Justin- The reason your loop temp is dropping when the furnace runs is because you are trying to force 100% of your boiler water through that little 3/4" plate. The restriction there is HUGE and severely hinders the flow through your main loop. That's what we'd call being piped "in series" which is what was being discussed above by Yoder and RSI. You are now limiting you main loop flow to 2-3 GPM instead of 6-7 like it should be. What pump do you have at the boiler for that loop? Looking at the WM specs on that plate, you're only able to transfer 30,000 BTU at 3 GPM on the boiler loop at 180 degrees. This would allow 100 degree return temps to raise to 120 across the plate at 2.5 GPM on the radiant side (which is about par for 3 loops). With 1000ft of floor space, you also have a tubing shortage in the floor with only 3 loops, but that's small potatoes here. I didn't realize you had thermometers on the manifolds so that is good to know. What do the return temps from the floor look like without the furnace running? Say 125 supply, where is the return at? Looks to be a fairly considerable difference from the little I can see of the thermometers. The perimeter insulation has a very large bearing on radiant slab performance; and if you don't have any may very well be a large portion of the issue here. Return temps will tell some of that story
Jon- I hadn't considered an indirect tank, it sounded to me like you were running your heated domestic through the floor for space heating as well. Some people do that, it's actually recommended by some big name radiant companies (cough, cough, BlueRidge, cough) and a very crappy idea. Glad you weren't suckered into that!