Oumina,
That's quite an interesting system you have! Unfortunately, I'm not aware of any way to model a heat pump that uses a building exhaust stream as a heat source/sink in eQuest. At the risk of understatement, it is not a commonly employed HVAC system. If you have to move forward I think your best approach would be to use eQuest's hourly reports to print out the hourly heating and cooling loads for your heat pump, the outside air temperature, and the temperature and flow rate of your exhaust stream. Then, using the heat pump performance curves provided in eQuest, calculate the cooling (or heating) COP for both the outdoor air temperature and the exhaust air temperature. Finally, use those two COPs to calculate the heat pump's electricity consumption for each scenario; the difference will be your (very roughly estimated) energy savings.
From a design perspective, I think this strategy carries some risks. Even if the exhaust flow rate is huge, a building exhaust will still have a very limited capacity to absorb or provide heat compared to all the outside air in the immediate vicinity of a heat pump condenser located outdoors. I could easily envision a scenario where the condenser raises or lowers the exhaust stream temperature beyond the heat pump's operational limits, which could "choke" the heat pump and limit its capacity. I don't mean to discourage you from investigating the idea; I just want to make you aware of the risks. You might be better served with another energy recovery strategy like using an energy recovery ventilator to preheat or precool makeup air.
Good luck!
