Does the supply pipe from the boiler go into the top of the duct coil or the bottom? It's a 3 row coil. The forced air blower fan will blow air upwards through the coil. I have seen it recommended both ways (and both ways were stated as the only way!) from the outdoor wood boiler websites. hmm.
Normally when running heat exchangers the two flows oppose each other. If your cool air is coming up from the bottom, your hot water would come down from the top. However with the 3 tube coil you have laying down, it shouldn't make much difference.