Just to summarize between what has been said already and to expose a potential problem, Sizing Factors can be entered:

• For zones

  • There is the global Sizing:Parameters >This object allows the user to specify global heating and cooling sizing ratios. These ratios will be applied at the zone level to all of the zone heating and cooling loads and air flow rates. These new loads and air flow rates are then used to calculate the system level flow rates and capacities and are used in all component sizing calculations
  • There is the Sizing:Zone parameters > >This input overrides the building level sizing factor input in the Sizing:Parameters object. And, of course, if this field is blank or zero, the global heating sizing factor from the Sizing:Parameters object is used. >

• For Plant equipment

  • There is the system Sizing Factor, for plant equipment (boiler, chiller, cooling towers). Example for a Chiller:Electric:EIR > This optional numeric field allows the user to specify a sizing factor for this component. The sizing factor is used when the component design inputs are autosized: the autosizing calculations are performed as usual and the results are multiplied by the sizing factor

All of this is summarized in the Component Autosizing section:

There are 3 places in the input where the user can impose sizing factors.

In Sizing Parameters (object: Sizing:Parameters), the user can specify an over-all sizing factor. This factor is applied to all the zone design loads and air flow rates resulting from the zone sizing calculations.

In Zone Sizing (object: Sizing:Zone), the user can specify a sizing factor for a specific zone. The factor is applied to the calculated zone design loads and air flow rates for the zone named in the Sizing:Zone object. This sizing factor overrides the global sizing factor. That is, a zone sizing factor, if specified, replaces the global sizing factor for the named zone.

For some plant components (basically all central chillers, boilers and cooling towers) the user can specify a sizing factor that modifies the autosized component capacity and flow rates. These factors are applied after the application of global or zone sizing factors. They are primarily used to split the design load between multiple components. These sizing factors can change the autosizing of the associated loops and pumps. The following rules are followed the effect of plant component sizing factors on loops and pumps.

So you need to be careful with the interaction between those.

If you define a global Sizing:Paremeters factor of 1.2, all of your zone loads will be multiplied by 1.2. If you have defined (or left default) the system Sizing Factor for your chiller at 1.0, you'll get a chiller that is 1.2 times what is really needed.

Now, if you define Sizing:Parameters at 1.2 and chiller Sizing Factor at 1.2, you'll get a chiller that's roughly 1.44 times the actual needed capacity