It doesn't look like the heat exchanger block would produce much cooling when there is no air flow, except for the amount of remaining air inside the heat exchanger volume. If the pipe diameter, length, and number are all reasonable, you may want to check the mdot at the A1 & B1 port and see if they make sense. The heat flow of the heat exchanger may also give you a clue.
If you can share the model, it can help the community diagnose your problem.
