The return from both furnaces, go into the mixing valve which is wide open, then into the rad heat manifolds. I went off from this drawing which shows the hot going into the water heater, then into the HX then into the manifold and out.
http://www.centralboiler.com/Tech/C190.pdfI just realized now though that it shows a note where the hot water splits and goes directly out to the OWB "and" to the rad heat manifolds. Really? I would think that since the rad heat lines are 1/2" and the return to the OWB is 1", the hot water would want to skip right past the rad heat manifold.
As far as height goes, my pump is about 5 feet below the ground my OWB is sitting on. the HX's are about 2-3 feet at most above the pump but the 3/4" PEX that feeds both HX goes up to the ceiling which is about 4' above the pump.
I do remember blowing through the second HX before I hooked it up, but didn't notice any restriction... of course I was just using my mouth.
The reason I split the two lines was I figured that the longer run would cool down faster than the shorter run, so if I ran them in series, the water would be alot cooler going into the floor lines. This way there would be a mix of hot water and really hot water going into the floor lines.
But again... I shut everything else down remember. Here's what I have for loops, approximate lengths:
1 main furnace run, 3/4", 20 feet total
1 second furnace run, 3/4", 190 feet total
1 garage rad heat loop, 1/2", 560 feet total
1 under garage rad heat loop, 1/2" 560 feet total
1 basement bedrooms rad heat loop, 1/2", 400 feet total
And it's been working great on all but the second furnace. The main HX stays hot, the floor lines stay hot, no problems.
So then I shut the valves to the main furnace and 2 of the long floor loops so now, all it was going through was:
1 second furnace run, 3/4", 190 feet total
1 basement bedrooms rad heat loop, 1/2", 400 feet total
and the fan cooled it down right away.