparing the two methods. The results demonstrate significant reductions in the peak heating demand when using the dynamic approach.
So why are steady-state calculations still being used?
Steady-state heating loads are far easier to calculate, with designers able to undertake these using spreadsheets or even hand calculations. Additionally as steady-state heating loads tend to come out higher than their dynamic counterparts (as the above table illustrates) they reduce the risk of under sizing plant and heat emitters. Under sizing means cold and unhappy occupants, which would be quickly noticed and come back to the engineer. However, dynamic thermal modelling is widely used across the industry and whilst it may be considered a more time intensive and by association expensive approach, it provides the highest levels of decision-making confidence during design stages.
Dynamic thermal analysis enables designers to design systems for the real world and offer numerous advantages over steady-state calculations, particularly in terms of plant sizing. By providing a more accurate representation of actual heating demands and considering dynamic factors, these calculations contribute to energy efficiency, cost savings, comfort, and adaptability in building heating systems.
Finally, whilst one of the leading professional engineering associations in building performance, CIBSE, do promote the use of dynamic thermal modelling, we don’t believe the use of steady-state calculations should be recommended during the detailed design of a development as per their illustration from Guide A: Environmental Design, which provides ‘guidance on the type of calculation method that may be used at different stages and for different design questions’.
