E+: Fan:OnOff consumes 1/3 of Fan:ConstantVolume

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In the process of trial and error linked to this question, I've set up two test cases where the only difference is: * First case, the AirLoopHVACUnitarySystem has a Fan:ConstantVolume, logically with a Supply Air Fan Operating Mode Schedule Name of always 1 (it won't accept anything else, see doc) * Second case, and the other has a Fan:OnOff, with a Supply Air Fan Operating Mode Schedule Name corresponding to my operating hours.

For some reason, my total EUI went down by 15%, with fan power lowering by 2/3 of what it was and some cooling savings (fraction of motor heat probably), and a small heating penalty.

Both of my fans are either running at a constant level or off, and they are exactly the same in both simulation.

Here's what the Zone Mechanical Ventilation Current Density Volume Flow Rate looks like for the same exact zone, for case 1 (left) and case 2 (right):

Here are the fans for two systems (the first one is the one represented on the above graph), but I've checked that all of them are the same between the two cases

Note that the Fan:OnOff has two modified curves: * Fan Power Ratio Function of Speed Ratio Curve Name => Set to only have a constant coefficient of 1, rest 0 * Fan Efficiency Ratio Function of Speed Ratio Curve Name => Also set Constant of 1, rest 0

Edit: I also tried removing the curve objects, and I get exactly the same results

Why would a Fan:OnOff consumes only 1/3 of the consumption of a Fan:ConstantVolume if they are used exactly the same?

Edit

I've checked the Fan Runtime Fraction for the Fan:OnOff: when on it's 1 in almost all zones, except a few that are around 0.75 (don't know why).

I've used the OpenStudio report for Outdoor Air, and multiplied the Zone Volume by the Avg Mechanical Ventilation (ACH), and summed that across all zones. The Fan:OnOff cases only has less ventilation by 2.5%, not nearly enough to justify saving 66% on fan energy.