The NCB 240eE's minimum modulated input is 18K, minimum output ~17K in condensing mode. That balances with about 85' of baseboard at condensing temps low enough to deliver 95% combustion efficiency. With lesser amounts of baseboard it will cycle, not modulate, based on the thermal mass of that zone, and the programmed temperature differential with which it operates.
If you combined the 53' of baseboard into one zone it would be 17,000 / 53= 320 BTU/hr per running foot, which balances at an average water temp (AWT) of ~140F. The best you can hope for is 150F out, 130F back. The entering water temperature (EWT) at the boiler needs to be below ~127F to condense
AT ALL.
At an AWT of about 130-135F it'll be at the beginning of condensing, 88-90% combustion efficeincy, and the 53' of baseboard would be emitting ~13-14,000 BTU/hr, which means there is still 3000 BTU/hr (50 BTU/minute) of excess heat going into the system. There is probably no more than 5lbs of thermal mass in that zone, so it'll be slewing on the order of 10F per minute. It's unlikely that the differential window is wider than 10F, so you're looking at sub-minute burns- it's short-cycling.
So, if what they "guarantee" violates the laws of physics, are you willing to take them up on it?
[edited to add]
In fact, as the temperature rises those 10 degrees out of the condensing zone the output is lower, and the heat rate coming out of the 53' of fin-tube rises, so the burn times would be longer than a minute, but the average combustion efficiency would be about 87%.
If instead of the combi boiler a
Navien NHB-55 or NHB-80 boiler were installed (along with an indirect hot water heater) either one of those boilers modulates down to 8K in, 7.6K out, and could modulate at 95% efficiency on a zone with only 38' of baseboard without cycling. Either one would also CLEARLY cover your 39K-45K heat load with margin, and would modulate well into the shoulder seasons, since the minimum output is 17-20% of the load at design condition.
With the combi boiler the minimum output is 37-45% of the design load, and will be forced in to cycling much of the time by the lack of sufficient load, even if there weren't the issue with the zone radiation being too small to operate in condensing mode (on at least the fin-tube zones.)
Regarding the magnitude of the heat load, even 39K is really pretty high for a house that size at Surrey's +25F-ish (~ -4C) 99% outside design temperature, and 45K would be an even bigger outlier for a tight 2x4 framed house, with clear-glass double-panes (or single-panes + storms). Most tight 2x4 framed houses that size would come in at about 30-35K @ -4C outdoors, +20C indoors. Is there a lot of window area (or single-pane window area) or something? Is the foundation insulated? Some amount of blower-door directed air-sealing and spot insulation upgrading might prove very cost effective. If the higher load is due to a lot of single-pane glass, low-E storm windows would cut the window losses by nearly 2/3, and would be very cost effective (far cheaper than replacement windows of similar performance.)