Chiller not meeting set point with waterside economiser

Chiller chilled water supply temperature does not meet set point when the heat exchanger (HeatExchanger:FluidToFluid) is not in operation. In fact it appears to be influenced by the heat exchanger nominal capacity even though it is not operating: [Chiller supply CHW temp] = [setpoint temp] + [dT that HX can provide]

Not sure why it controls this way. There is still capacity in the chiller and condenser loop to meet set point, and even a lower setpoint - so don't believe it is a chiller or cooling tower size issue.

Using Design Builder / Energy Plus.

I've had trouble with HeatExchanger:FluidToFluid control is DesignBuilder as well. A few things I'd suggest, some of which you may have already done:

Check the Control Type on the HX dialog. Based on your description, I'm assuming you'd want it to be 7-Cooling setpoint modulated.
Check the staging of the chiller and HX at the loop level under "Plant Equipment Operation"

If those don't work, sometimes I've had to put the HX in series with the other pieces of equipment. This may require adding a separate "pseudo-condenser loop" in your model that just connects to the HX. Otherwise, since the HX is in series with the chiller, the HX will provide cooling all year round.

If you want to provide a little more description of your system, I may have some better ideas, or I may not. In the past, I've filled out a support ticket at the DesignBuilder help desk for certain HX issues.

Hope this helps, sorry I couldn't give a more detailed solution.