Adding zone to forced hot water heat system

That will work. You are fortunate that you can split it that way so easily!

Here is the best way to do this. Have a valve near the zone valve inlet location, and then have a valve at the outlet location. That way if you need to service the valve that requires opening up and letting the water out, you do not need to drain the whole system. And you only need to add 2 valves since the existing valves were installed with very short copper pipe between the fittings. And since those crimp fittings are a one and done i would suggest that you add longer pipe stubs to those crimp fittings for future use in the event you need to cut some off in the future, You don't need to throw those new valves away.

Or you can add the valves on the PEX side, with valves above and below.

I would not install the zone valves with PEX on either side, but that is a personal choice for me. No rule against it.

Higher the Cv number the less restriction. Honeywell V8403 was my go to valve. Many others liked the Taco 570 series, however I have found in the brief time at a local supply house, that many plumbers experienced many wiring problems.

For almost 100 years we have been operating what is today called "micro zones" with standard pumps, I don't see the need for the ECM Pump in this case. Would you benefit? Of course you would. Lower operating cost and perhaps less noise when only the smaller zone is calling for heat. Also optimizing the proper flow rate can't hurt either. How to do that may be a fixed ∆T or ∆P would be the best plan. Then when one zone finishes and the other zone starts, or both zones call at the same time, the pump can adjust to each change in real time.

yes there is an easy way to do it. I’m watching the Philadelphia Eagles kill Dallas right now. So after the game is over, I’ll draw your picture.

This has all the needed valves numbered for purging air from the boiler and from the system loops

Label all these valves and print these instructions and hang them near the boiler

To Purge air from the boiler primary loop

1. Close valve A7
2. Connect garden hose to valve A6 and run it to a floor drain, laundry drain or outside.
3. Open valve A5
4. Lift fast fill lever on F1. (CAUTION here. Watch the boiler pressure gauge and be ready to close A5 if the pressure goes above 25 PSI)
5. Open Valve A6 and listed for air in the boiler to be leaving the system. When you hear the air stop leaving for a count of 10 seconds, then close A6 and A5
6. Open valve A7, the primary loop should be air free now.
7. Set F1 fast fill lever to automatic
8. Open valve A5
9. If the boiler pressure is above 15 PSI, then open A6 to let the pressure drop below 15 PSI. then close A6

To Purge air from the system loops

1. Connect garden hose to valve A3 and run it to a floor drain, laundry drain or outside.
2. Close Valve A4 and A2
3. Set Z1 to manual open position
4. Open valve A5
5. Lift fast fill lever on F1. (Caution here. Watch the boiler pressure gauge and be ready to close A5 if the pressure goes above 25 PSI)
6. Open valve A3 and listed for air to leave thru garden hose. when you hear the air stop leaving for a count of 10 seconds, then close A3 and A5
7. Zone 1 is air free now.
8. Close valve A1
9. Set Z2 to manual
10. Open valve A5 (CAUTION here: F1 is still on fast fill here so keep your eye on the pressure gauge as above)
11. Open valve A3 and listed for air to leave thru garden hose. when you hear the air stop leaving for a count of 10 seconds, then close A3 and A5
12. Zone 2 is now air free.
13. Set both Z1 and Z2 to automatic operation.
14. Set F1 fast fill lever to automatic.
15. Open valve A5
16. If the boiler pressure is above 15 PSI, then open A3 to let the pressure drop below 15 PSI. then close A6
17. System is ready for operation.

If you feel that all the air is not purged after this is completed. then you must repeat the steps. when you have valve A3 or A6 open for more that 20 seconds, check to see that the boiler pressure is above 20 PSI during the air purge. If the pressure drop below 10 PSI during the air purge, then close A3 or A6 in order to build up more pressure. Then open A3 orA6 at 25 PSI to fast flush the air out. That higher pressure will force air to flow down with the faster flowing water and exit the purge hose. If the water is not moving fast enough, the air bubbles may be too buoyant to get entrained in the flow going strait down a pipe.