That short term drop you are seeing is probably just all the water between OWB and FPHX, happens to me all the time; cause I only system the pump on when there is a call for heat in the house. In less than 5 minutes, the lines (50' run each way of 1" PEX) are up to temperature and I can begin pulling heat off of stove.
How about this for a setup (MINE
):
Water comes off of OWB (pumped by SS Alpha, set to AutoAdapt and happily pumps at 3 GPM), thru thermostatic bypass (sends water back to OWB if below 140) then to 60 plate FPHX
*On other side of FPHE, mechanical mixing valve (with outdoor reset, max temp into slab is 110f), thru a different Cast Iron Alpha (in AutoAdapt) and out to 2-zone (6 & 8 loops, heating 4000sf)
**ONLY COMPLAINT, was from wife the week it was -40f and the house kept losing a degree a day (til it was about 65, in the house) at which point kicked pump up to HIGH between FPHX and slab and everyone was happy again, after a few hours