Question-and-Answer Resource for the Building Energy Modeling Community
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Based on the comments, it sounds like the increased temperature difference between the hot slab (which should be controlled to the same temperature in both climates) is the reasons. Pittsburgh is much colder, so you have a lot of heat loss, meaning that even though the system is more efficient, you are wasting extra heat by injecting it down into the ground. Atlanta, which has much warmer ground temperatures, doesn't have as much wastage and is able to send more heat upward into the space in comparison. You should be able to confirm this by looking at the heat transfer from the slab into the space (by radiation and convection) compared to the heat transfer from the slab into the ground (by conduction).
I think that's you're answer, but just to be complete, the reason you are seeing this is because it's a poor design for a radiant slab to not have insulation. If you add even a little bit of insulation under the radiant system, you prevent that heat from going into the ground (being wasted) which allows all of it to go up into the space. I suspect that a comparison like this would show a similar percentage heating energy savings, but higher absolute savings in locations, like PA, with colder temperatures.
