Opus Environmental

Producing equivalent standard Guide A Section 5 winter and summer design calculations with Tas dynamic simulation software.

How do the procedures compare?

The main difference between the CIBSE steady state heat loss / admittance procedures and dynamic simulation is the ability dynamic simulation to take account of variations in weather over a number of days.

Steady state heat loss is the equivalent of running a 24 hour heated building with no internal heat gains for a long period of weather at a constant outside air temperature and no solar income. Remove the internal heat gains from a Tas dynamic simulation model. Run the model using a weather file with constant outside air temperature and no sunshine. At the end of a 30 day period the simulated heating load is exactly the same as the steady state heat loss calculation.

The admittance procedure uses a 24 hour harmonic to predict summer design day performance using idealised weather data for the design day. The procedure does not have the ability to carry forward the influence on performance of previous days. In fact, the procedure assumes that the design day has been preceded by an infinite number of identical days. To reproduce this type of analysis with Tas dynamic simulation software an extended period of weather data is used. This weather data contains repeated days of the same idealised weather used in the admittance procedure. At the end of a 30 day simulation on repeated day weather the simulated performance is very close to that predicted by the admittance method.

It is therefore possible using idealised weather to perform the equivalent of standard heat loss / admittance calculations.

The following examples from Guide A Section 5 show how closely simulation reproduces standard CIBSE calculations

Heat Loss

Guide A Section 5 example 5.1 Calculation of steady state heat losses

A small factory is heated to a resultant temperature of 19°C with an external air temperature of –1°C. Details of constructions and occupation can be found below.

 

The Tas model has been set up with the same construction details and internal heat gains as the example building. A weather file has been specified with 30 days of constant –1°C outside air temperature and no sunshine.

The Tas model was run for the 30 days and the convective heating demand on the final day was a steady 8.56 kW. This compares with the example calculation of 8.72 kW.

The model was re-run with radiant heating to give a heat loss of 8.76 kW. This compares with the example calculation of 8.81 kW