bjl, It sounds like you are making some condensate when your Units are running, as it should be with a cold evaporator. Heating on a cold day will take more loop flow than cooling the house. Your loop will get colder as the winter progresses, but you'll have to wait until next winter to see that. When you have things running the way you want them, you should consider the auto air removal Watts float valve. It may save your system from air binding one day. Not being back filled would cause an initial warm water flow, but that should be pushed through the unit without causing a performance issue. It could be a safety issue if someone stepped into the hole or a pet got stuck. Check your septic aerator for air flow into the house. I'll have to check how they operate, but I don't believe the air should be allowed to go into your house. That could become a health issue. When you have more flow capacity, then you can check to see if the larger Unit somehow interferes with the smaller Unit. There are several people on this website that have 2 Units and they have a lot of questions about how they interact with each other over time. This becomes a question when the house won't stay warm after a few years of operation with no problems. My Brother bought a digital amp meter and mounted it on the wall next to his Unit in order to monitor compressor performance at a glance. He bought the Daikin Inverter/Variable speed compressor which is the latest in Geothermal design, so he has all the temperatures, pressures and flows on the wall. He was telling me how this newer design is the way to go, but few HVAC people know enough to work on them, so he has made his Unit into a Science Project. He doesn't have the automatic variable speed loop pumps that are available since he is using an open loop design where we are using the closed loop design. The Inverter Geothermal units have a 1 ton limit above the unit rating, so if you have a 5 ton, it can produce 6 ton. The Daikin was over $8,000 but should pay for itself in a very short period of time since it was a Hobby Install not a "$43,000 gonna get yur money back from the govment" with a 10 year payback deal. I'm running blind for now with my Trane. I have cheap temperature probes that I put in the insulation to monitor water temperature and a few extra for monitoring air temperatures. I didn't like the idea of installing a football air valve/peach valve as some pros call it, in my unit loop inlet & outlets. Keep me posted on how the Brutes work out, and don't let someone talk you into what they have on hand, buy them on line after spending some time watching the net for a good deal.