Replacing cast iron rads with fin-tube baseboard is almost always a mistake. Fin tube has really non-linear output at water temps below ~115F or so, whereas rads are fairly linear all the way down to 80F water. The thermal mass of the water volume & cast iron also limits the amount of short cycling. Fin tube is extremely low-mass. To emit the full 25,000 BTU/hr min-fire output of your boiler without cycling takes about 125' of baseboard. With anything less than 100' it's likely to short-cycle due to the limited thermal mass. Even if the baseboard replaced radiators of equivalent output, the radiators are likely to have had 10x the thermal mass to work with, and far far fewer burn cycles per hour would be occurring.
97% efficiency is possible, but only if it can be run at sufficiently low temperature without short cycling. That's probably what the contractor was trying to tell you about improving the heat emitters.
With a perfectly dialed-in outdoor reset curve the zones would be running almost continuously during cooler weather. To some that may seem like it's "struggling to keep up" or "... doing squat...", but as long as the room temperatures aren't dropping it doesn't really matter if it takes a couple of hours (or even a couple of days) of run time before the thermostat is satisfied. A rapidly satisfied thermostat means the reset curve is set too high.
Morgan Audetat, a guy from Minnesota who used to post here fairly regularly, had one system that ran without a thermostat, only a perfectly dialed-in reset curve for control that ran constantly all winter. The indoor temp stayed within a few degree window all winter long, probably varying a bit based on the sunny vs cloudy, calm vs. windy conditions, but ran the pumps at a 100% duty cycle for months on end. (Most of us "semi-normal" people never tune the curves that closely. )
The storage temperature of the domestic hot water typically has to be at least 20F below the boiler output temp. If the boiler was set up for 190F during calls for hot water that might still be fine, but the delta-T across the boiler needs to be checked. Most boilers can't tolerate a difference of more than 50F between it's output and entering water temperature indefinitely. (I'm not sure what the specs are for this boiler.)
It's almost never the case that any heating zone would ever need 180F out of the reset curve. Most houses previously heated with an oil boiler that had been set to a high-limit of 180F do just fine with 140F max at the 99% outside design temp or lower. Does the zone with the recessed fin tube convectors and cast iron baseboard keep up, or does it actually lose ground at the lower curve setting?
How much of what type of radiation is there on the allegedly short-cycling zone 4?
To limit short cycling the boilers are set to trip off when the water temp goes too far above the reset target temp, and don't re-fire until it's several degrees below the target temp. If the curve's target setpoint was 90F and it tripped off at 100F, turning back on at 82F the boiler is behaving correctly, with an ~8F overshoot/~8F undershoot for the ~91F average. What that indicates is that there isn't sufficient zone radiation to emit the minimum-fire output of the boiler with ~90F water. The amount of thermal mass on the zone plumbing & radiation determines how quickly it slews from 82F to 100F at that water temperature.
For example, say there's 100lbs of water (or water equivalent thermal mass) in the zone radiation and it's only emitting 5000 BTU/hr at 90F, and that the 5000BTU/hr emitted is actuall the heat load- the room doesn't drop in temperature. But the min-fire output of the the boiler is 25,000 BTU/hr, so there is 20,000 BTU/hr of excess (= 333 BTU per minute) going into the system that isn't being emitted, thus the temperature begins to rise at 333/100lbs = 3.33F per minute.
With a +/-8F hysteresis around the setpoint (16F total swing) the burn times will be about 16F/3.33= 4.8 minutes. If set up perfectly the heat load is 5000 BTU/hr and the minimum fire output is 25,000 BTU/hr, so the duty cycle on the burner will be 5000/25,000= 20%, or about 12 minutes of burn time per hour. At 4.8 minutes per burn that's fewer than 3 full cycles per hour, which is not at all abusive of the system, efficiency stays high, only 2-3 ignition cycles & flue purges per hour.
But if there happens that there is only 10 lbs of thermal mass in the zone it'll have the same duty cycle, but 10x the numbers of cycles, with a boiler & efficiency destroying 0.48 minute (29 seconds) per burn time per cycle and a couple dozen burn cycles per hour.
Your reality is probably somewhere between those numbers, but it's worth figuring out what the numbers really are. Anything over 3 minutes is fine, and anything fewer than 5 cycles per hour is still pretty good.
