old cast iron radiators primary/secondary layout question

Colinmcm

New Utica combi boiler in basement. I am reinstalling three old cast iron radiators on third floor for 600 sq ft tenant apartment (bedroom, bathroom, LR/kitchen). The rads had previously been supplied by steel pipes exposed in rooms running vertical through first and second floors (brick house). Old pipes were removed as we redesigned and changed heating system in our unit, which is on floors 1 and 2, adding central air with a hydrocoil and forced air floor ducts as well as hyper heat minisplits for the bedrooms. 
I plan to run 2 lengths of 3/4" orange pex through a chase in the walls, vertical up to 3rd floor. These will be tied into the primary loop (1" copper) off the boiler in basement with a monoflow tee, which had been used for each radiator previously on only the return side. Question: do I need a monoflow tee on both supply and return in this case, since it's a pretty far distance, about 25 feet vertical rise? Then I want to create a supply manifold with 3 supply branches off it in the floor joist space of the 3rd floor, and a return manifold with 3 return branches, for the 3 rads. The old rads had large steel pipe reduced down to 1/2" copper on both supply and return sides. Question is do I need monoflow tees on those branches off the manifolds? And what kind of spacing between branches is permissible? Also how far horizontally can I plan to run in the floor joist space, to get to the radiator locations, without running into trouble with heat distribution?
Does this plan sound reasonable? Anything else I should add to the system? Thanks for any advice.

pedmec

i would try and create a separate zone for the third floor rads. Trying to get flow thru the branch of a monoflo tee is a tricky game. especially considering you are using pex piping. pex fittings are very restrictive. they are not full bore fittings so they will increase the resistance on the monoflo branch circuit. then you want to create another restrictive area by creating a manifold to feed the individual rads. i would forgo the monoflo and put a circulator on the pex loop. create a direct return system and Put a 1 x 3/4 tees in the primary. Separate zone control relay to control the circulator and to initiate boiler call.

You can't control nor determine the flow thru the branch of the monoflo without doing serious calculations. you can balance the flows out at the manifold. By setting it up as a direct return is your best chance of success.

EBEBRATT-Ed

I agree with @pedmec

Nothing wrong with Monoflo in the right application but since the old pipe was bigger you may not get the flow you want. Make it a separate zone especially with a tenant.

Colinmcm

Thank you for the responses. 3rd floor (1BR apt) is going to be the only heat load on that combi boiler. Our owner occupied unit has its own boiler so the tenant will pay their own gas bill for heat and hot water. In that case I don't need any signal relay correct, just the thermostat? The copper primary loop previously went all the way around the basement of the house but I'm making it smaller now, will only be about 25 linear feet of 1" copper with the boiler in the middle of loop. So you're advising I add a circulating pump by the start of the supply 1 by 3/4 pex branch? How do I make that pump talk to the boiler? What should I look for when I go buy a pump? Are the manifolds I proposed unnecessary? What would be the method of balancing the flows in the manifold? Can I make the manifold out of the same 3/4 pex? Is there a simpler design that will work instead of manifolds, like running the pex through the radiator in the LR/kitchen first (biggest space) then the rad in bedroom, then bathroom, just in a chain of reducing square footage? You mentioned direct return, but I read elsewhere that reverse return is better for balanced flow and pressure. Thoughts on that?

Colinmcm

I just made a quick drawing of the reverse return layout as I understand it with the circulator
pump you recommend adding... One other complication is that the vertical chase I have available to pass through the 1st and 2nd floors is very close to the locations of the BR and WC rads on 3rd floor, but not that close to the LR rad, the largest space. So no matter what I will have a pretty long horizontal run of piping to that one rad's supply and return connections through the floor joists. Any concerns with that?