Question-and-Answer Resource for the Building Energy Modeling Community
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In response to the hints that you have been given, there needs to be some clarifications made as to what the EnergyPlus radiant model is doing.
The notion that there is no feedback between the radiant system, the wall, and the zone is absolutely false. There is feedback. While the radiant system, the wall, and the zone are modeled in different parts of the code, the results of the radiant system simulation is incorporated into the wall and zone calculation automatically via iteration. This is necessary to avoid any heat imbalances in the overall EnergyPlus heat balance based solution. This was one of the many advances over its predecessor programs like BLAST and DOE-2--that EnergyPlus integrated various solution parts. The EnergyPlus radiant model has always been integrated with both the zone and HVAC portions of the simulation.
The CTF method does approach some limits when dealing with very massive elements and other unusual constructions. The question is: how massive is your slab and what is the rest of your construction? Are you getting any warnings about not completing the warm-up period? The CTF approach as well as its extension to include heat sources/sinks present in radiant systems is fairly robust so I'm not sure that is the source of any differences you are seeing.
I completely agree with the comment about making sure that EnergyPlus is actually using the manual inputs you are trying to feed it. This is where I would start--making sure that your flow and water temperature data is matching what EnergyPlus is using. If it isn't, there shouldn't be an expectation that EnergyPlus surface temperatures are in agreement with your data.
