Why energy consumption is not proportional with Outdoor air

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There are several reasons why the building energy consumption would not be proportional with the outside air rate: (1) There are other loads in a building such as conduction, solar, internal heat gain, etc. If you lump those as A and the outside air load as B, would A+B be proportional to A+2B, A+3B, etc.? (2) There may be a deadband in the building thermostat setting, such as heating at 16C and cooling at 28C. If the building floats throughout the day, such as during the swing season, the heat flows due to the fresh air will never appear in the building energy consumption at all. (3) Differences in HVAC sizing relative to the ventilation loads can have a big impact on the building energy consumption. Since you're varying the outside air rate from 0 to 20, the equipment will be sized for the non-ventilation loads in the first case, and maybe completely on the ventilation loads in the second case. On the other hand, the HVAC might not be sized at all for the ventilation load, since many sizing routines don't even consider the ventilation load!

Lastly, have you checked to make sure that the Fresh Air Amount is truly as you've entered?

When you change the amount of outdoor air, the mixed air temperature entering the coils also changes (at least up to 100% outdoor air). I don't see exhaust fans in your simulation so any exhaust air is removed at the outdoor air mixing box and a mixture of outdoor air and return air determines the mixed air temperature. As the mixed air temperature changes the coil performance changes. The incremental changes from 1 to 2, or 2 to 3 are not all that different. Nor are the changes from 5 to 10, 10 to 15 and 15 to 20.

Check the mixed air temperature for these simulations to see how it is affected by increasing outdoor air quantity. Also, have you checked the zone temperatures to make sure they are the same between these simulations?