CIBSE Guide A Environmental Design Calculations
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
|
Example 5.1 convective |
Tas simulation convective |
Example 5.1 radiant |
Tas simulation radiant |
| Heat Loss kW |
8.72 |
8.56 |
8.81 |
8.76 |
| Resultant Temperature °C |
19 |
19 |
19 |
19 |
| Air Temperature °C |
21.45 |
21.5 |
17.23 |
17.25 |
| Mean Surface Temperature °C |
16.55 |
16.54 |
20.77 |
20.77 |
Correction for intermittent heating Example 5.4
Tas has automatic procedures for sizing heating loads with optimum start control of
intermittent heating. When the Tas model is set up to operate for 8 hours with a 3 hour
preheat and night setback of 10oC, the performance is as shown in the following two sets
of graphs.
In the example 5.4 the heating capacity required for intermittent operation is calculated
to be 14.21 kW. The Tas simulation required 14.2 kW.
Air temperature and heating demand for - 1°C outside
Mean radiant (surface) and resultant temperatures
Summer temperatures and cooling loads
Guide A Section 5.2 Determination of overheat risk
A small office is specified with construction and internal heat gain details as found
on page 5-19. The calculation predicts resultant temperature between 11am and noon for
a sunny period in August.
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 repeated
warm sunny weather as per the August design day specified in Guide A.
The Tas model was run for the 30 days and the 24 hour performance curves are shown below.
The predicted resultant temperature at 11.30 am was 37.2°C for the example and 37.68°C for
Tas with the repeated hot day simulation.
Internal and external air temperatures
Mean radiant (surface) and resultant temperatures
Guide A Section 5 cooling load calculations
These procedures are not yet implemented in design software such as Hevacomp or Cymap.
To be of more direct value the comparison made here is between Tas predicted cooling
loads using the repeated sequence of hot design days, and the heat gain method currently
implemented in commonly used software.
The example office is used with cooling set to maintain 21°C internal air temperature.
The Tas simulation predicts a peak cooling load of 1.95 kW, whilst the CIBSE heat gain
method predicts 2.2 kW. The simulation loads with repeated hot days is ~10% less than the
standard heat gain method.
Conclusions
By using simulation with repeated design days the results for heating loads, cooling
loads and summer temperatures closely match loads and temperatures predicted by the
CIBSE steady state and admittance procedures.
This means that Tas models created for dynamic simulation based design analysis can also
be used to generate standard heating and cooling loads. The procedures to do this,
including the generation of repeated day weather data have been automated within Tas.