Zone heating - turning on a zone

I have a typical 1960's baseboard oil heating hot water system that is on three zones. Two are for the house and the third is for a detached greenhouse. This is in connecticut.
I have kept the greenhouse zone off for about the last 4 years since we moved in. To shut it off I disconnected the thermostat wires and shut the water feed to this zone. From the return and send pipes there are two valves for draining which is located in the basement and this is where I drained the water and attempted to blow out the lines.
I now want to get it operational again.
I am wondering what is the best method to refill the pipes, especially so it doesn't mess up the other two zones?
This is what I was thinking of doing: I'll shut the water to the main two house zones and then turn the water to the greenhouse until it flows from one of the drains, then shut the drain and start it up with the thermostat. If things are very rusty should I just keep flushing till relatively clear so it doesn't go into the rest of the house or does that not matter? Are there any issues with air and how would I get rid of that?
Thanks much!

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