Question-and-Answer Resource for the Building Energy Modeling Community
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Radiant systems in E+ are unusual in that the temperature set point isn't controlled by the thermostat (as you have noticed), but instead by schedules in the radiant objects themselves. You can also control what type of temperatures (Mean Air, Mean Radiant, Operative) the setpoint schedule represents using this field.
I suggest checking out these schedules, and then plotting the zone temperature (whichever type you set as the control type) vs. time to see if the setpoint is being met. If the temperature is way below setpoint, you've found your issue. If the temperature looks OK, I would start by isolating the loads (using Ideal Loads). If the loads look like what you expect using Ideal Loads, you'll know it's an HVAC problem. If the Ideal Loads look too low, check out the outdoor air, the infiltration, and the envelope performance.
