Contractors who would blindly hook up a mod-con with a min-fire output of 18K to a multizoned sytem with only 35-50' of low-mass baseboard on individual zones is clearly not great hydronic designers, even if it turns out they are decent plumbers. They should be able to talk you through the tweaking adjustments too, and should have started you out on a sort-of-reasonable K-factor that at least would heat the place.
Did they at least plumb & pump it primary/secondary, as shown in the manual?
I'm on the same page with mage182 on the relative merits of toe-kick heaters, wall coils, and their ilk. Toe kick heater increase the wind-chill on feet in the air path when the water temps are at low condensing levels, and when the call for heat is satisfied and zone kicks off the room chill begins immediately. With high mass radiation the average radiation temp in the room is higher, and when the flow stops it's still radiating, slowly cooling off- it's WAY more comfortable.
If brand new low temp panel radiators are too pricey, there are plenty of really great antique rads out there heading for scrap yards (cast iron baseboard too) at nearly scrap rates, and the usually clean up easily. Even if there isn't room for radiators, replacing and extending the existing cheapo baseboard convectors with high-mass cast iron baseboards will lengthen the minimum burn times measurably, and will provide more even heat than the fin-tube. Search the materials section of your local craigslist until you find the right radiation at the right price. Run room-by-room heat load calculations and figure out roughly how much radiation you'd be needing. It's fine to go big on radiation, but be sure to oversize it at the same proportion for every room on a given zone to keep the room-to-room temperatures relatively close.
The Burnham Radiant and Arco SunRad series low-profile convecting radiators are only 5" deep, ~20" tall, and put out a decent amount of heat due to their high "equivalent direct radiation" (EDR) per running foot of radiator. They're designed to fit in kneewalls and are low enough to fit under windows, which is a good location to place them, since the radiant temp of the window is pretty low compared to wall, and the higher radiant temp of the radiator counterbalances that, as well as counteracting the cold convection draft of chilled air falling down the face of the window. They don't eat up a lot of floor space, and it's easy to build out a narrow enclosure for the sides & top to meet an antique house aesthetic, or if painted on the top & sides they're not too ugly to just mount an inch out from the wall without enclosing them :
specs for Radiant EDR here.
A big old SunRad (feel free to re-paint 'em- they're not all this ugly when you get 'em- some are even worse!
) Specs for Sunrad EDR here.
These show up regularly at junk-radiator dealers if you can't find 'em on craiglist. Burham still makes the Radiant, but they're pretty pricy new. A handful of years ago I picked up a 56" & 40" pair of SunRads for a whopping total of ~$100 for an architecturally appropriate retrofit project in an attic space at my home. I only needed the 56" radiator to be able to heat the space at condensing temps, but they insisted I take both. I've seen them both cheaper & more expensive than that on CL, they usually more expensive at antique dealers, but they're around. They clean up really well, and are essentially bulletproof.
From a condensing temp output point of view, every square foot of EDR is worth 50 BTU/hr @ 120F AWT, which is where you'd like to be for condensing efficiency, and about 90 BTU/hr @ 140F AWT, which would still be OK if that's what it needs at the 99% outside design temp as long as you're using outdoor reset, but ideally it would be lower. So if a room had a 99% heat load of say, 3000 BTU/hr, you'd want at least 3000/90= 33 ft^2 EDR of radiator in that room, but 3000/50= 60 ft^2 EDR in that room would mean can deliver the heat at condensing temps 100% of the time.
At about a foot of EDR per inch of radiator length for these old-girls, it still takes decent sized radiator to get condensing temps 100% of the time, (60 ft^2 EDR takes a 60" wide Radiant) but anything wider than 33" could still be "condensing mostly" using outdoor reset. Fin-tube baseboard is only good for about 215 BTU/hr per foot @ 120F AWT. SunRad/Radiant at 1'EDR per running inch delivers about 12' EDR x 50BTU/ft^2= 600 BTU/hr per running foot @ 120F. So each linear foot of this type of short-thin rad is "worth" roughly 3 feet of baseboard- it's fairly wall-length efficient for something that still fits under windows and doesn't stick out into the room too much.
But the thermal mass of both the water volume and cast iron (did I mention they're wicked heavy?) means that even if the zone can't quite emit the full 18,000 BTU/hr at condensing temps, the burn lengths are still reasonably long, and burns per hour numbers are a fraction of what they would be if it had the same amount of output using fin-tube baseboard.
What is that "...R-6 siding..." you have installed? How deep are the framing cavities? The answers are important since it affects what you can and cannot safely get away with for wall insulation.
