Dana says fin-tube baseboard emits about 200BTUs per foot. Isn't is more than double that, depending on GPM? Slant-Fin says 510BTUs at 170 degrees and 1GPM and 540 at 4GPM. Where are you getting 200BTUs per foot? The only way I see it in the 200BTUs neighborhood is when the temp is 120 or less (which I think is radiant floor territory)
The conditional phrase is "...at condensing temperatures...", not 170F, not 140F. (The conditional phrase was in the preceding sentence.)
Take a peek at
the specs for a typical fin-tube baseboard product.
Compare the output of the fin tube to the entering water temps of a representative condensing boiler curve:
At an average water temp of <115F entering water temp it takes to climb into the 90s for combustion efficiency Slantfin Fine Line 30 fin tube is putting out a bit less than 200BTU/hr per running foot. Say 122F out, 115F back, 118F average water temp (AWT). Interpolate the 110F and 120F specs on the baseboard if you like, but it's ~200 BTU/foot.
Changing it from 1gpm to 4gpm doesn't move the needle much when the water temps are that low, since it's not losing much temperature over the whole loop even at 1gpm. At 4 gpm the output comes up at most 10%, but the entering water temperature at the boiler goes up a lot, dropping the efficiency to 90% or less.
And yes, that is indeed staple-up radiant-floor territory, but that is what is necessary to get condensing efficiency out of the boiler. Most of the time the radiant floor would not need 120F water, and neither would the fin-tube. But low-height convector output becomes very non-linear when the AWT drops below 115F.
Baseboard is cheap, and in this non-modulating situation where the design heat load is probably less than the minimum fire output of the boiler it's best to to just install enough baseboard to ensure it'll condense at the min-fire output so that it won't short-cycle itself into low efficiency and an early grave.
If efficiency and comfort isn't important, go ahead and set it up to deliver the minimum firing rate output at a higher temp, just never turn the water temperature down, and don't expect it to be more efficient than a crummy cast iron beast. If it short cycles on the stubby run of fin tube crank the output temp up until the short cycling stops.
It's bad enough to run a modulating condensing boiler on a load so small that it never modulates, worse still to run it at temperatures that never condense. But if REALLY going for the trifecta, install just enough baseboard so that it can heat the space at high temp, even though the output of the boiler is 2x what the stub of fin tube is emitting so that it will be guaranteed to short-cycle for even lower efficiency and higher maintenance.
With glass in the windows and doors that shut, with even modest amounts of fluff in the walls and ceiling you're looking at a design heat load of ~7500 BTU/hr or maybe less in most NJ locations. So with 14' of baseboard and an EWT of 180F it would be covered. Many combi-boilers have a minimum output north of 18000 BTU/hr at non-condensing temps, so there would be 10KBTU/hr , over 150 BTU/minute of heat going into less than 20 lbs of water mass. The temperature of the system water would be rising at about 150/20 = 7.5F per minute, so even with the differential opened up to 15F (not always programmable with combi-boilers) the burn times are only 2 minutes with over a dozen burns per hour. Most combi boilers would be looking at 1 minute burns.
Fin tube is cheap. Buy lots of it, and do at least some napkin math and some quality time with the boiler dialing it in once it's installed.
