Best Of

Air comes out of solution best at the hottest point, so as a general recommendation air purgers go at the supply. But that is not what is causing your issues.

Looks like some work was done here? A check added to the bypass?? Someone shooting the parts cannon at it?

Think of a primary loop as a conveyer belt on your system.

The boiler loads heat into that conveyer. To take a portion of heat out you tap into the conveyer belt.

By using two tees close together you create a hydraulic separation between the conveyer and the zone. So the individual zones just pull off the conveyer what they need. And yes, in your example each set of tees down the line receives lower water temperature.

With low temperature zones that is not a concern. You have mixing valves to adjust the temperature you need.

I don't know how deep you want to dive into theory?

This issue 19 we took the most common mistakes made in piping and tried to explain and show the fix. Many are missed attempts at primary secondary. I think you will find your exact piping in here, sorry.

https://www.caleffi.com/en-us/magazine/19-proven-hydronic-distribution-systems

Another good read by our host

Here is a way. hand sketch, I think you could correct this with the least amount of repiping.

Connect the upper red line with the lower blue line, red line shown. Now you have you zone loop.

Next the boiler needs to feed a supply of hot into that loop, but also a return back. Closely spaced tees MAKE a primary secondary piping system.

Now system and pumps are isolated from one another. Boiler pump no longer interferes with the zone flow, they are isolated, hydraulically speaking.

It will be a messy version of this "horseshoe" piping method.