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My shop is 32x60 with 18' walls. Walls are 4" of rigid foam. Floor is 6" slab with vapor barrier and 4" of foam. Have 8 300' 1/2 per loops in slab. Using a toyotomi OM-128 oil tankless boiler. Have 2 zones in system with 15-58 grundfos circ pumps. One zone has 5 loops the other 3. Thermostat are set at 54. Delta runs between 50-70 no matter what I have done. On the 5 loop zone manifold reads .5gpm per loop. If I close 4 it will go to 1gpm. Any input would be great.