Help/Advice requested, undoing some iffy "modernizations" of a 1920s water radiator system.

Thanks for the suggestion, I really appreciate it. That does sound like a pretty solid bet. A couple questions in relation to that plan:

1. Do you know if 3/4" PEX would be sufficient for the main distribution loop on a 2 pipe reverse return system? I was unsure if Monoflow or 2-pipe is more demanding in that regard, as Monoflow has more obstructions in the tees, but has less total feet of pipe.

2. Do you have any idea of if my existing Taco 007 Circulator would be up to the task of the 2-pipe reverse return?

3. I've been trying to educate myself on the pros and cons of Direct Return vs. Reverse Return on 2-pipe systems. I found this quote in a guide on hydronic heat system design:

"The designer should attempt to select all terminal units with nearly-equal pressure drops if a reverse-return arrangement is to be used. If the above considerations cannot be met, the primary advantage of a reverse-return piping arrangement will be lost, and the use of a lower-cost direct-return system would be more practical."

"If terminal units have widely varying pressure drops or if throttling-type valves are used to control flow rates, it is generally advisable to use a direct return piping arrangement."

Given that I am looking to continue to use my existing cast iron radiators, I don't have any control over the pressure drops, and would be balancing the radiators heat output using valves on the outflow from each one, which is a sort of throttling valve I think, then do you think Reverse Return is still advantageous for me over Direct Return?

Yeah, I agree fully regarding the series loop/radiator combo! I didn't know jack about this stuff when I bought this house, and was working in a day job out of state, so I just trusted the plumber who my parents had used for years to do me right. He told me he thought the heat "should balance out fine", and when the first Winter hit, and it didn't, and I'd realized how it was a series loop, I called him up. I had the other house with the Fluidflow that worked great, and I asked him why he hadn't done it using that or a 2 pipe, to better distribute the heat. He actually told me "No one does it like that any more, the fittings cost way too much". Now, after struggling with it for the past 15yrs, I've actually looked up the fittings and see it would have cost me an extra $100-200, on a system that cost somewhere around $10k! So for 1-2% extra cost, I could have avoided 15 years of hassles, tenant complaints, freeze issues, etc…!😡

Yeah man, I hear ya on the butchering! You are correct that the house originally had a single boiler. Not sure what the original 1920s fuel source was, but when I bought it it had an oil boiler, that was certainly 50+ years old, but not sure if it was 90yrs old. I also saw signs of earlier old tanks, so it was on at least it's 2nd tank.

Wow, that suggestion about the hot water tanks is blowing my mind. I have never even heard of that, but I can see how it could be uniquely helpful in certain circumstances. I see you are talking about using one for each boiler, but you also mentioned "You could use these tanks for a single bottom fed gravity hot water system for both living spaces as buffer tanks on the return side of each boiler." Sorry if it's a dumb question, but I'm unfamiliar with this concept. Would that intermingle the fuel consumption for the 2 living spaces? I want to be sure to keep the utilities separate. Also, would that hot water tank setup be useful even with an electric circulator, or just if we converted it to a gravity fed system? The 2nd floor has the same 1/2" copper running from the basement up to each radiator, and from what I'm reading Gravity systems need large pipes, so not sure if that would work without a pretty major retrofit.

Regarding the orifice discs, I was unfamiliar with them, but just looked them up. Seems that they were/are used to balance flow to different radiators, and I can certainly see how having them in place could mess with a lot of retrofit configurations that differ from the original. Is it likely that they would have used them if the radiators also have balancing valves on the return branch, or would the valves have negated the need and offered more adjustability? I am attaching a pic of the valve in case that's helpful.

Thanks for the suggestion and diagram. Am I correct in thinking that if I were to split my current single series loop output into 2, I would be getting half the flow rate on each of the smaller series loops? If my issue is that I'm not getting heat past halfway around my current circuit, would splitting the circuit into 2, but halving the flow rate not result in the same issue at the latter radiators in each of the series loops (i.e. the water takes so dang long to get there that it has lost all its heat)?

Even still, I could see splitting things up into 2 loops being helpful just in terms of layout, so if I ended up doing a 2-pipe reverse return setup, having 2 separate loops of that seems like it could aid the layout while the bypasses on each radiator would allow the water to get to each of the radiators in the series faster.

@Jamie Hall Thank you for juxtaposing the various options discussed above! As you saw, when I'd first inquired, I was leaning towards swapping my series loop for either a 1-pipe Fluidflow, or a 2-pipe setup. Given that no-one here seems to be advocating for a new Fluidflow installation, and you made a strong case for 2-pipe reverse return, I was leaning towards that option.

