Advice on pumping and radiator valve upgrades

Hi HeatingHelp,

I'm looking for some advice on some hydronic system upgrades I am contemplating. You can see some details of my system in some comments I made in this post but I'll summarize the details here, apologies for how long this story is!

The TLDR: I have a cast iron boiler with a boiler protection valuve in a gravity conversion system, and I suspect my current pumping arrangement is insufficient to get all my radiators warm except in very long heating calls (e.g. balancing issues). I am going to have my system drained this spring for other reasons, and will get my numerous seized radiator valves replaced so that I can at least try to balance things better. While this happens should I consider some "fancy" upgrades - TRVs and/or a better pumping arrangement?

The long story:
I have a Burnham X-209 (280,000 btu/hr input, 84% efficient, so ~ 235,000 btu/hr output). My installer estimates my design day heating load was about 200,000 btu/hr. Luckily we had design day temperatures a few days after this boiler was installed, and my ecobee shows that it ran for no more than 18 hours in any consecutive 24 hour period during that time, so my actual design day heating load is probably closer to 175,000 btu/hr.

I believe my piping is a gravity conversion. The supply mains right after the boiler output are quite large - at least 3 inches, maybe 4, and most of the runs to individual radiators are between 1 and 1.5 inches, at least those that I can see from the basement.

I have a ton of cast iron radiators, about 2200 sq ft EDR. This means that the radiators can emit considerably more than my boiler can produce (I think its about 350-375k btu/hr). As a result, my sytem runs cold. When the boiler was originally installed with direct piping (no primary/secondary, and no boiler protection), I was seeing supply temperatures that maxed out at 140F, and frequently were in the 120-130F range. When I say "cold" I don't mean that he house is cold, I just mean that the water temperatures are in principle too low for reliable operation of a cast iron boiler over many years. All of the radiators were definitely warm to the touch in this state of the world, but I was nervous about boiler longevity.

After noticing the low temperatures, and doing much research on this forum about this issue, I requested that my installer add a bypass loop and a Caleffi 280, the 1 1/4 inch variant. Initially, I had the 140F cartridge in this valve, and I was seeing extreme system imbalance issues - the closest radiators were quite hot and the other ~2/3 of them were lukewarm or cold, even an hour or two into a call for heat. I self-diagnosed this as being driven by the fact that the 140F cartridge was diverting more flow than was really needed, and ordered and installed the 130F valve myself, which did improve balance in my system. Today, I'd say 2/3 of my radiators get warm or hot immediately, 1/6 will eventually get warm/lukewarm after an hour or two of runtime, and 1/6 are still basically cold except after extremely long heating cycles. The cold radiators are, unsurprisingly, all at the end of their respective lines, which is unfortunate because a bunch of these are in or near my kitchen, where we spend lots of time. I (and my installer) have checked for air everywhere and at this point I think the air is gone.

I've installed temperature measurement on all 3 ports of the Caleffi valve, and estimated based on the heat balance equation that (1) my circulator - an NRF-33 - is pumping about 20 gpm, and (2) long into a call for heat the protection valve is still diverting 1/3 to 1/2 of this flow back into the boiler.

Here is the math behind these conclusions: about an hour into a call for heat, the boiler is sending out 155F degree water, and the mixed port on the valve is putting 131F water into the boiler, and the system return is about 110F. I think this implies ~20 gpm through the valve (235000/(500 * (155-131)) = 19.6), and that the bypass is taking 47% of it. If I wait another hour or two, the boiler and valve stay at similar temps, while the system return rises to 118F, meaning that the bypass is only taking 35%.

I think this means that best case my radiators are getting like 13-14 gpm. I never had especially careful temperature measurement before getting the bypass and valve installed, but my hacky checks prior suggested a delta T of 12-17F, implying that the pump was previously pushing about 30-33 gpm. I think this tracks with the NRF-33's pump curve at like 1-2 ft of head, which is what I think a gravity conversion ought to have, absent a boiler protection valve.

My installer's position on this situation is just that I need to do better balancing - throttle the radiators closer to the boiler and everything will be fine. I didn't get a chance to do that this heating season because most of my radiator valves are old and seized up. So while I waited this season out, I started doing the above measurement and math. When I presented these calculations to my installer, he didn't really have a response besides "you are just imbalanced", which to me means he either thinks I messed them up and wanted to politely avoid that, or doesn't get my point.

This spring I am going to get a radiator installed in a currently unheated room, and so while the system is drained for this work, I have the opportunity to get a bunch of these valves replaced, plus possibly other stuff.

I'm seeking your expert advice on what other stuff actually makes sense. Here are the ideas I'm contemplating.

Option 1: just do what my installer says, get these valves replaced, stick with manual balancing valves, and see if balancing is indeed all I needed. Do nothing on the boiler side. This is the cheapest, but to me it seems least likely to work, since 14 gpm seems too low for both the boiler and for the piping and radiators I've got.

Option 2: get valves replaced, but get some (or many) TRVs. If I do this I guess I would need to replace my NRF-33 with an ECM, and also get a dirt/magnetic filter installed. This is a bit more. I know if I do this I need to make sure to keep manual valves on the radiators closest to the main thermostat, etc. I don't see how this would solve my low flow issues, but I can see how it would get my hot radiators chilled out a bit sooner, leaving some flow available for the other currently cool radiators earlier in a heating cycle. Again this wouldn't change my pumping capacity, assuming I don't get an ECM that pumps substantially more than the NRF-33 does.

Option 3: get valves replaced, stick with manual valves, and separately deal with my pumping situation by getting a bigger pump. My guess is that this is feasible at not a ton of money, since the NRF-36 exists and fits in exactly the same space as my NRF-33. My napkin math suggests I could get a lot more flow this way: the 280 has a Cv of 14, and my system head is probably about 3ft. However, at the ~20gpm I currently push through this, I am already hearing some noise at the start of a call for heat (the first 2 mins or so). If I had the NRF-36 on speed 2, I think I'd probably see about 23-24 gpm, and at speed 3 maybe 28. Maybe this would solve my flow problems but make the noise way worse?

Option 4(a,b): get the valves replaced + get someone to come do primary/secondary piping for me, either using closely spaced tees or using a hydraulic separator. If I did manual valves (option 4a), I could keep the NRF-33 I already own on the system side, and then get a smaller pump for the boiler side (maybe an NRF-25 on speed 2?). If I did TRV's (option 4b) I'd need a different pump. While this is obviously a bunch more money, it seems "right" in the sense that I would decouple my system's pumping needs from what the boiler + protection valve need.

What do you think, HeatingHelp experts? What should I do? Is there another option you think I should pursue?

Thanks in advance for sharing your wisdom.