Long Simulation runtime with Radiant Systems

One of the most common causes of a long simulation runtime is the very large number of thermal zones that contain radiant surfaces. Slow simulation performance in models with extensive use of radiant systems is a well-documented limitation in EnergyPlus. This occurs because, for each timestep, EnergyPlus must solve additional coupled heat-balance equations for every radiant surface, significantly increasing the computational load as the number of zones and radiant surfaces grows.

For general advice on handling large models, see the Modelling Advice

Working with Large Models and Speeding Up Simulations. 

How to Speed Up Simulations

Use these recommended approaches:

• Reduce the number of zones served by radiant systems. Zones with identical or very similar thermal and operational characteristics can be merged to decrease the total number of radiant surfaces. Note that excessive zone merging can increase geometric complexity and lead to unintended simulation slowdowns. Use this feature cautiously, as it may complicate the model and reduce simulation efficiency.
• Replace the hot-water radiant surfaces with HW convectors. This approach can substantially improve simulation speed, as convectors are modeled using simpler heat-transfer algorithms and do not require the additional radiant surface heat-balance calculations.
  
  

Key Differences and Limitations

Hot-water convectors and radiators are both hydronic heating devices but differ thermally:

• Radiators emit a substantial portion of heat through radiation, directly affecting surface temperatures and mean radiant temperature.
• Convectors deliver heat almost exclusively through convection to zone air.

In EnergyPlus, radiant systems require surface-level heat-balance calculations, significantly increasing computational cost. Therefore, replacing radiators with convectors improves simulation speed but introduces a simplification that reduces fidelity in radiant heat transfer and comfort representation.