The ability of the system to shut off for long periods of time means it has been running at a much higher than optimal water temperature, and even overshooting the thermostat's setpoint. That's a significant efficiency hit with a condensing boiler.
For highest efficiency it needs to run at the lowest temperature, with as few on/off cycles as possible, but with long burn-cycles. When the entering water temperature (EWT) at the boiler is above 130F there is zero condensing, and it'll be doing about 86-87% combustion efficiency. When it's above 150F it'll be more like 85%. The efficiency sweet spot starts at 125F EWT where it hits ~90%, and it increases rapidly as the EWT drops to about 115F, but more slowly thereafter.
To hit 95% efficiency the entering water temperature at the boiler needs to be ~120F or less, and burning at the lowest firing rate. With 40' of baseboard on a zone you can run the system at 125F out and probably get there. A 120F can emit about 8000 BTU/hr, which is about 20% less than the minimum fire output of the boiler, which is fine if you program the DIFFERENTIAL to 15-20F or so, and the OFFSET to 0-5F or so. With a 20F or higher temperature swing to slew through from the bottom of DIFFERENTIAL to the top of OFFSET the boiler will cycle/off when only one zone is calling for heat, but it probably won't short-cycle. You'll be able to play around with that and tweak it to where the burn times and cycles per hour are reasonable.
With fin-tube baseboard the output becomes very non-linear with water temperature once it's below 115-120F, so something like 120F-125F should be set up as your minimum operating setpoint independent of the curve.
To figure out a starting point for the cold outdoor temp/high water temp at the other end of the curve, first,
run a fuel-use based load calculation to come up with a BTUs per degree-hour constant, and from that derive about how many BTU/hr it takes to heat the place at +5F (your 99% outside design temperature), and how many BTU/hr it takes to heat the place at whatever point you have picked on the curve set-up. (If you've been running the boiler mostly above 140F, assume a boiler efficiency of 87% for those calculations.) Then divide the BTU/hr number by the number of feet of baseboard. The BTU/hr per foot of baseboard will determine the average water temperature (AWT) needed for the baseboard to emit that much heat. Most baseboard will deliver about 500-525 BTU/hr per running foot at an AWT of 170F, which in most systems corresponds to an EWT of about 180F. At an AWT of 120F it'll deliver about 200-225 BTU/hr per foot, which happens at an EWT of about 125F. You can either look up the specs on YOUR baseboard if you know the manufacturer & model, or
use this ratings chart as a rough guide for ball-parking it.
For example, say over a wintertime billing period you went through 250 therms, and from a nearby weatherstation data there were 1200 heating degree-days (base 65F) between meter reading dates. That means you used 250/1200= 0.21 therms per degree-day (rounding to 2 significant digits), which at 100,000 BTU/therm is 21,000 BTU/degree-day But at 87% efficiency only 0.87 x 21,000 = 18,270 BTU of heat stayed inside the heating system and house (the other 13% went up the flue).
An a 24 hour day, that means the house uses a 18,270/24= 677 BTU per degree-hour. Since we used base 65F heating degree days, the presumptive temperature at which the heat load is zero is 65F. Your 99% outside design temp is +5F, so you have 65F-5F= 60F heating degrees.
So to keep the place comfortable at +5F outdoors takes about 677 BTU/degree-hour x 60F heating degrees= 40,620 BTU/hr.
The load of 40,620 BTU/hr divided by 80' of fin-tube baseboard is 508 BTU/hr, which if you refer to that ratings chart happens at an AWT of about 170F, which takes a EWT of ~180F. So as a starting point for tweaking the curve you'd program the low end of the curve for 180F out at +5F, then see if it keeps up on cold days. If it does, try backing off the boiler temp by 5F every cold winter day until you wake up in the AM and the boiler has been running 100% of the time, and the house is a degree or three below the thermostat setpoint. Then bump up the water temp back up by 4F- it should eventually catch up and satisfy the thermostats. If it does, back off 1F every night until you wake up in the AM and the house is within 1F of the thermostat's setpoint.
For the warmer weather end of the curve, in condensing mode the WHN055 is good for about 10,000 BTU/hr. So with the 677BTU/degree-hour constant it means the outdoor temperature above-which the boiler has to cycle be 10,000/677= ~15F cooler than the 65F balance point, or ~50F. The ratio of 10,000 BTU/hr into 80' of baseboard is 125 BTU/hr per foot, which fin-tube would theoretically deliver at 95-100F AWT, but it's so non-linear that can't be counted on, so we'll attack it from another angle.
If one sets the minimum boiler temp to be 120F, the AWT will be about 115F, and at that AWT the baseboard emits about 175 BTU/hr per foot. So for 80' of baseboard that delivers about 14,000 BTU/hr at your chosen minimum water temperature. At 677BTU/degree-hour the load of 14K happens about 14,000/677 BTU/hr = 21F cooler than the presumptive 65F zero-load point, or about 39F. So setting the minimum output temperature be 120F any time it's above 39F would be about right.
With these starting parameters, the boiler output temp will ramp smoothly from 120F at 39F up to 180F as the outdoor temperatures fall to 5F, and it should keep up pretty much most of the time. During colder weather do your curve-tweaking adjustments only on the cold-temp end of the curve, but when it gets to be above freezing at night, if it's not quite keeping up move the outdoor temperature for the crossover to minimum water temperature up 5F at a time, then start backing down until you find the perfect balance where the house temperatuer just barely keeps up at night without satisfying the thermostats quickly.
When the outdoor temperatures are in the 50F range you can start looking at tweaking in DIFFERENTIAL and OFFSET numbers. Initially set the OFFSET to 0F, and the DIFFERENTIAL to 20F. If the burns during light-load days with just one zone calling for heat are shorter than 3 minutes, increase the DIFFERENTIAL by 5F at a time, but not to more than 30F (which would mean the system has to cool to 90F or lower before it re-fires, which may take too long and the house might cool off). If that doesn't quite get it to where the burn times are at least 3 minutes and <10 burns per hour, start bumping up the OFFSET. With a minimum boiler temp of 120F, taking the OFFSET higher than 10F would take it out of the condensing range at the end of a burn, so hopefully you won't have to go there.
