Given that it seems to be a Heating Loop serving baseboard, and that you have a PUMP:CONSTANTSPEED:PUMP CONSTANT SPEED 1, and that it gets too hot in the summer, my guess is that it is likely caused by pump heat building up over time indeed.

I can offer two leads:

• Switch the Pump:ConstantSpeed Control Type from Continous (I'm guessing it's the current control type) to Intermittent. Read the I/O ref for this specific field, which does mention pump heat buildup too: https://bigladdersoftware.com/epx/docs/9-4/input-output-reference/group-pumps.html#field-pump-control-type-1-000

• Use an AvailabilityManager to switch off your PlantLoop in the summer months when no heat is required.

  • You could do it seasonally with na AvailabilityManager:ScheduledOff to switch off the HW loop between say May and October.
  • You could alternatively use an AvailabilityManager:HighTemperatureTurnOff to switch off the PlantLoop whenever the outside air temperature is above a certain value (eg: 15°C).