3/4 Hot Water Feed to 2 1/2 "heat zones"

Question

Let me start by thanking you for looking at my question.

My oil burner has a 3/4 inch line from the circulator to the heating pipes that feed the downstairs. The 3/4 line splits into Zone A and Zone B which drops to 1/2. During a reno this summer, zone B was cut and capped as a project to come back to. Since the temp outside is near freezing now is as good a time as any.

Should I run the currently capped pipe back to where the junction was and do a 1/2 x 1/2 x 3/4 inch T (Idea B) or can I run the 1/2 inch into the 3/4 and because its reduced later down the line I'll have enough pressure to still feed Zone B (Idea A). Picture attached because I dont think I am explaining myself well.

Answer 1

If Zones A&B each have a zone valve or similar control, any of these works, so do the most convenient.

What you might need if they don't have individual zoning, or even if they do, for when both zones are calling for heat, would be balancing valves to adjust the relative flow in each zone. I assume that zone B is going "out of the picture;" if it has become much, much shorter than zone A, you might need a balancing valve you can adjust so it does not rob zone A whenever it wants heat, unless that's a behavior you desire.

If the relative change in overall pipe length is minor (there's a lot of B out of the picture) you might not need any balancing valve, or it might be needed in zone A, if that's shorter. Or perhaps there are already balancing valves in the existing piping. They are a bit non-obvious if you don't know what to look for.

e.g. this post has a balancing valve typical of the ones I know: Is this a bleeder? Why won't it close?