Have about 7 weeks on my new installation of a Portage and Main 250 Optimizer furnace. Heating 9000 sq ft plus basement.Love the unit... really something to view the blue hot burn in the bottom!! and absolutly no visable smoke coming out the stack. I ran 60' of logstor 1 1/4" pipe into the house, pump is at furnace, Grundfos cast iron UPS26-99FC 3 speed model. I built an "H" looking header system to be flexable in that I can run some flow thru a filter to keep return temp up, or bypass house all together if needed thru a 1" bypass. First in my loop is a water to air heat exchanger, then on to a sidearm heat exchanger on a hot water heater, then on to where I have another identical furnace in the attic to feed another water to air heat exchanger, after this furnace water passes back down to basement where I turn and go to return side of my "H" loop to return to furnace. this is all 1" Pex inside. I have developed what is best described as a "thumping sound" that comes every 12 -15 seconds from the down pipe on the return side of this attic furance. It is 30-32' vertical rise to this unit in piping system. I know it's from air or to me seems like my pump can't shove enough around fast enough to keep air out. System is "open" at stove to atmospere. I removed the check valve from pump... thought it would restrict flow.... I have tried a pump that pumps 125' of head to try and bleed any entraped air out of pipes/heat exchanger but to no avail. Anyone have an idea of what causes this and how to resolve getting rid of it? Thanks for any help. Been a pretty mild winter but have had a few cold weeks so far... oh... 128-132 deg F out of registers, and 165 deg F on hot water. Run furnace between 180-185 to keep desolved O2 out of system, PH 9.6 East Tennessee.