ApacheCalc does not converge - '50 iterations' error message?
ApacheCalc (CIBSE Loads) occasionally reports a problem with convergence and repeatedly asks if you want to perform a further 50 iterations. This scenario tends to occur for buildings with closely-coupled internal spaces.
This can be dealt with as follows:
1. Find out whether the problem lies in Heat Loss or Heat Gain by turning one calculation off, you can do this on the calculation settings dialog before running the calculations.
2. In the case of a problem in Heat Gain (Cooling loads calculation), find out which month the problem occurs by running the months individually.
3. Continue iterating by choosing to continue for a total of (say) 300 iterations. If the results converge then these results can be relied on, else we need to proceed to step 4 and check them in more detail.
4. Rename the results file (*.htg for Heat Loss, *.clg for Heat Gain) and run again, this time with 350 iterations.
You can now compare the results of the 300 iterations and 350 iterations cases to look for any instability. This will typically manifest as fluctuating room temperatures, the software has stringent tests for this so will be sensitive and err on the side of caution by throwing the error message to allow the user to investigate further. You should pay particular attention to the coupled spaces and details of the interior design which could lead to these fluxes if adjacent rooms are being conditioned differently for instance.