Plus, I have the aforementioned bias toward passive low tech systems vs. adding additional powered circulators for secondary loops, which some of these primary/secondary diagrams I'm seeing involve.

After @hot_rod mentioned splitting it into 2 series loops, I started thinking of the layout, and for some reason the idea of 2 loops sounds appealing, although I'd still want to do the 2-pipe reverse return on each of them for the individual radiator/room control. If Jamie is saying that there's no harm in doing 2 loops vs. 1 (and potentially even a benefit), I'm now thinking about splitting my current output into a loop that runs from the #1 through #5 radiators on the diagram in my original post, and then a 2nd loop that runs backwards from the #10 to the #6 radiators. Both would be reverse return, and with the flow split between 2 loops, it seems like 3/4" PEX may be sufficient as I could run up to 4gpm through each loop, which is comparable in capacity to the 1" loop @Kaos had suggested for a single loop system, and it fits with @Jamie Hall's point about 3/4" PEX maxxing out at about 40k BTUh, so 2 individual 3/4" loops could handle my Max boiler output of 77 MBH. I'm looking to make it work well, but also get it done quickly, as I have cranky tenants and our temps have been in the single digits the last few nights, so getting it to 80% of optimal is good enough for all of our needs.

In my own living space, I can see how a manifold/home run system that I can fine tune for my preferences could be a great setup, but I rent mostly to student tenants from the local universities, so in this case I'd prefer to have a setup where they can regulate each radiator individually from their living space. That way, they're not needing to go to the basement to try to adjust the heat delivery for their loop, and if one roommate wants it to be 80f in their room and another likes to optimize their sleep with a 60f room, they can self manage with the valves on each radiator (which has not been possible with the current series loop system).

3 questions for y'all:

1. Does the above 2 loop 3/4inch 2-pipe per loop reverse return system seem like a "80% of optimal" solution to you?
2. Should I remove the super small/tight elbow following the flowcheck in the upper right of the following pic when I set up the new system? It seems unnecessarily restrictive. My inclination would be to place the 3/4" wye or tee to split into the 2 loops right after the flowcheck to avoid the bottleneck, and then just flex the PEX to head off in the proper direction, and avoid unnecessary restrictions.
   
3. Will the valves in the following pic, that I'd thought were balancing valves, work to modulate heat to the individual rooms? @leonz informed me that they are actually shutoff valves, and so I'm assuming they don't offer the same level of fidelity as proper balancing valves. These valves will go 1/2 turn, lock to lock, so fine tuning is assuredly a bit more difficult but, assuming that they can be set to intermediate points between open and closed, I am having trouble seeing why they can't be used for modulating the heat delivery to individual rooms with the above spec, 2-pipe 2 loop, system.
   

Thanks again, everyone, for your contributions. You're really helping me narrow down the myriad options to get to something that will get us not just back in action, but restored to proper function.

@mattmia2 Thank you for your input. On your first post, you said "whoever "repaired" it didn't understand hydronics." Yeah, fully agreed, and every winter I get more and more angry at that idiot/slacker (who's name happens to be Tim). He went to tech school, and grew up/inherited the plumbing business from his dad, plus my parents had relied on their plumbing biz for years, so I'd assumed he knew what he was doing, but I never got a good vibe from him when we were talking face to face. I've moved on to doing things myself with a great "old timer" handyman, but have the urge to call Tim up and give him a piece of my mind every time I end up dealing with particularly irksome situations on this system.

Your description of someone messing with a radiator valve and shutting down the whole system rings very true to me, as I've been there and done that. We took the knobs off all the shutoff/balancing valves, to deter that, but that doesn't keep someone from using a wrench on the flats of the stem. It's particularly difficult with student renters, due to the high turnover, different move-in dates of each roommate, and possible language barriers with international students (of which, there are an increasing amount). That is why I was originally looking at a swap to Fluidflow or 2-pipe, as that is how I was assuming the system was originally configured, and restoring the bypasses around each radiator would be very forgiving of a single roommate turning their radiator off (for whatever reason). Basically, idiot proofing is always a consideration for me

Regarding your second post, thank you for the suggestions about specific pipe diameters, and reassurance that I'm not crazy with the 2-pipe split-loop idea. I love the idea of optimizing fuel usage, but as long as my tenants are happy I am less concerned about Short Cycling. so that layout is gaining steam with me.