Piping individual zones to upper floors of a multi unit

Adk1guy

I have a 3 story 6 unit, 2 on each floor. The heating systems was a funny story when I got it (later)
Presently my two ground floor units each have individual zones with standing cast iron hydronic, and the remaining 4 on the second and third floor are all on one zone. They don't balance well and a lot of heat is wasted.
I have a second floor bathroom gutted and am replacing the failing cast iron sewer stack from basement to roof top, and replacing brass domestic water with pex. This bathroom and the one above on the 3rd floor are stacked and identical. While I am at it and have a pathway open I can run heating risers up the wet wall which is almost dead center. The radiators are along one wall. I could run pex or copper on the second floor ceiling in a soffit and catch the 3rd floor radiators, and possibly the 2nd floor but risers would have to drop. I might be able to run pipe along the 1st floor ceiling and box them in a soffit.
Any one have experience or good ideas? A direct return design would save pipe but be inbalanced. An Indirect return would take a lot of pipe. Dropping from the ceiling down isn't a good idea, the radiator are in front of windows and would have to be purged from time to time.

Now for the funny story. When I got ahold of the building in the 90's, the previous owner had a half a@@sed plumber repipe the basement gravity piping. I know him. Nice guy. He could sweat copper but had no idea of design. Another story. he looped the risers and 1st floor radiators together. So as you can imagine the first radiator was very hot and the last radiator on the loop not very. He broke them into random zones for each apartment and put thermosets randomly one in each unit. I don't know how he chose, but at the thermostat affected at least one radiator in the unit and also the radiator above and below. So one apartment might have a need heat in other rooms but their thermostat was satisficed by a radiator controlled from a different apartment. The colder it got the more windows were opened. I drove by one night around 8pm, 20 below zero absolute not wind chill, and the windows were wide open in a 2nd floor apartment. I was outraged, pounded on their door, made an idiot of myself scolding them and slammed the windows closed. As I continued to lecture them I felt the heat in the room, like standing too close to a blast furnace. I said something like "holy cow it's really hot in here". I apologized and re opened the windows and said "I have to fix this". But that took some planning and more wherewithal than I had to do during a cold snap. Every apartment had to have some windows open in some rooms while they were too cold in others. I guess it's funny now to have windows wide open when it's 20 below zero, my money pouring out the windows. At least since I owned a fuel company I was getting my oil wholesale.

hot_rod

Well if you can get from A to B, it should work. Basically need to know the heat load to size the tube properly. 1/2" pex at those lengths could handle 15,000 btu at a common 20∆

EdTheHeaterMan

Can you get PEX home runs from each radiator to the renovated bathroom(s) while maintaining a somewhat downward slope from the Rad to the common location? Use a PEX manifold to connect all those home runs to a pair of riser from the boiler room. Zone valves in the boiler room to each zone supply (or return... pick one) riser. Cover the manifolds with a soffit or cabinet looking box for future service access. also place a drain pipe for purging the zones in that same soffit or cabinet. That way you don't need to run a garden hose thru the home every time you need to purge some air. You don't want to put that hose on the new carpet the day after you used it in a sloppy boiler room floor from yesterday's job. The new helper will be glad to learn from that mistake at your expense.

this diagram shows the concept that I might be talking about. Although the manifolds are located in the apartment below the actual user, the concept is sound and if you plan it properly with the proper isolation valves on the supply (shown) and the return (not shown) there will be minimum need to access the manifolds once the initial balancing is completed. All purging of the air from each radiator will be completed with an individual air vent on each radiator. All zone valves can be located in the basement/boiler room.

EdTheHeaterMan

I had another thought. What is the possibility that TRVs on each radiator can solve the balancing problem to the point of each room or radiator will have individual control. You will never get that complaint about "I set the thermostat on 70° and some rooms are 71° and other rooms are 69°. When can you come over to fix that?" because the thermostat adjustment is not a room temperature. it is a dial from 1 to 6 or A to H. The best kind of thermostat in my opinion>>> If you are cold turn it up, if you are hot turn it down.https://www.amazon.com/Dasing-Thermostatic-Radiator-Automatic-Temperature/dp/B0BJCXZVHB/ref=sr_1_3?keywords=thermostatic+radiator+valve&qid=1680894126&sr=8-3