ApacheSim reports an instability. How do I prevent this?
Surface Temperature instability shows up when you have one of just a few situations:
- A very, very small thermal zone (relative to the connected mechanical system capacity and/or internal/external thermal loads);
- A conditioned thermal zone with one very small dimension, relative to its other dimensions (e.g., a chilled or heated floor slab);
- A thermal zone with insufficient space conditioning relative to the rate of heat gain or loss in that space;
- A thermal zone or surface element thereof that is subject to very high solar gains;
- A thermal zone or surface element thereof that, a result of highly conductive materials, is subject to extreme thermal losses or gains;
- Internal gains or HVAC terminal units with a high radiant fraction;
- Numerous, complex, or substantial MacroFlo openings and associated controls;
- Very small or non-existent temperature control deadbands;
- Space temperature controls based upon sensed variable with a high radiant fraction.
As you know the error message provides the reasons/ remedies for the issue -
- add thermal capacity to constructions
- Reduce time step (From 10 minutes to 6 minutes, then 2 minutes etc..)
- set control radiant fraction to 0
- check gain and ventilation levels
- reduce macroflo opening areas
Instability problems are most commonly caused by:
1. MacroFlo openings that are too large (eg 50% openable area).
2. Very large gains turned on suddenly.
3. Setting the Control Temperature Radiant Fraction (Apache View -> Settings -> Building) to a value greater than 0. We recommend using the default setting (control based on air temperature).
4. Extremely lightweight structures - eg large areas of walls or roof made entirely of glass. These can give rise to a 'surface instability'. Adding a small amount of thermal mass to the construction by increasing the density of a layer usually overcomes the problem.
Occasionally the instability is a result of over complicated or flawed geometry. The Instability error message will give the ID of the problematic room, when the above options have been checked it is often an idea to review the problem room and attempt to simplify the geometry or separate it into smaller volumes (especially if the room is a composite space consisting of one or more subspaces that have been Connected)
