Getting my 1924 system going again

And just a side note - if the system is really at slightly over 7 psi, that's roughly the static head from the 2nd floor. It's roughly 15 to 18 feet higher than the boiler pressure gauge. And 2.31 feet per psi, that's 18 / 2.31 = 7.8 psi.

Yesterday, I found the electric power source for the boiler control unit, which in turn connects to the thermostat and the BASO A850-1. Photo of control unit nameplate below. The transformer had 20.5 VAC where it connects to the blue and white wires. I didn't get to checking voltages on the 5 or 6 terminals inside the dome on top of the gas control, nor at the thermostat and the BASO A850-1. Will do that soon.

Today's job was to get the pilot lit. It's been, um, a while. Forgot where it was. Finally opened the bottom door ("Clinker Door" on the photo below) of the boiler and there it was, still happily burning since probably the 1990s. I guess that answers whether the thermocouple and BASO A850-1 are still working. The pilot has a big flame, but I guess that's to help the boiler light quickly. On to that.

The boiler burners form a circle. See the photo below. It appears that the incoming gas is split into two separate feeds for the left and right halves of the circle. There are big pipes just below the two semicircular cutouts visible in the "non-burning" photo. They reach all the way out to just below the electric control unit, where there's some kind of manifold to connect them to the single inlet.

I used the manual lever on the boiler control unit (is that the correct name for it?) to light the boiler main burners. The right semi-circle lit quickly, front and back almost simultaneously. The left side took a while, one or two nozzles at a time. When it finally settled, the flames on the left side were smaller than the right and the back 1/4 of the circle on the left side never lit, even after 2 or 3 minutes. Door open all the time. Photo below.

Intuitively, it seems that the boiler would heat more efficiently with the whole circle burning. Is there a way to fix that - me or an experienced boiler guy? Or just leave it alone and carry on? Unless this is a serious safety concern, the cost of a complete boiler replacement is probably a deal breaker. It is, after all, my backup system.

And finally, what's the big diaphragm-looking thing between the gas feed pipe and the control unit?

Big thanks to all, especially to @mattmia2 @delcrossv and @EdTheHeaterMan. I really appreciate you sharing your knowledge and experience, plus taking the time to write thoughtful responses. Much appreciated!

I should have said at the beginning that I'm a degreed and registered mechanical engineer. I've had a long career in auto racing, blending hands-on work and science. By nature and experience, I'm methodical and analytical. And I've learned the hard way that just because you think something is true, that may not be the actual case. So - the things I can safely and correctly do myself, I will, because that's me. The rest, I'll be bringing someone in. More on that shortly.

The plan at the moment:

1-Finish restoring the thermostat. Confirm its wiring is good. Install it. Test to see if it will start and turn off the boiler, just running the boiler briefly. (me)

2-Get the parts to assemble the "hose bib pressure gauge". Use it to confirm the pressure gauge on the boiler. If both read roughly 7 psi, the system isn't pressurized, it just has the static head from the upstairs radiators and plumbing. That would seem to confirm that the PRV is stuck shut. (me)

A neighbor, also in an old house, highly recommends a "boiler guy" who maintains the hydronic systems in his large-ish house and the over-and-under duplex he owns next door. Both roughly the same vintage as my house. "Boiler guy" also maintains the systems in several large old nearby multi-story apartment buildings. So next, bring in the boiler guy.

3-General assessment of the system (Boiler guy)

4-Look at the boiler burners. I expect there are 3 ways this can go. #1 He says he can fix it and the price isn't a deal breaker, #2 He says he can't fix it, but it's safe, just not as efficient as it could be, or #3 He says it's unsafe and he can't fix it. If we wind up with #1 or #2, we proceed. (Boiler guy)

5-Replace the PRV and relief valve and install a new shutoff valve upstream of them. Thanks @delcrossv for mentioning that the current B&P are bronze. New flow: Water source>shutoff>PRV>relief>radiator return to boiler. Fix the weeping copper pipe at the hot water heater feed. (Boiler guy)

I can't find it now, but I think I read in Classic Hydronics that he thought it would be good for the boiler feed to come from the hot water heater, to avoid shocking the boiler. If that's a good idea, this would be the perfect time to do it.

6-Do whatever else the Boiler Guy found in step 3 (Boiler guy)

7-Fill, bleed, and test the system (me and Boiler guy together, so I'll know how to do it)

Your thoughts?

And if you need any help with suspension geometry, onboard data logging, pit stop strategy, spec'ing shock absorber damping, spring rates, tuning your aero downforce, cooling carbon fiber brakes, designing a roll cage, running a lap time simulation, or any of that stuff, I'm your guy. 🏁

Yup, strongly agree. My regular HVAC company says "we work on radiator systems". I'm inherently suspicious of that. That's why I'm leaning toward my neighbor's guy, if his background and experience is as billed.