A sidenote for those thinking of switching to this type of heat. My floors are cooler now because my basement is colder. The reason is that my old 1974 boiler leaked heat into my basement,
the new CH-240 does not .. therefore my basement is colder and my floors are colder. Something I never thought of !!! No bog deal - just a point of information. I have a temp gun
and my floors are 5 degrees colder than normal.
Running a lossy jacked high-mass oversized boiler hot enough to keep the basment hot is one EXPENSIVE way to make radiant floors, eh? ;-)
If you haven't already, consider...
A: Air sealing and insulating the foundation sills & rim joists, and sealing up all unused flues into the basment, fixing all the window & door weatherstripping, etc.
B: Insulating the basement walls n such a way that the foundation continues to dry toward the interior rather than forcing the moisture higher in the foundation to spall on the above-grade exterior, but with enough foam between the foundation and any interior wood or wallboard that the mold hazard stays low. Some suggestions on ways to go about it live
here. (The thermo-hygric analyses are for Minneapolis, so you could get away with slightly thinner foam in the foam/fiberglass stackups, but don't go less than an inch of XPS, or less than 3" if EPS. Don't use any rigid foam with foil/poly/vinyl facers, since they're too vapor retardent.)
When I air sealed the basement and put ~ R20 on 90% of the foundation walls it reduced my heating fuel use ~ 15-20%, and the basement stays 65F or above even after having converting to a low-mass tankless for a boiler. The uninsulated slab is still on the cool side, but the basement feels GOOD, and the first floors are quite reasonable in bare feet- always 65-67F in a 68-70F room. (Except for my radiant-floor zones, which are of course even BETTER in bare feet.
)
With old fashioned high-mass radiators setting your K to 3.7 would be a ridiculously high ramp which is almost certainly leading to the high hysteresis in your room temps. It's starting out with a huge delta-T between the mass of cold water coming from the cold radiators with a high firing rate, then the radiators end up dumping too much heat in into the rooms overshooting the setpoint, the whole place is then too hot for long enough that the radiators are then dead cold again, repeat. With the curve set correctly the radiators will warm up and cool down, but in a much narrower temperature range, and they'll never be super-hot or room-temp-cold to the touch when there's a moderate heat load.
Crank the K-curve down to 1 or 1.5 then bump it up or down depending on how it's behaving. Again, you're looking for long, even continuous burns between calls for heat from the thermostat. If your thermostat has a programmable hysteresis or anticipation, as zl700 suggests, you might want to adjust that for a more-even heat, once you're close to the right curve. It might be worth spending a few bucks on a Tekmar 500-series T-stat that uses a PID algorithm for locking the room temp dead-on. (They're often useful for to managing the thermal mass of concrete slab radiant floors with low-mass boilers to reduce under/overshoots on the room temp.)
Also, DON'T use deep overnight setbacks- if you do it takes forever to heat up in the AM unless you set the curve unnecessarily high, in which case you're blowing any savings that you might have gained in the setback period by spending more time with the burner running higher than condensing temps. You'll be more comfortable and use less fuel if you either keep a constant temp and dial in the curve.
Also, high mass radiators are GREAT for running low-temp water- far better than fin-tube baseboard. Fin-tube craps out and becomes unpredicatble at out temps below ~120F, so you end up having to set the temps higher during the shoulder seasons and running ~88-90% efficiency when you COULD be getting 95% out it. With the curve set right those old-school radiators should be able to deliver even better comfort than it did with the high-mass boiler running in bang-bang on/off mode. The installer that told you to set the curve to 3.7 either knows something I don't, or is completely clueless and just picked a number toward the high side of the middle of the programmable range figuring you at least wouldn't get too cold.