If I’m understanding your question correctly, I think the ventilation losses you are getting are correct as from a thermodynamic perspective, you can only recover a fraction of the heat from exhaust air.
For example, if you have a system where your exhaust air is recirculated into a mixing chamber with fresh outdoor air, and assuming the box is perfectly insulated, you will have the following relationship between the input and output air:
- MassAir1 + MassAir2 = MassAir3
- MassAir1 * Enthalpy1 + MassAir2 * Enthalpy2 = MassAir3*Enthalpy3
Rearranging 2 and substituting 1 for MassAir3 we get:
Enthalpy3 = (MassAir1 * Enthalpy1 + MassAir2 * Enthalpy2)/ (MassAir1 + MassAir2)
So the resulting enthalpy is the weighted average of the exhaust and outdoor air, by their air mass flow rate (same thing also applies to temperature assuming the same specific heat capacity, and humidity ratio). Enthalpy is the total heat content of our air, so this equation proves that you can’t transfer all your heat from one air mass to another. Assuming equal exhaust and outdoor air rates, the best you can recover is half the heat content.
In your bar chart, you can also see that going from 0 to 99.99% heat recovery is reducing the mechanical ventilation more then 50% which is something I can’t explain. I’m not an expert on HVAC systems, so there may be something I’m missing on how mechanical heat recovery systems work, but this is how I would think about it.
S