To account for capacitance, E+ adds a well-stirred tank to the inlet of each half loop (on for supply, one for demand). This Section of the engineering ref manual is of interest:
http://bigladdersoftware.com/epx/docs/8-7/engineering-reference/plant-condenser-loops.html#loop-capacitance-and-pump-heat
"The total plant loop volume is separated into two tanks, on on each
half-loop inlet. For normal loops (without common pipes) each tank is
one half of the plant loop volume. For common pipe plant loops, the tank
on the supply side inlet has three fourths of the volume and the tank
on the demand side inlet has one fourth. Each plant loop is assigned a
total fluid volume as user input or an autocalculate routine based on
the design flow rate. The size of the thermal capacitance affects the
speed of recovery from situations where the setpoint was not maintained.
The user must estimate a fluid volume based on the size of the pipes in
the loop. Note that rough estimates seem to be sufficient. Loop capacitance (m3) could be calculated from pipe size data but this is not usually known"
Best,
Julien