The M-100-AGB is a 100KBTU- in/ 83KBTU-out boiler, probably similar to the
MGB series (?) and ridiculously oversized for a house that size. The I=B=R net 72K rating would only be relevant if the boiler is fully outside of conditioned space, say, in a garage separated from the conditioned heated space by an insulated wall, or in a ventilated crawlspace under an insulated floor. It looks like yours is in a basement(?), where at least some of the standby losses still accrue to offsetting some of the heat load. (If the basement walls are insulated to the current code minimum virtually all of those standby losses would be heating the house, not the ground, and not "the Great outdoors". Check back on how to do it without creating a mold farm if that's a project you want to take on.)
Is the boiler also heating the domestic hot water with an embedded tankless coil or indirect fired tank? (I'm guessing not if the boiler was off for the summer.)
A dripping vent will only keep dripping if there's enough pressure on it. If the auto-fill is isolated from the potable supply the pressure will drop, and so will the drip, as the system pressure drops.
The circulator direction isn't going to affect the leak, only the amount of cavitation if it's mis-located relative to the expansion tank. If the pipe run into the circulator is pretty straight for at least a foot or three the cavitation issues can be pretty low even if pumping the "wrong" way .
The way the expansion tank is mounted in the picture looks like a disaster waiting to happen unless there is something mechanically supporting the weight of the expansion tank. That's quite a moment-arm of torque bearing down on that iron tee, and over time with temperature cycling & vibration it's only a matter of "when", not "if" it's going to break.
At 180F out the average water temp (AWT) through the radiation would be about 170F (unless way over-pumped, which it might be. At an AWT of 170F
typical fin-tube baseboard delivers about 500 BTU/hr per running foot. So with 75' of baseboard you only have enough heat emitter for 500 x 75' = 37,500 BTU/hr, which makes that boiler more than 2x oversized for the radiation even if it were operated as a single zone, but 4x oversized for any single zone, which means even at the coldest outdoor temps it's doing lot of unnecessary on/off cycling taking a toll on both efficiency and putting wear & tear on the ignition components.
A heat purging economizer would reduce the total number of cycles and lower the average operating & idling temperature of the boiler, saving something like half the standby & distribution losses, cutting fuel use by something like 10-20%. But if the boiler is actually leaking between the heat exchanger plates it may not be "worth it".
The design heat load of a 2x4 framed 860' rancher with single pane windows + clear storm windows over a full basement at at a typical +10F inland-NJ type
99% outside design temp would be 13-15,000 BTU/hr if it's pretty tight and the basement walls are insulated or 18-20,000 BTU/hr with no foundation insulation and/or leaking excessive air. That makes even the
zone radiation 2x oversized for the design load, and the boiler 8x oversized for any given zone, which means a very low duty-cycle with lots of standby loss.
With that low duty cycle the fact is it has relatively few burner hours compared to the same boiler serving a design heat load of ~60K, (which would have made it right-sized.) But it's also probably seen 4-5x as many ignition cycles as a right-sized boiler would have over the same number of years. From an ongoing reliability issue point of view the basic boiler probably still has a lot of life left to it, but the controls and ignition stuff hanging on it may have to be repaired/updated. But there may be other reasons for changing it out even if it's otherwise in good shape:
Run the fuel-use load numbers, but if the 75' of baseboard only needs to deliver 20,000 BTU/hr even at design condition, that's only 268 BTU/hr per foot, which it could deliver at an AWT of 130F (or less), which means a right sized modulating condensing boiler or a condensing water heater (isolated from the potable water by a heat exchanger) would improve the overall efficiency by quite a lot! There are some pretty good easy-to-retrofit stainless fire-tube boilers out there for under $2K. Bricking up the old flue port would also reduce a 24/365 stack-effect outdoor air infiltration driver that raises both the heating and cooling loads for the house.
Do you have central air? If yes, are the ducts in the basement, or did some idiot (as is all too common) install them in the attic above the insulation, where it increase both the heating and cooling loads for the house?
If not, do you WANT central air? The typical cooling load of a house that size is about 3/4 ton- 1 ton, and there are modulating ducted cold-climate heat pumps in the 3/4-1.5 ton range that are capable of delivering enough heat for a house that size (if sufficiently tightened up) at good to excellent efficiency. If it's an open floor plan even higher efficiency (and capacity) can be had with ductless solutions (often at an even lower upfront cost), but heat distribution to doored-off rooms has to be looked at carefully. In most markets high efficiency heat pumps still have a somewhat higher operating cost than condensing natural gas boilers, but not all. In low-priced electricity markets it can be less expensive.
With the fuel-use heat load calculation numbers we'll have a better idea of what makes sense in the near term. Most boilers can tolerate even 210F without undue stress, so even though the reason for the temperature overshoot to 190F isn't clear (could be an aquastat calibration issue, could be the aquastat's sensor isn't well coupled to the sensor well- some need to use a thermal goop work well), it's not doing anything bad to the boiler. The bigger issue is the constant on/off cycling.
What is the boiler's low-limit temperature set (or the differential, if it's the type of aquastat that is set to refire when xx degrees below the high temp)?
When fire up the boiler, start measuring the burn times and duty cycle when it's cycling on/off during an extended call for heat.
In the near term it's fine to just lower the boiler temperature and keep the high/low difference as wide as possible to keep the burn times as long as possible, utilizing the thermal mass of the boiler to the best extent possible with dumb aquastat controls. For low money (really low, as a DIY) a smart heat purging economizer control would take that a step further and fully optimize the use of the available thermal mass, if you decide you'll be keeping the boiler for several more years.
