We need to know the total area of the walls (less the windows & doors) to estimate the heat load. The basic arithmetic is:
U-factor x square feet x temperature difference= BTU/hour
To that we have to insert a WAG or fudge-factor for air infiltration.
Are the slab edges insulated, as well as below?
The
99% outside design temp in Red Wing is about -12F (Minneapolis' is -11F, Eau Claire's is -13F, Rochester's is -12F.), so assuming a 68F interior design temp that's a temperature difference of 80F.
Your U-factors are approximately:
2x6/R21 wall (assuming 20% framing fraction): 0.065 BTU/hr per square foot per degree-F
R40 attic (assuming blown insulation in trusses 24" o.c. with 2x6 chords): 0.028 BTU/hr-F-ft^2
R8 door (assuming 2" thick foam, with wood spline structure): 0.17 BTU/hr-F-ft^2
Dual-pane argon single low-E windows w/vinyl frame: 0.34 BTU/hr-F-ft^2
So your door losses are:
0.17 BTU/hr-F-ft^2 x (18' x 8') x 80F= 806 BTU/hr
The window losses are:
0.34 BTU/hr-F-ft^2 x 96' x 80F= 2,611 BTU/hr
Assuming a 1200' ceiling the ceiling losses are:
0.028 BTU/hr-F-ft^2 x 1200' x 80F= 2688 BTU/hr
That's 6105 BTU/hr so far.
The remaining losses to estimate are primarily wall loss, infiltration loss, and slab loss. The infiltration is going to be WAG, the slab losses are too, but dependent upon the rough estimate of the slab temperature needed to support the load.
It's a bit difficult to figure out the near-boiler plumbing from that picture, but the expansion tank appears to have a less-than-ideal placement relative to the pumps.
Assuming the existing radiation pump has been adequate and is pumped direct with the electric boiler (rather than primary/secondary), what has the temperature difference between the return water and boiler output been, and what is the make/model of the pump?