CIBSE Steady State Calculations

The environmental performance of a building is dependent on a correctly selected and operated HVAC system.

Thermal modelling and simulation can be used by design teams to accurately predict heat losses and heat gains. Heat is lost from a building in two ways – either by transmission through the building fabric (fabric losses determined by U-values), or by air infiltration and ventilation.

The fabric of modern buildings, together with improved levels of airtightness means that many commercial heating systems are incorrectly sized. This can lead to increased emissions, higher capital costs and ineffective operational performance.

Advanced simulation techniques allow us to specify modular heating systems which allow higher efficiencies and part load operation with separately  pumped boilers (for example) as load demands.

CIBSE Guidance

We undertake checks that follow CIBSE procedures, giving heat losses equivalent to those in their steady state methodology (CIBSE Guide A 2015).

If mechanical cooling is present, heat gains can also be calculated in line with CIBSE steady cyclic methodology.

We work with M&E teams, architects and designers on many commercial and public buildings across the UK. We help them to not only meet their planning and compliance obligations, but to deliver buildings which are going to perform long into the future.

Steady state calculations are a standard deliverable at RIBA Stage 3–4 on most commercial and public sector projects. They are produced to support M&E system design, allowing engineers to size heating and cooling plant before detailed specifications are committed. On projects requiring a Part L compliance report, steady state calculations form part of the underlying thermal model used to demonstrate that proposed systems meet minimum efficiency targets under Approved Document L.

BREEAM assessments also reference heating and cooling loads when evaluating energy system performance under the Ene 04 credit. Planning authorities increasingly require energy statements that set out peak demand figures, which are drawn directly from steady state outputs. For projects in Greater London, the GLA energy assessment process requires load calculations to support the demand-side analysis.

Steady state calculations are also used in existing building projects – for example when a building undergoes change of use, significant extension, or M&E system replacement. In these cases the calculation establishes current and proposed load profiles, supporting both compliance demonstrations and investment decisions on system capacity.

A completed CIBSE steady state calculation report sets out peak heat loss and heat gain figures for each space and for the building as a whole. Fabric heat loss is calculated room by room using design U-values, areas, and the CIBSE Guide A design temperature difference for the project location. Ventilation heat loss accounts for infiltration and fresh air supply rates, typically derived from the proposed mechanical ventilation strategy or default infiltration assumptions.

Cooling load calculations include solar gain through glazing (assessed against orientation and shading), casual gains from occupants, lighting, and equipment, and the contribution of fresh air supply. The results are presented in a format that M&E engineers can use directly for equipment selection – boiler and chiller sizing, air handling unit capacities, and distribution pipe and duct sizing.

Build Energy delivers steady state calculation reports in PDF and Excel formats. Reports include a full schedule of room-by-room results, the design assumptions and input data used, and summary tables for each building section. Where calculations are being used to support a planning or BREEAM submission, results can be formatted to meet the specific reporting requirements of those processes.

What are CIBSE steady state calculations?

CIBSE steady state calculations are heat loss and heat gain analyses carried out in accordance with CIBSE Guide A. They determine the maximum heating and cooling loads a building is likely to experience under design conditions, enabling engineers to correctly size HVAC plant and systems. Build Energy follows the CIBSE steady state methodology to ensure calculations are consistent with industry standards and acceptable to building services engineers and planners.

Why does accurate heat loss calculation matter?

Many commercial heating systems are oversized because of inaccurate or overly conservative heat loss calculations. Oversized plant costs more to install, runs inefficiently at part load, and produces unnecessary carbon emissions. Accurate steady state calculations allow engineers to specify right-sized, modular systems that operate efficiently across all load conditions.

When are steady state calculations required?

Steady state calculations are typically required at detailed design stage (RIBA Stage 4) to support mechanical and electrical engineering design. They underpin HVAC equipment schedules and form part of the building services design package. Some planning conditions and BREEAM assessments also require evidence of accurate heating and cooling load calculations.

How do steady state calculations differ from dynamic simulation modelling?

Steady state calculations assess peak design loads under fixed worst-case conditions – they give a single maximum figure for heating or cooling demand. Dynamic simulation modelling (DSM) goes further, simulating hour-by-hour performance over a full year to model seasonal variation, occupancy patterns, and real-world conditions. Both methods are used at different stages of design and serve different purposes.