Didn't get notified about PM...apparently I had a setting turned off, hopefully fixed now.
I went back and looked over your photo from post #250, and your system looks pretty straightforward, other than the mixing valve, which I already noted. It looks like an air separator is right above the pressure tank. If it were me, I'd get rid of the mixing valve (the thing right above the pump). Other than that item (which would be needed if you had a boiler, but not for a tankless) it looks like everything is by the book, though it doesn't have a great location for fill ports. I also don't see a check valve after the pump, but those Grundfoss's often have them built-in. There is a nice valve kit that makes filling very easy:
58613 - Webstone 58613 - 3/4" Sweat Purge & Fill Ball Valve
You turn the handle and now it diverts the flow from the closed loop to the hose ports. So you feed in water on the hot side and purge it out on the cold (return) side and it ensures one-way flow. This allows you to get air out by purging through. But you don't have this...
Make sure you shut off/unplug your heater before you start any of this. You can't use the ports on the heater flush valves as those only work for flushing the heater. The only option I see is the hose fittings on the ends of the manifold. You would need to shut off the valves feeding those manifolds first (red & blue). Connect your pressure hose to the one on the lower manifold, and connect the return hose to the upper hose port. Purging should be done one zone at a time to be effective. To do this, you need to isolate each zone, one by one, and it gets tedious. Close both zone valves on ALL zones of the manifold (the white ones on the upper, and the flow valves on the lower) and leave one pair open (same zone, 1 in, 1 out). Flush that zone until it runs clear with no air for a few minutes. It may be best to leave off the return line filter fabric I mentioned above for this as it makes it hard to see air. When you are satisfied with that zone, close those valves and open the next pair. Always keep pressure on the system once you start. When you are all done, open up the manifold inlet valves, close the upper hose port, open all zone valves and pressurize to whatever your setting is (looks like 20-25 psi). You cannot avoid a tiny bit of air, but that is what the air separator valve is for.
Attached is a picture of filling/flushing my shop the first time. I had to use a pump and cart water in in buckets as i had no water source at the time. I was filling from the pump in the green bucket and dumping into the black tub, but eventually just put the pump in the black tub and recirculated it around that way. This system has a fill valve setup that is similar to the one I linked, but not as nice. Hard to see, but it is where the hoses are attached.
It's not hard to do, just tedious. If you have plain water, it is easier as you can use a hose from the well pump to do a lot of this. If you have antifreeze, you will need a pump with a hose fitting and really need to recirculate/capture the fluid.
Hope this helps.
-Dave
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